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Photoprotection mechanisms are ubiquitous among photosynthetic organisms. The photoprotection capacity of the green alga Chlamydomonas reinhardtii is correlated with protein levels of stress-related light-harvesting complex (LHCSR) proteins, which are strongly induced by high light (HL). However, the dynamic response of overall thylakoid structure during acclimation to growth in HL has not been fully understood. Here, we combined live-cell super-resolution microscopy and analytical membrane subfractionation to investigate macroscale structural changes of thylakoid membranes during HL acclimation in Chlamydomonas. Subdiffraction-resolution live-cell imaging revealed that the overall thylakoid structures became thinned and shrunken during HL acclimation. The stromal space around the pyrenoid also became enlarged. Analytical density-dependent membrane fractionation indicated that the structural changes were partly a consequence of membrane unstacking. The analysis of both an LHCSR loss-of-function mutant, npq4 lhcsr1, and a regulatory mutant that over-expresses LHCSR, spa1-1, showed that structural changes occurred independently of LHCSR protein levels, demonstrating that LHCSR was neither necessary nor sufficient to induce the thylakoid structural changes associated with HL acclimation. In contrast, stt7-9, a mutant lacking a kinase of major light-harvesting antenna proteins, had a slower thylakoid structural response to HL relative to all other lines tested but still showed membrane unstacking. These results indicate that neither LHCSR- nor antenna-phosphorylation-dependent HL acclimation are required for the observed macroscale structural changes of thylakoid membranes in HL conditions.

Reef-building corals have a variety of green fluorescent protein (GFP)-like proteins, also known as fluorescent proteins (FPs). These proteins have broad spectral properties covering most of the visible spectrum, with fluorophores fluorescing from cyan to red. However, the role of FPs is still a topic of debate and requires further investigation, particularly in the direction of mapping these FPs within the coral tissue and describing their cell- and tissue-level distributions. This study applied confocal laser scanning microscopy (CLSM) to investigate species-specific differences in the distribution of FPs in three coral taxa (Stylophora sp., Acropora sp., Echinopora sp.), combined with their photoacclimation response and that of associated symbiotic algae to light gradients in a controlled aquarium experiment. CLSM produced high-resolution images that enabled the identification of different FPs, their tissue distribution and quantification of their fluorescence intensity, as well as quantification of symbiont chlorophyll a (chl- a) fluorescence. Emission scans revealed three emission peaks between 490 - 501 nm (cyan, CFPs), 510 - 515 nm (green, GFPs), and 679 nm (chl- a fluorescence signal; Fchl) shared by all three studied species. The distribution of GFPs in Stylophora was concentrated in the intermesenterial muscle bands of the polyp, whereas CFPs were typically located at the tips of the tentacles. In contrast, Acropora and Echinopora exhibited agglomeration of CFPs and GFPs primarily in the epidermis. In general, species-specific differences in FP distribution remained unaltered during the experiment. However, linear regression models showed a significant negative relationship between CFP fluorescence intensity and light irradiance in Stylophora , whereas Echinopora exhibited a negative relation between chlorophyll fluorescence (Fchl) and light. In summary, the CLSM methodology provided a high-resolution tool to study coral FP patterns and symbiont response to irradiance, revealing ecophysiological differences among coral species at the tissue and cellular levels. CLSM has the potential to elucidate the intricacies of coral photobiology within the natural environment and to discern their adaptive responses in situ. [ABSTRACT FROM AUTHOR]

Primary production in the Southern Ocean is strongly influenced by the availability of light and iron (Fe). To examine the response of two distinct natural Antarctic phytoplankton communities (diatom vs. flagellates) to increasing light and Fe availability, we conducted two shipboard incubation experiments during late summer and exposed each community to increasing light intensities (30, 80, and 150 µmol photons m−2 s−1) with or without Fe amendment. Our results show clearly that both communities were Fe-limited since Fe addition resulted in higher particulate organic carbon (POC) production rates. The magnitude of the Fe-dependent increase in POC production, however, varied between the two stations being higher in the diatom-dominated community relative to the flagellate-dominated community. This differential response to increasing Fe supply could be attributed to the higher Fe requirement of the flagellate-dominated assemblage relative to the diatom-dominated assemblage. Irrespective of Fe availability, light also strongly stimulated the POC production of both communities between low and medium light supply (30 versus 80 µmol photons m−2 s−1), indicating that both assemblages were light-limited in situ. However, since POC production of both communities did not increase further at the highest light intensity (150 µmol photons m−2 s−1) even under high Fe supply, this suggests that light supply was saturated or that other conditions must be fulfilled (e.g., availability of trace metals other than Fe) in order for the communities to benefit from the higher light and Fe conditions. [ABSTRACT FROM AUTHOR]

Cryptophytes are abundant and ubiquitous microalgae that constitute a major plastid source for kleptoplastidic ciliates and dinoflagellates. Despite their ecological significance, the understanding of their light preferences and photophysiology remains limited. Here, we provide a comprehensive study of the response of the haploid strain Teleaulax amphioxeia (Cr10EHU) to varying light irradiance. This strain is capable of growing under a wide range of irradiance levels, notably by finely tuning the different pigments bound to the membrane light‐harvesting proteins. Analysis of the luminal phycoerythrin content revealed remarkable flexibility, with phycoerythrin emerging as a pivotal protein facilitating acclimation to varying light levels. Detailed ultrastructure examinations unveiled that this adaptability was supported by the synthesis of large thylakoidal vesicles, likely enhancing the capture of green photons efficiently under low light, a phenomenon previously undocumented. Teleaulax amphioxeia Cr10EHU effectively regulated light utilization by using a cryptophyte state transition‐like process, with a larger amplitude observed under high growth irradiance. Furthermore, our results revealed the establishment of growth irradiance‐dependent non‐photochemical quenching of fluorescence, likely inducing the dissipation of excess light. This study underscores the particularities and the significant photoadaptability of the plastid of the haploid form of T. amphioxeia. It constitutes a comprehensive photophysiological characterization of the Cr10EHU strain that paves the way for future studies of the kleptoplastidy process. [ABSTRACT FROM AUTHOR]

Reef-building corals have a variety of green fluorescent protein (GFP)-like proteins, also known as fluorescent proteins (FPs). These proteins have broad spectral properties covering most of the visible spectrum, with fluorophores fluorescing from cyan to red. However, the role of FPs is still a topic of debate and requires further investigation, particularly in the direction of mapping these FPs within the coral tissue and describing their cell- and tissue-level distributions. This study applied confocal laser scanning microscopy (CLSM) to investigate species-specific differences in the distribution of FPs in three coral taxa (Stylophora sp., Acropora sp., Echinopora sp.), combined with their photoacclimation response and that of associated symbiotic algae to light gradients in a controlled aquarium experiment. CLSM produced high-resolution images that enabled the identification of different FPs, their tissue distribution and quantification of their fluorescence intensity, as well as quantification of symbiont chlorophyll a (chl-a) fluorescence. Emission scans revealed three emission peaks between 490 - 501 nm (cyan, CFPs), 510 - 515 nm (green, GFPs), and 679 nm (chl-a fluorescence signal; Fchl) shared by all three studied species. The distribution of GFPs in Stylophora was concentrated in the intermesenterial muscle bands of the polyp, whereas CFPs were typically located at the tips of the tentacles. In contrast, Acropora and Echinopora exhibited agglomeration of CFPs and GFPs primarily in the epidermis. In general, species-specific differences in FP distribution remained unaltered during the experiment. However, linear regression models showed a significant negative relationship between CFP fluorescence intensity and light irradiance in Stylophora, whereas Echinopora exhibited a negative relation between chlorophyll fluorescence (Fchl) and light. In summary, the CLSM methodology provided a high-resolution tool to study coral FP patterns and symbiont response to irradiance, revealing ecophysiological differences among coral species at the tissue and cellular levels. CLSM has the potential to elucidate the intricacies of coral photobiology within the natural environment and to discern their adaptive responses in situ.

Primary production in the Southern Ocean is strongly influenced by the availability of light and iron (Fe). To examine the response of two distinct natural Antarctic phytoplankton communities (diatom vs. flagellates) to increasing light and Fe availability, we conducted two shipboard incubation experiments during late summer and exposed each community to increasing light intensities (30, 80, and 150 µmol photons m−2 s−1) with or without Fe amendment. Our results show clearly that both communities were Fe-limited since Fe addition resulted in higher particulate organic carbon (POC) production rates. The magnitude of the Fe-dependent increase in POC production, however, varied between the two stations being higher in the diatom-dominated community relative to the flagellate-dominated community. This differential response to increasing Fe supply could be attributed to the higher Fe requirement of the flagellate-dominated assemblage relative to the diatom-dominated assemblage. Irrespective of Fe availability, light also strongly stimulated the POC production of both communities between low and medium light supply (30 versus 80 µmol photons m−2 s−1), indicating that both assemblages were light-limited in situ. However, since POC production of both communities did not increase further at the highest light intensity (150 µmol photons m−2 s−1) even under high Fe supply, this suggests that light supply was saturated or that other conditions must be fulfilled (e.g., availability of trace metals other than Fe) in order for the communities to benefit from the higher light and Fe conditions.

Four strains of green microalgae (Scenedesmus acutus, Scenedesmus vacuolatus, Chlorella sorokiniana, and Chlamydomonas reinhardtii) were compared to determine growth and pigment composition under photoautotrophic or heterotrophic conditions. Batch growth experiments were performed in multicultivators with online monitoring of optical density. For photoautotrophic growth, light-limited (CO2-sufficient) growth was analyzed under different light intensities during the exponential and deceleration growth phases. The specific growth rate, measured during the exponential phase, and the maximal biomass productivity, measured during the deceleration phase, were not related to each other when different light intensities and different species were considered. This indicates species-dependent photoacclimation effects during cultivation time, which was confirmed by light-dependent changes in pigment content and composition when exponential and deceleration phases were compared. Except for C. reinhardtii, which does not grow on glucose, heterotrophic growth was promoted to similar extents by acetate and by glucose; however, these two substrates led to different pigment compositions. Weak light increased the pigment content during heterotrophy in the four species but was efficient in promoting growth only in S. acutus. C. sorokiniana, and S. vacuolatus exhibited the best potential for heterotrophic biomass productivities, both on glucose and acetate, with carotenoid (lutein) content being the highest in the former. [ABSTRACT FROM AUTHOR]

Mesodinium rubrum is a kleptoplastidic ciliate that sequesters the chloroplasts and nuclei of cryptophyte algae to perform photosynthesis. Blooms of M. rubrum can cause red tides in coastal oceans worldwide. Such red tides are detectable by remote sensing, and studying M. rubrum pigments and optical properties is a crucial step toward characterizing its blooms using satellite observation. Previous studies have shown that M. rubrum photoacclimates, modifying its pigment content depending on irradiance. Using cultures at different irradiance levels, we observed that photoacclimation in M. rubrum closely resembles that of its cryptophyte prey Teleaulax amphioxeia , leading to substantial phenotypic variability. In both species, phycoerythrin 545 cellular concentrations increased 3-fold between the highest and lowest irradiance, suggesting a major role in photoacclimation. Absorption cross-section decreased, and pigment-specific absorption coefficients increased with irradiance at the peak absorption wavelengths of chlorophyll a and phycoerythrin 545. After assessing the variability of absorption properties in M. rubrum , we combined field measurements and high-resolution Sentinel-2 satellite images to estimate chlorophyll a concentration of a coastal red tide and document small-scale spatio-temporal features. This work provides an overview of pigment photoacclimation in a peculiar phytoplankter and suggests guidelines for future studies of M. rubrum blooms. [ABSTRACT FROM AUTHOR]

Mixotrophic protists combine photosynthesis and phagotrophy to obtain energy and nutrients. Because mixotrophs can act as either primary producers or consumers, they have a complex role in marine food webs and biogeochemical cycles. Many mixotrophs are also phenotypically plastic and can adjust their metabolic investments in response to resource availability. Thus, a single species's ecological role may vary with environmental conditions. Here, we quantified how light and food availability impacted the growth rates, energy acquisition rates, and metabolic investment strategies of eight strains of the mixotrophic chrysophyte, Ochromonas. All eight Ochromonas strains photoacclimated by decreasing chlorophyll content as light intensity increased. Some strains were obligate phototrophs that required light for growth, while other strains showed stronger metabolic responses to prey availability. When prey availability was high, all eight strains exhibited accelerated growth rates and decreased their investments in both photosynthesis and phagotrophy. Photosynthesis and phagotrophy generally produced additive benefits: In low‐prey environments, Ochromonas growth rates increased to maximum, light‐saturated rates with increasing light but increased further with the addition of abundant bacterial prey. The additive benefits observed between photosynthesis and phagotrophy in Ochromonas suggest that the two metabolic modes provide nonsubstitutable resources, which may explain why a tradeoff between phagotrophic and phototrophic investments emerged in some but not all strains. [ABSTRACT FROM AUTHOR]

Abstract Background The light-harvesting antennae of photosystem (PS) I and PSII are pigment-protein complexes responsible of the initial steps of sunlight conversion into chemical energy. In natural environments plants are constantly confronted with the variability of the photosynthetically active light spectrum. PSII and PSI operate in series but have different optimal excitation wavelengths. The prompt adjustment of light absorption by photosystems is thus crucial to ensure efficient electron flow needed to sustain downstream carbon fixing reactions. Fast structural rearrangements equilibrate the partition of excitation pressure between PSII and PSI following the enrichment in the red (PSII-favoring) or far-red (PSI-favoring) spectra. Redox imbalances trigger state transitions (ST), a photoacclimation mechanism which involves the reversible phosphorylation/dephosphorylation of light harvesting complex II (LHCII) proteins by the antagonistic activities of the State Transition 7 (STN7) kinase/TAP38 phosphatase enzyme pair. During ST, a mobile PSII antenna pool associates with PSI increasing its absorption cross section. LHCII consists of assorted trimeric assemblies of Lhcb1, Lhcb2 and Lhcb3 protein isoforms (LHCII), several being substrates of STN7. However, the precise roles of Lhcb phosphorylation during ST remain largely elusive. Results We inactivated the complete Lhcb1 and Lhcb2 gene clades in Arabidopsis thaliana and reintroduced either wild type Lhcb1.3 and Lhcb2.1 isoforms, respectively, or versions lacking N-terminal phosphorylatable residues proposed to mediate state transitions. While the substitution of Lhcb2.1 Thr-40 prevented the formation of the PSI-LHCI-LHCII complex, replacement of Lhcb1.3 Thr-38 did not affect the formation of this supercomplex, nor did influence the amplitude or kinetics of PSII fluorescence quenching upon state 1—state 2 transition. Conclusions Phosphorylation of Lhcb2 Thr-40 by STN7 alone accounts for ≈ 60% of PSII fluorescence quenching during state transitions. Instead, the presence of Thr-38 phosphosite in Lhcb1.3 was not required for the formation of the PSI-LHCI-LHCII supercomplex nor for re-equilibration of the plastoquinone redox state. The Lhcb2 phosphomutant was still capable of ≈ 40% residual fluorescence quenching, implying that a yet uncharacterized, STN7-dependent, component of state transitions, which is unrelated to Lhcb2 Thr-40 phosphorylation and to the formation of the PSI-LHCI-LHCII supercomplex, contributes to the equilibration of the PSI/PSII excitation pressure upon plastoquinone over-reduction.

Background and Aims Kelp forests are increasingly considered blue carbon habitats for ocean-based biological carbon dioxide removal, but knowledge gaps remain in our understanding of their carbon cycle. Of particular interest is the remineralization of detritus, which can remain photosynthetically active. Here, we study a widespread, thermotolerant kelp (Ecklonia radiata) to explore detrital photosynthesis as a mechanism underlying temperature and light as two key drivers of remineralization. Methods We used meta-analysis to constrain the thermal optimum (T opt) of E. radiata. Temperature and light were subsequently controlled over a 119-day ex situ decomposition experiment. Flow-through experimental tanks were kept in darkness at 15 °C or under a subcompensating maximal irradiance of 8 µmol photons m−2 s−1 at 15, 20 or 25 °C. Photosynthesis of laterals (analogues to leaves) was estimated using closed-chamber oxygen evolution in darkness and under a saturating irradiance of 420 µmol photons m−2 s−1. Key Results T opt of E. radiata is 18 °C across performance variables (photosynthesis, growth, abundance, size, mass and fertility), life stages (gametophyte and sporophyte) and populations. Our models predict that a temperature of >15 °C reduces the potential for E. radiata detritus to be photosynthetically viable, hence detrital T opt ≤ 15 °C. Detritus is viable under subcompensating irradiance, where it performs better than in darkness. Comparison of net and gross photosynthesis indicates that elevated temperature primarily decreases detrital photosynthesis, whereas darkness primarily increases detrital respiration compared with optimal experimental conditions, in which detrital photosynthesis can persist for ≥119 days. Conclusions T opt of kelp detritus is ≥3 °C colder than that of the intact plant. Given that E. radiata is one of the most temperature-tolerant kelps, this suggests that photosynthesis is generally more thermosensitive in the detrital phase, which partly explains the enhancing effect of temperature on remineralization. In contrast to darkness, even subcompensating irradiance maintains detrital viability, elucidating the accelerating effect of depth and its concomitant light reduction on remineralization to some extent. Detrital photosynthesis is a meaningful mechanism underlying at least two drivers of remineralization, even below the photoenvironment inhabited by the attached alga. [ABSTRACT FROM AUTHOR]

Phytoplankton is the most important primary producer in the ocean, owing to its pivotal role at the base of the food chain and in the ocean carbon cycle. To better estimate ocean productivity, the retrieval of accurate observations of phytoplankton carbon (Cphy) biomass from ocean color satellites is of the utmost importance. When Cphy is retrieved using the backscattering signal generated by marine particles, the signal due to non‐algal particles—NAP (bbNAP) must be removed. However, algorithms currently used to retrieve bbNAP do not work globally, especially in the large mid‐ocean subtropical gyres. Here, we propose a new model to derive Cphy globally, including all subtropical gyres, that accounts for the estimation of phytoplankton photoacclimation when retrieving bbNAP. The new model shows more valid retrievable bbNAP coefficients than the previously published methods and values mainly around 0.0011 m−1, consistent with coefficients estimated with in situ observations. Moreover, the retrieved Cphy concentrations show a relative error of 7% with respect to in situ measured picophytoplankton carbon data. Our research indicates that the photoacclimation status of phytoplankton must be addressed to confidently retrieve bbNAP and Cphy at the global scale from ocean color measurements. Plain Language Summary: Chlorophyll‐a is often considered as the best proxy for phytoplankton biomass in the ocean. However, in low‐nutrient surface waters of the open oceans, the relationship between chlorophyll‐a and phytoplankton biomass can be weak or even negative. To get a better measure of phytoplankton biomass, researchers have started to use particulate backscattering coefficient (bbp). But before this can be used to measure phytoplankton biomass, the non‐algal particle fraction (bbNAP) needs to be deducted from bbp. In this study, we propose a model to more accurately determine bbNAP, and use satellite data to estimate sea‐surface phytoplankton biomass in the ocean. Our model can be used in subtropical regions, where other methods may not work. This will help us to better understand the variability of phytoplankton biomass in the global ocean. Key Points: A photoacclimation model is employed to retrieve both phytoplankton carbon and non‐algal particles from ocean color dataThe new model performs well in open oceans, especially in subtropical gyres, validated by an in situ phytoplankton carbon biomass data setPrevious studies using a constant backscattering of non‐algal particles notably overestimated phytoplankton carbon in subtropical gyres [ABSTRACT FROM AUTHOR]

Sea ice algae have a broad salinity tolerance but can experience stress during rapid decreases in salinity that occur with seasonal ice melt and during ice sample melt. This study investigated the impact of salinity on the photophysiological responses of bottom-ice algal communities from two Svalbard fjords (Tempelfjorden and Van Mijenfjorden). To further investigate the impact of salinity alone, and particularly to rapid freshening, the responses of a lab-cultured ice algal community from Van Mijenfjorden were assessed. Photophysiological responses were mainly determined via 14C-based incubations which provided photosynthesis-irradiance curves. Main findings showed that i) the bottom-ice algal community in Tempelfjorden was characterized by lower photosynthetic efficiency and chlorophyll a biomass than the Van Mijenfjorden communities, and ii) a lab-cultured ice algal community from Van Mijenfjorden dominated by pennate diatoms had significantly lower photosynthetic efficiency, maximum photosynthesis and photoacclimation index after a decrease in salinity from 33 to 10. The lower photosynthetic efficiency and chlorophyll a biomass at Tempelfjorden may be attributed to the almost two-fold lower bulk-ice salinity in Tempelfjorden compared to Van Mijenfjorden, which was likely associated with freshwater inputs from the tidewater glacier Tunabreen during sea ice formation. Other factors such as under-ice light intensities, brine volume fraction and brine nutrient concentrations likely also contributed to variability in ice algal response. Furthermore, experimental results indicated that the cultured Van Mijenfjorden community was negatively impacted by a rapid (within 4 to 24 h) reduction in salinity from 33 to 10. We further documented a significant start of recovery of these algae after 168 h. From this work, we surmise that decreases in surface water salinity, for example arising from the intensifying freshening of fjord waters, may only cause temporary changes in ice algal photoacclimation state and thus in chlorophyll a biomass. Further, this study also supports the need for salinity buffered melt of sea ice samples to reduce artificial bias in biological measurements. [ABSTRACT FROM AUTHOR]

Sea ice algae have a broad salinity tolerance but can experience stress during rapid decreases in salinity that occur with seasonal ice melt and during ice sample melt. This study investigated the impact of salinity on the photophysiological responses of bottom-ice algal communities from two Svalbard fjords (Tempelfjorden and Van Mijenfjorden). To further investigate the impact of salinity alone, and particularly to rapid freshening, the responses of a lab-cultured ice algal community from Van Mijenfjorden were assessed. Photophysiological responses were mainly determined via 14C-based incubations which provided photosynthesis-irradiance curves. Main findings showed that i) the bottom-ice algal community in Tempelfjorden was characterized by lower photosynthetic efficiency and chlorophyll a biomass than the Van Mijenfjorden communities, and ii) a lab-cultured ice algal community from Van Mijenfjorden dominated by pennate diatoms had significantly lower photosynthetic efficiency, maximum photosynthesis and photoacclimation index after a decrease in salinity from 33 to 10. The lower photosynthetic efficiency and chlorophyll a biomass at Tempelfjorden may be attributed to the almost two-fold lower bulk-ice salinity in Tempelfjorden compared to Van Mijenfjorden, which was likely associated with freshwater inputs from the tidewater glacier Tunabreen during sea ice formation. Other factors such as under-ice light intensities, brine volume fraction and brine nutrient concentrations likely also contributed to variability in ice algal response. Furthermore, experimental results indicated that the cultured Van Mijenfjorden community was negatively impacted by a rapid (within 4 to 24 h) reduction in salinity from 33 to 10. We further documented a significant start of recovery of these algae after 168 h. From this work, we surmise that decreases in surface water salinity, for example arising from the intensifying freshening of fjord waters, may only cause temporary changes in ice algal photoacclimation state and thus in chlorophyll a biomass. Further, this study also supports the need for salinity buffered melt of sea ice samples to reduce artificial bias in biological measurements.

Photophysiological characteristics of the Southern Ocean phytoplankton species Phaeocystis antarctica, Geminigera cryophila, and Chaetoceros simplex were assessed during 7 weeks of darkness and subsequent recovery after darkness at 4 and 7°C. Chlorophyll a fluorescence and maximum quantum efficiency of PSII decreased during long darkness in a species‐specific manner, whereas chlorophyll a concentration remained mostly unchanged. Phaeocystis antarctica showed the strongest decline in photosynthetic fitness during darkness, which coincided with a reduced capacity to recover after darkness, suggesting a loss of functional photosystem II (PSII) reaction centers. The diatom C. simplex at 4°C showed the strongest capacity to resume photosynthesis and active growth during 7 weeks of darkness. In all species, the maintenance of photosynthetic fitness during darkness was clearly temperature dependent as shown by the stronger decline of photosynthetic fitness at 7°C compared to 4°C. Although we lack direct evidence for this, we suggest that temperature‐enhanced respiration rates cause stronger depletion of energy reserves that subsequently interferes with the maintenance of photosynthetic fitness during long darkness. Therefore, the current low temperatures in the coastal Southern Ocean may aid the maintenance of photosynthetic fitness during the austral winter. Further experiments should examine to what extent the species‐specific differences in dark survival are relevant for future temperature scenarios for the coastal Southern Ocean. [ABSTRACT FROM AUTHOR]

Background: The light-harvesting antennae of photosystem (PS) I and PSII are pigment-protein complexes responsible of the initial steps of sunlight conversion into chemical energy. In natural environments plants are constantly confronted with the variability of the photosynthetically active light spectrum. PSII and PSI operate in series but have different optimal excitation wavelengths. The prompt adjustment of light absorption by photosystems is thus crucial to ensure efficient electron flow needed to sustain downstream carbon fixing reactions. Fast structural rearrangements equilibrate the partition of excitation pressure between PSII and PSI following the enrichment in the red (PSII-favoring) or far-red (PSI-favoring) spectra. Redox imbalances trigger state transitions (ST), a photoacclimation mechanism which involves the reversible phosphorylation/dephosphorylation of light harvesting complex II (LHCII) proteins by the antagonistic activities of the State Transition 7 (STN7) kinase/TAP38 phosphatase enzyme pair. During ST, a mobile PSII antenna pool associates with PSI increasing its absorption cross section. LHCII consists of assorted trimeric assemblies of Lhcb1, Lhcb2 and Lhcb3 protein isoforms (LHCII), several being substrates of STN7. However, the precise roles of Lhcb phosphorylation during ST remain largely elusive. Results: We inactivated the complete Lhcb1 and Lhcb2 gene clades in Arabidopsis thaliana and reintroduced either wild type Lhcb1.3 and Lhcb2.1 isoforms, respectively, or versions lacking N-terminal phosphorylatable residues proposed to mediate state transitions. While the substitution of Lhcb2.1 Thr-40 prevented the formation of the PSI-LHCI-LHCII complex, replacement of Lhcb1.3 Thr-38 did not affect the formation of this supercomplex, nor did influence the amplitude or kinetics of PSII fluorescence quenching upon state 1—state 2 transition. Conclusions: Phosphorylation of Lhcb2 Thr-40 by STN7 alone accounts for ≈ 60% of PSII fluorescence quenching during state transitions. Instead, the presence of Thr-38 phosphosite in Lhcb1.3 was not required for the formation of the PSI-LHCI-LHCII supercomplex nor for re-equilibration of the plastoquinone redox state. The Lhcb2 phosphomutant was still capable of ≈ 40% residual fluorescence quenching, implying that a yet uncharacterized, STN7-dependent, component of state transitions, which is unrelated to Lhcb2 Thr-40 phosphorylation and to the formation of the PSI-LHCI-LHCII supercomplex, contributes to the equilibration of the PSI/PSII excitation pressure upon plastoquinone over-reduction. [ABSTRACT FROM AUTHOR]

A green alga, Chlorococcum sp. (Chlorococcales, Chlorophyceae), was grown from scrapings of the travertine rocks from the outlet stream of the Emerald Pool, Khlong Thom Nuea, Khlong Thom District Krabi province in Southern Thailand – a classic karstic stream habitat. It was a recalcitrant alga: organic solvents did not extract chlorophyll (Chl) from the algae very well but heated Dimethylsulphoxide (DMSO) was effective. Rapid light curve PAM experiments were used to quantify photosynthetic parameters: Eopt = 275 ± 9.7 µmol photons m–2 s–1, ETRmax = 277 ± 5.99 µmol e– g–1 Chl a s–1, photosynthetic efficiency (Alpha, α0) = 2.248 ± 0.112 e– g–1 Chl a. For Chlorococcum sp. deliberately grown in very low light, Eopt = 290 ± 4.89 µmol photons m–2 s–1 but ETRmax = 109 ± 0.97 µmol e– g–1 Chl a s–1, Alpha (α0) = 1.010 ± 0.192 e– g–1 Chl a. Overall, the amount of Chl b compared with Chl a was relatively low for most Chl a + b organisms (Chl b/a ≈ 0.21) but the ratio was not strongly affected by irradiance. The changes in ETRmax and α0 indicate that the Chlorococcum sp, responded to very low irradiance by increasing total Chl as Chl a, a marginal decrease in Chl b/a ratio but no substantial change in far-red absorption. The spectrum stripping technique confirmed Chlorococcum sp, is a Chl a + b organism albeit with a low Chl b content. Photosynthesis in this species was not very sensitive to pH in the range pH 5–9 but was inhibited at pH 10, indicating that it was very capable of using HCO3– as an inorganic carbon source. Chlorococcum has a carbon concentrating mechanism (CCM), however based on its insensitivity to the depolarizing effects of high [K+], it is unlikely to be dependent on the membrane potential across the cell membrane. The alga is homiochlorophyllous and recovers rapidly from dessication without synthesis of new chlorophyll.

Coastal kelp forests produce substantial marine carbon due to high annual net primary production (NPP) rates, but upscaling of NPP estimates over time and space remains difficult. We investigated the impact of variable underwater photosynthetically active radiation (PAR) and photosynthetic parameters on photosynthetic oxygen production of Laminaria hyperborea, the dominant NE‐Atlantic kelp species, throughout summer 2014. Collection depth of kelp had no effect on chlorophyll a content, pointing to a high photoacclimation potential of L. hyperborea towards incident light. However, chlorophyll a and photosynthesis versus irradiance parameters differed significantly along the blade gradient when normalized to fresh mass, potentially introducing large uncertainties in NPP upscaling to whole thalli. Therefore, we recommend a normalization to kelp tissue area, which is stable over the blade gradient. Continuous PAR measurements revealed a highly variable underwater light climate at our study site (Helgoland, North Sea) in summer 2014, reflected by PAR attenuation coefficients (Kd) between 0.28 and 0.87 m−1. Our data highlight the importance of continuous underwater light measurements or representative average values using a weighted Kd to account for large PAR variability in NPP calculations. Strong winds in August increased turbidity, resulting in a negative carbon balance at depths >3–4 m over several weeks, considerably impacting kelp productivity. Estimated daily summer NPP over all four depths was 1.48 ± 0.97 g C · m−2 seafloor · d−1 for the Helgolandic kelp forest, which is in the range of other kelp forests along European coastlines. [ABSTRACT FROM AUTHOR]

Acclimation of corals to light is known to rely on multiple strategies working at different timescales. Among them, photosynthetic alternative electron flows (AEFs) could act as photoprotective mechanisms under fluctuating light intensities. In this work, we first compared the use of AEFs in shallow and mesophotic colonies of the coral Stylophora pistillata by carrying out joint measurements of oxygen exchange and photosystems quantum yields. We observed similar capacities to re-route photosynthetically derived electrons toward oxygen (Mehler reaction) and to perform cyclic electron flow around photosystem I under high light intensity in both colony types. But in contrast to mesophotic colonies that hosted Cladocopium, the photosynthetic apparatus of Symbiodinium microadriaticum hosted by their shallow counterparts was notably able to drive a higher number of electrons, displayed a higher thermal dissipation of absorbed light energy. Then, a short-term light stress was applied to evaluate the plasticity of the photosynthetic apparatus. Both shallow and mesophotic colonies showed fast acclimation to the low light regime. In contrast, under the high light regime, mesophotic colonies showed a limited capacity to dissipate light energy and were strongly photoinhibited, though their PSI activity was partly preserved and likely involved cyclic electron flow. This study shows how important the photosynthetic alternative electron flows are in acclimation processes to light and how the plasticity of the photosynthetic processes in Symbiodiniaceae may shape the vertical distribution of the coral holobionts. [ABSTRACT FROM AUTHOR]

Four strains of green microalgae (Scenedesmus acutus, Scenedesmus vacuolatus, Chlorella sorokiniana, and Chlamydomonas reinhardtii) were compared to determine growth and pigment composition under photoautotrophic or heterotrophic conditions. Batch growth experiments were performed in multicultivators with online monitoring of optical density. For photoautotrophic growth, light-limited (CO2-sufficient) growth was analyzed under different light intensities during the exponential and deceleration growth phases. The specific growth rate, measured during the exponential phase, and the maximal biomass productivity, measured during the deceleration phase, were not related to each other when different light intensities and different species were considered. This indicates species-dependent photoacclimation effects during cultivation time, which was confirmed by light-dependent changes in pigment content and composition when exponential and deceleration phases were compared. Except for C. reinhardtii, which does not grow on glucose, heterotrophic growth was promoted to similar extents by acetate and by glucose; however, these two substrates led to different pigment compositions. Weak light increased the pigment content during heterotrophy in the four species but was efficient in promoting growth only in S. acutus. C. sorokiniana, and S. vacuolatus exhibited the best potential for heterotrophic biomass productivities, both on glucose and acetate, with carotenoid (lutein) content being the highest in the former.

By combining physiological/biochemical and transcriptional analysis, the inhibition and recovery mechanisms of Phaeodactylum tricornutum in response to extreme high light stress (1300 μmol photons · m−2 · s−1) were elucidated. The population growth was inhibited in the first 24 h and started to recover from 48 h. At 24 h, photoinhibition was exhibited as the changes of PSII photosynthetic parameters and decrease in cellular pigments, corresponding to the downregulation of genes encoding light‐harvesting complex and pigments synthesis. Changes in those photosynthetic parameters and genes were kept until 96 h, indicating that the decrease of light absorption abilities might be one strategy for photoacclimation. In the meanwhile, we observed elevated cellular ROS levels, dead cells proportions, and upregulation of genes encoding antioxidant materials and proteasome pathway at 24 h. Those stress‐related parameters and genes recovered to the controls at 96 h, indicating a stable intracellular environment after photoacclimation. Finally, genes involving carbon metabolisms were upregulated from 24 to 96 h, which ensured the energy supply for keeping high base and nucleotide excision repair abilities, leading to the recovery of cell cycle progression. We concluded that P. tricornutum could overcome photoinhibition by decreasing light‐harvesting abilities, enhancing carbon metabolisms, activating anti‐oxidative functions, and elevating repair abilities. The parameters of light harvesting, carbon metabolisms, and repair processes were responsible for the recovery phase, which could be considered long‐term adaptive strategies for diatoms under high light stress. [ABSTRACT FROM AUTHOR]

Eddies modulate open ocean productivity, and this influence depends on both eddy source and evolution. Southeast Indian Ocean eddies are important pathways for the westward transport of biogeochemical anomalies from the Leeuwin current into the central oligotrophic South Indian Ocean (SIO). Eddy processes at the base of the mixed layer may stimulate and sustain phytoplankton, allowing these eddy impacts to persist over thousands of kilometers. We present 4 months of high‐frequency profiles from autonomous floats in one anticyclonic and one cyclonic eddy in the SIO. At the start of observations, from September to October, particulate organic carbon (POC) and especially chlorophyll were higher in the cyclone, and evenly distributed throughout the mixed layers in both eddies. As spring progressed and the eddies were transported westward, chlorophyll and POC concentrated at the base of the mixed layer at depths invisible to satellites, likely reflecting nutrient depletion in overlying waters. In the anticyclone, the increased chlorophyll at depth occurred as POC decreased, suggesting photo‐acclimation and thus both light and nutrient stress. In contrast, in the cyclone chlorophyll to POC ratios remained close to constant as their subsurface maxima formed. In both eddies, the subsurface biomass maxima exhibited no significant change in oxygen saturation state over several months suggesting these communities are sustained by low ongoing productivity in balance with community respiration. Thus, deep biomass layers may represent a mechanism for long‐distance transfer of eddy plankton communities which is not reflected in satellite remote sensing. Plain Language Summary: Eddies are spinning vortices of water about 100–200 km across. They are everywhere in the ocean, and they move heat, salt, nutrients, and biomass both vertically and horizontally. In the southeast Indian Ocean, eddies are created in the Leeuwin Current along the coast of Western Australia. These eddies are unusual because the clockwise‐spinning (cyclonic) eddies have lower surface productivity than their anticyclonic counterparts. Both types of eddies last for months to years and move westward over time. Satellite measurements of surface chlorophyll, a proxy for ocean productivity, indicate decreased chlorophyll in both types of eddies, during the transition to summer. Our measurements from robotic floats show that the satellites see decreased chlorophyll because it is redistributing deeper (>30–50 m) out of view of the satellites. This redistribution appears to also affect other components of the ecosystem, because particulate organic carbon levels, estimated from light scattering sensors, also become focused into subsurface layers. At the base of the ocean surface mixed layer, about 100–150 m, these layers of plankton biomass exhibit a balance between oxygen production from photosynthesis and oxygen consumption by other trophic levels, suggesting the layers remain active over long periods. Key Points: In both cyclonic and anticyclonic eddies, phytoplankton biomass concentrates deeper in the mixed layer in summerThere is evidence of photoacclimation in the deep layers of the anticyclone, and growth of high‐carbon phytoplankton in the cycloneIn these deep layers, stable oxygen concentrations over at least a month indicate a balance between photosynthesis and respiration [ABSTRACT FROM AUTHOR]

Despite their global distribution moss physiology is not well understood, particularly in tropical environments. Photosynthetic Electron Transport Rate (ETR) of Hyophila involuta was measured using PAM technology. The plants were growing in a heavily shaded habitat with irregular sunflecks of full sunlight. Two models were used for estimating photosynthetic electron transport (ETR), the Waiting-in-Line and Eilers-Peeters models. Both take photoinhibition into account and gave similar results. The population studied was growing on bricks and stonework and so periodically dried out. The same species is sometimes found in waterfall environments where it is semi-aquatic. ETRmax was low at dawn, increased to an early morning maxima and then decreased during the course of the day, partially reversed by rainstorms. Mid-morning (10:30 solar time): Eopt ≈ 521 ± 77 µmol photon m–2 s–1 which is higher than might be expected in a shaded sun-fleck environment, ETRmax ≈ 28.3 ± 2.5 µmol e- m–2 s–1 on a projected surface area basis or ≈ 57.7 ± 5.1 (µmol e- g–1 Chl a s–1), photosynthetic efficiency (Alpha, α0) ≈ 0.301 (e- photon–1 g–1 Chl a). Chl a content of Hyophila was ≈ 453 mg Chl a m–2 with Chl b/a ≈ 0.371. The O2 electrode-based respiration rate was R = 6.78 ± 0.698 µmol O2 g–1 Chl a s–1, the ETRmax is roughly equivalent to Gross photosynthesis (Pg) ≈ 14.4 ± 1.28 µmol O2 g–1 Chl a s–1 (4 e-/ O2), so the optimum Pg/R ratio is about 2.12 ± 0.289 and net photosynthesis (Pn) is ≈ 7.62 ± 1.46 µmol O2 g–1 Chl a s–1. Thus even under optimum irradiance net photosynthesis was actually rather low. pH experiments showed that Hyophila used both CO2 & HCO3- inorganic carbon sources, suggesting that a concentrating mechanism (CCM) is present. Hyophila is partially homiochlorophyllous, recovering well from desiccation after 2h of lighted rehydration and recovers more after 24h but lost ≈30% of its Chl a. The moss retained its chlorophyll content if desiccated in the dark. The moss lost ≈ 2/3 of its Chl a during the monsoonal dry season. [ABSTRACT FROM AUTHOR]

Information on the intracellular content and functional diversity of phytoplankton pigments can provide valuable insight on the ecophysiological state of primary producers and the flow of energy within aquatic ecosystems. Combined global datasets of analytical flow cytometry (AFC) cell counts and High-Performance Liquid Chromatography (HPLC) pigment concentrations were used to examine vertical and seasonal variability in the ratios of phytoplankton pigments in relation to indices of cellular photoacclimation. Across all open ocean datasets, the weight-to-weight ratio of photoprotective to photosynthetic pigments showed a strong depth dependence that tracked the vertical decline in the relative availability of light. The Bermuda Atlantic Time-series Study (BATS) dataset revealed a general increase in surface values of the relative concentrations of photoprotective carotenoids from the winter-spring phytoplankton communities dominated by low-light acclimated eukaryotic microalgae to the summer and early autumn communities dominated by high-light acclimated picocyanobacteria. In Prochlorococcus-dominated waters, the vertical decline in the relative contribution of photoprotective pigments to total pigment concentration could be attributed in large part to changes in the cellular content of photosynthetic pigments (PSP) rather than photoprotective pigments (PPP), as evidenced by a depth-dependent increase of the intracellular concentration of the divinyl chlorophyll-a (DVChl-a) whilst the intracellular concentration of the PPP zeaxanthin remained relatively uniform with depth. The ability of Prochlorococcus cells to adjust their DVChl-a cell-1 over a large gradient in light intensity was reflected in more highly variable estimates of carbon-to-Chl-a ratio compared to those reported for other phytoplankton groups. This cellular property is likely the combined result of photoacclimatory changes at the cellular level and a shift in dominant ecotypes. Developing a mechanistic understanding of sources of variability in pigmentation of picocyanobacteria is critical if the pigment markers and bio-optical properties of these cells are to be used to map their biogeography and serve as indicators of photoacclimatory state of subtropical phytoplankton communities more broadly. It would also allow better assessment of effects on, and adaptability of phytoplankton communities in the tropical/subtropical ocean due to climate change.

Phytochrome photoreceptors control plant growth, development, and the shade avoidance response that limits crop yield in high-density agricultural plantings. Cyanobacteriochromes (CBCRs) are distantly related photosensory proteins that control cyanobacterial metabolism and behavior in response to light. Photoreceptors in both families reversibly photoconvert between two photostates via photoisomerization of linear tetrapyrrole (bilin) chromophores. Spectroscopic and biochemical studies have demonstrated heterogeneity in both photostates, but the structural basis for such heterogeneity remains unclear. We report solution NMR structures for both photostates of the red/green CBCR NpR6012g4 from Nostoc punctiforme In addition to identifying structural changes accompanying photoconversion, these structures reveal structural heterogeneity for residues Trp655 and Asp657 in the red-absorbing NpR6012g4 dark state, yielding two distinct environments for the phycocyanobilin chromophore. We use site-directed mutagenesis and fluorescence and absorbance spectroscopy to assign an orange-absorbing population in the NpR6012g4 dark state to the minority configuration for Asp657. This population does not undergo full, productive photoconversion, as shown by time-resolved spectroscopy and absorption spectroscopy at cryogenic temperature. Our studies thus elucidate the spectral and photochemical consequences of structural heterogeneity in a member of the phytochrome superfamily, insights that should inform efforts to improve photochemical or fluorescence quantum yields in the phytochrome superfamily.

Using microalgae to treat biogas slurry can realize wastewater treatment and nutrients recovery. However, the dark color of biogas slurry causes serious light attenuation. Flashing light was used to solve light attenuation, while improving biomass production and pollutant removal. Higher frequency (f) and lower duty cycle (φ) flashing light conditions increased biomass concentration and pollutant removal. With 100 Hz-0.3 φ-60 μmol photons m−2 s−1, the biomass concentration, biomass productivity and NH4+-N removal reached the highest values of 255 mg L−1, 8.45 mg L−1 day−1 and 55.2%, which were 35.8%, 33.6% and 44.2% higher than those in continuous light (100 Hz-1.0 φ-60 μmol photons m−2 s−1), respectively. With 100 Hz-0.3 φ-60 μmol photons m−2 s−1, the total chlorophyll content increased by 25.4% and the Chl a/b ratio decreased by 35.1% compared with that in continuous light at the fourth day, which showed stronger photoacclimation. The photoacclimation and the higher luminous intensity caused the enhancement of biomass concentration in dark color biogas slurry, and f had more effect than φ. In addition, the same quantity of biomass produced under the condition of 100 Hz-0.3 φ-60 μmol photons m−2 s−1 decreased 25.1% light energy consumption compared with continuous light. Flashing light could be used to solve the light attenuation effect caused by dark color biogas slurry. It provided a special method for light provision when using microalgae to treat dark color biogas slurry. [ABSTRACT FROM AUTHOR]

The chlorophyll‐specific absorption spectrum of phytoplankton aPH∗(λ) $\left[{a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)\right]$ multiplied with phytoplankton chlorophyll provides the total absorption coefficient of phytoplankton [aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$], a fundamental quantity with significance in many marine biogeochemical (BGC) and environmental processes. Representing accurately the sources of variability of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ in BGC ocean models is a crucial task. The two main sources of variability in aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ are changes in the pigment composition of the phytoplankton community and the size‐dependent constraints to pigment packaging. Therefore, changes in community structure and physiological state impact aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ and consequently aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$. The objective of this work is to improve estimates of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ in a BGC model of the global ocean by portraying the variability of aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ driven by the variable content in photoprotective carotenoids (PPCs) in the phytoplankton community. We used a three‐dimensional spectrally‐resolved BGC model to simulate the inherent and apparent optical properties of the global ocean based on its content on optically active constituents. The aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ for each phytoplankton type represented in the model were made variable as a function of the type‐specific content in PPCs. By comparing model‐derived aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ to satellite retrievals and an extensive field data set of optical and BGC observations, we concluded that photoprotective pigments content impacted significantly the contribution of the aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ to the total non‐water absorption in the ocean. Pigment‐impacted aPH∗(λ) ${a}_{\mathrm{P}\mathrm{H}}^{\ast }(\lambda)$ contributed to reproduce the global variability of aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ as well as the observed bio‐optical relationship between aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ and chlorophyll. The improved representation of the aPH(λ) ${a}_{\mathrm{P}\mathrm{H}}(\lambda)$ of the phytoplankton community influenced model simulations in terms of water‐leaving radiances. Plain Language Summary: The multifaceted interactions between light and particles suspended in the ocean translate to complex light paths at larger scale, such as the color of the light that leaves the ocean, can be detected by satellites and is used to map the concentration of phytoplankton. In this work, we have developed an Earth‐system biogeochemical model that represents how different colors of light travel though the ocean and interact with phytoplankton and other suspended matter. As phytoplankton cells contain a variety of algal pigments and different pigments absorb different colors of light, the amount and diversity of pigments found in the phytoplankton community determines how much and which color of light they absorb. In our model, the pigments that phytoplankton accumulate to protect themselves from the harmful effects of light alter the amount of light that the cells absorb in the blue‐green region of the spectrum. Our model was able to better represent how much light phytoplankton absorbed when compared with many observations collected by expeditions and satellites in the global ocean. Our proposed model structure can have high potential implications in the future, from providing insight to the causes of changes in the color of the ocean to refining model estimates of phytoplankton production. Key Points: Ocean biogeochemical model represented the impact of photoprotective carotenoids (PPCs) in the light absorption coefficients of phytoplanktonProposed model characterized the global variability of phytoplankton light absorption when compared to in situ and satellite observationsThe impact of PPCs in phytoplankton light absorption reduced the dependence of reflectance on total chlorophyll [ABSTRACT FROM AUTHOR]

Under ideal conditions, Chlamydomonas reinhardtii can photoacclimate to excess light through various short- and long-term mechanisms. However, how microalgae handle excess light stress once they exit exponential growth, and especially in stationary phase, is less understood. Our study explored C. reinhardtii's photoprotection capacity and acclimation strategies during high-light stress once batch culture growth reached stationary phase. We monitored cultures of wildtype strain (CC125) over five days once they reached stationary phase under both low-light (LL) and high-light (HL) conditions. Under HL, many photosynthetic proteins were degraded but the stress-related light harvesting complex protein (LHCSR) was rapidly induced and contributed to the rapid activation of nonphotochemical quenching (NPQ). However, the LHCSR3-defective mutant (CC4614, npq4) lacked the rapid induction of quenching typical of post-exponential cultures, indicating that LHCSR3 is required for this response in stationary phase. Collectively, the main strategy for photoacclimation in stationary phase appears to be a dramatic reduction of photosystems while maintaining LHCII-LHCSR antenna complexes that prime the antenna for rapid activation of quenching upon light exposure. Part of this response to HL involves a resumption of cell growth after two days, that we hypothesized is due to the stimulation of growth-regulating pathways due to increased metabolite pools from the HL-induced protein turnover in the cell, something that remains to be tested. These findings demonstrate how C. reinhardtii manages high-light stress during stationary phases to maximize longevity., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Abstract The chlorophyll‐specific absorption spectrum of phytoplankton aPH∗(λ) multiplied with phytoplankton chlorophyll provides the total absorption coefficient of phytoplankton [aPH(λ)], a fundamental quantity with significance in many marine biogeochemical (BGC) and environmental processes. Representing accurately the sources of variability of aPH(λ) in BGC ocean models is a crucial task. The two main sources of variability in aPH∗(λ) are changes in the pigment composition of the phytoplankton community and the size‐dependent constraints to pigment packaging. Therefore, changes in community structure and physiological state impact aPH∗(λ) and consequently aPH(λ). The objective of this work is to improve estimates of aPH(λ) in a BGC model of the global ocean by portraying the variability of aPH∗(λ) driven by the variable content in photoprotective carotenoids (PPCs) in the phytoplankton community. We used a three‐dimensional spectrally‐resolved BGC model to simulate the inherent and apparent optical properties of the global ocean based on its content on optically active constituents. The aPH∗(λ) for each phytoplankton type represented in the model were made variable as a function of the type‐specific content in PPCs. By comparing model‐derived aPH(λ) to satellite retrievals and an extensive field data set of optical and BGC observations, we concluded that photoprotective pigments content impacted significantly the contribution of the aPH(λ) to the total non‐water absorption in the ocean. Pigment‐impacted aPH∗(λ) contributed to reproduce the global variability of aPH(λ) as well as the observed bio‐optical relationship between aPH(λ) and chlorophyll. The improved representation of the aPH(λ) of the phytoplankton community influenced model simulations in terms of water‐leaving radiances.

Unicellular photoautotrophs adapt to variations in light intensity by changing the abundance of light harvest pigment-protein complexes (LHCs) on time scales of hours to days. This process requires a feedback signal between the plastid (where light intensity is sensed) to the nucleus (where the genes for LHCs are encoded). The signals must include heretofore unidentified transcription factors that modify the expression level of the LHCs. Analysis of the nuclear genome of the model diatom Phaeodactylum tricornutum revealed that all the lhc genes have potential binding sites for transcription factors belonging to the MYB-family proteins. Functional studies involving antisense RNA interference of a hypothetical protein with a MYB DNA-binding domain were performed. The resultant strains with altered photosynthetic and physiological characteristics lost their ability to acclimate to changes in irradiance; i.e., cellular chlorophyll content became independent of growth irradiance. Our results strongly suggest that the inter-organellar signaling cascade was disrupted, and the cell could no longer communicate the environmental signal from the plastid to the nucleus. Here, we identify, for the first time, an LHC Regulating Myb (LRM) transcription factor, which we propose is involved in lhc gene regulation and photoacclimation mechanisms in response to changes in light intensity. [ABSTRACT FROM AUTHOR]

Sargassum horneri (Turner) C. Agardh is a genus of brown algae and plays an important role in marine ecosystem. However, the inhabiting area of S. horneri has been decreasing sharply in China. To understand the photoacclimation and photoregulation strategies of S. horneri in responses to temperature and light, S. horneri was cultured under different temperatures [18℃ (LT) and 26℃ (HT)] and light intensities [60 μmol(photon) m-2 s-1 (LL) and 120 μmol(photon) m-2 s-1 (HL)] for 7 d, and then the chlorophyll a fluorescence parameters were measured. The results showed that the maximum electron transfer rate occurred at low temperature and high light (LT-HL) condition. The high temperature was the predominant factor for causing inhibition of PSII, lowering the effective quantum yield of PSII, and reducing the nonphotochemical quenching (NPQ). However, high light could improve the photoprotective ability via enhancing the NPQ. On the other hand, a strong linear relationship was observed between NPQ and the electron transport efficiency (α); the increase of NPQ could reduce the α value and avoid damage from high light stress to PSII. Therefore, S. horneri was found to be well adapted to grow under LT-HL conditions.

The marine environment is a crucial component of global biogeochemical cycles. Recent BGC-Argo observations provide new opportunities to study the profiles of biogeochemical parameters. The study analyzed the diurnal variations of temperature, salinity, chlorophyll-a and dissolved oxygen using a high-frequency (∼5 h) cycle BGC-Argo float in the Bay of Bengal. The hydrography showed the existence of a strong barrier layer with a thickness of around 30 m, with fresh water on top and an inversion layer within it. Analysis showed that the Mixed Layer Depth (MLD) was dominated by diffuse convection, while the Isothermal Layer Depth (ILD) exhibited salt-fingering regimes. In the upper layer (0–60 m), temperature showed significant variation on a daily scale; however, notable changes were not observed for salinity. Additionally, temperature and chlorophyll-a were found to be strongly linked to solar insolation. The mean chlorophyll-a in the upper layer increased from 0600 h and peaked around 1800 h local time. However, surface chlorophyll-a increased only from 1100 h to 1800 h. It is suggested that this difference between surface and mean chlorophyll-a during high availability of sunlight was due to the process of photoacclimation. The dissolved oxygen cycle closely followed the variability of biomass production. The similarity between dissolved oxygen and the difference between the surface and mean chlorophyll-a further indicated photoacclimation variations on a diurnal scale. The Sverdrup model was used to indicate luminosity and an accumulation time of 14 h was used to show a strong correlation with diel chlorophyll-a variation. The work highlights the importance of having high-frequency BGC-Argo floats with finer vertical resolution and the need for time-series observations of biological parameters in the Bay of Bengal. [ABSTRACT FROM AUTHOR]

Freshwater filamentous algae have potential for wastewater bioremediation and bioproduct generation. This study investigated the separate and combined effects of growth irradiance regime (300 ± 25 μmol.m−2.s−1 with 13:11 dark:light cycle or 775 ± 25 μmol.m−2.s−1 with 8:16 dark:light cycle) and temperature (15 ± 1 or 25 ± 1 °C) during acclimation on the adaptive biochemistry and photosynthetic activity of Oedogonium with the higher and lower levels representing average outdoor conditions in summer and winter in Melbourne, Australia, respectively. Photoprotective pigments were upregulated in response to either high irradiance regime or temperature, while the chlorophyll content was also reduced when both stressors were combined. The upregulation of protective adaptations slightly lowered photosynthetic efficiency, which was more dramatically impaired by the reduced chlorophyll at high temperature and irradiance. The polar lipid content increased from ~10% to ~30% of total lipid content, the protein content decreased by ~10% and the starch content increased by 30% in response to higher irradiance and temperature, with implications for biomass utilisation. These changes in biochemical composition due to long-term acclimation suggests the potential for compositional stratification in stable floating filamentous algae mats due to the presence of self-shading. Further, the shading created by the upper layers in the mat can be expected to provide further protection to the biomass at the lower levels against photooxidative stress. The results reveal the impact of variations on seasonal growth conditions and filamentous mat depth on the composition and productivity of the algae. [Display omitted] • Filamentous alga Oedogonium shows compositional changes under different conditions. • Separate and combined effects of growth light regime and temperature were studied. • Photooxidative and heat stress increased protective pigments & decreased chlorophyll. • Protective biochemical adaptations reduced photosynthetic efficiency and productivity. • Both seasonal fluctuations and algae mat depth affect biomass growth and utilisation. [ABSTRACT FROM AUTHOR]

Photosynthetic organisms adapt to a variety of light conditions. Codium fragile, a macrosiphonous green alga, binds a unique carbonyl carotenoid, siphonaxanthin, to its major photosynthetic light-harvesting complexes, allowing it to utilize dim blue-green light for photosynthesis. Here, we describe the absolute chemical structure of a novel siphonaxanthin biosynthetic precursor, 19-deoxysiphonaxanthin, that accumulates specifically in the photosynthetic antenna only when cultivated under blue-green light. The action spectra of pigment accumulation suggest that siphonaxanthin biosynthesis is regulated by a specific wavelength profile. The results provide clues to a new acclimation mechanism to withstand hours of intense light at low tide and why siphonous algae have been growing invasively on the world's coasts for more than a century. [ABSTRACT FROM AUTHOR]

Diatoms of the Arctic Ocean annually experience extreme changes of light environment linked to photoperiodic cycles and seasonal variations of the snow and sea‐ice cover extent and thickness which attenuate light penetration in the water column. Arctic diatom communities exploit this complex seasonal dynamic through a well‐documented species succession during spring, beginning in sea‐ice and culminating in massive phytoplankton blooms underneath sea‐ice and in the marginal ice zone. The pattern of diatom taxa sequentially dominating this succession is relatively well conserved interannually, and taxonomic shifts seem to align with habitat transitions.To understand whether differential photoadaptation strategies among diatom taxa explain these recurring succession sequences, we coupled laboratory experiments with field work in Baffin Bay at 67.5°N. Based on field data, we selected five diatom species typical of different ecological niches and measured their growth rates under light intensity ranges representative of their natural habitats. To characterize their photoacclimative responses, we sampled pigments and total particulate carbon, and conducted 14C‐uptake photosynthesis response curves and variable fluorescence measurements.We documented a gradient in species respective light intensity for maximal growth suggesting divergent light response plasticity, which for the most part align with species sequential dominance. Other photophysiological parameters supported this ecophysiological framing, although contrasts were always clear only between succession endmembers, Nitzschia frigida and Chaetoceros neogracilis. To validate that these photoacclimative responses are representative of in situ dynamics, we compared them to the chlorophyll a‐specific light‐limited slope (α*) and saturated rate of photosynthesis (PM*), monitored in Baffin Bay on sea‐ice and planktonic communities. This complementary approach confirmed that unusual responses in α* and PM* as a function of light history intensity are similar between sentinel sympagic species N. frigida and natural ice‐core communities. While no light‐history‐dependent trends were observed in planktonic communities, their α* and PM* values were in the range of measurements from our monospecific cultures.Synthesis. Our results suggest that Arctic diatoms species photoadaptation strategy is tuned to the light environment of the habitats in which they dominate and indeed drives the seasonal taxonomic succession. [ABSTRACT FROM AUTHOR]

Induction of non-photochemical quenching (NPQ) of chlorophyll fluorescence in leaves affords photoprotection to the photosynthetic apparatus when, for whatever reason, photon capture in the antennae of photosystems exceeds their capacity to utilise this excitation in photochemistry and ultimately in CO2 assimilation. Here we augment traditional monitoring of NPQ using the fast time resolution, remote and relatively non-intrusive light induced fluorescence transient (LIFT) technique (Kolber et al . 2005 ; Osmond et al . 2017) that allows direct measurement of functional (σ'PSII) and optical cross-sections (a'PSII) of PSII in situ, and calculates the half saturation light intensity for ETR (Ek). These parameters are obtained from the saturation and relaxation phases of fluorescence transients elicited by a sequence of 270, high intensity 1 µs flashlets at controlled time intervals over a period of 30 ms in the QA flash at intervals of a few seconds. We report that although σ'PSII undergoes large transient increases after transfer from dark to strong white light (WL) it declines little in steady-state as NPQ is induced in shade- and sun-grown spinach and Arabidopsis genotypes Col, OEpsbs, pgr 5bkg, stn 7 and stn 7/8. In contrast, σ'PSII increases by ~30% when induction of NPQ in spinach is inhibited by dithiothreitol and by inhibition of NPQ in Arabidopsis npq 1, npq 4 and pgr 5. We propose this increase in σ'PSII arises as some excitation from closed PSII reaction centres is transferred to open centres when excitation partitioning to photochemistry (YII) and NPQ (YNP) declines, and is indicated by an increased excitation dissipation from closed PSII centres (Y NO, including fluorescence emission). Although E k increases following dissipation of excitation as heat when NPQ is engaged, it declines when NPQ is inhibited. Evidently photochemistry becomes more easily light saturated when excitation is transferred from closed RCIIs to open centres with larger σ'PSII . The NPQ mutant pgr 5 is an exception; Ek increases markedly in strong light as electron transport QA → PQ and PQ → PSI accelerate and the PQ pool becomes strongly reduced. These novel in situ observations are discussed in the context of contemporary evidence for functional and structural changes in the photosynthetic apparatus during induction of NPQ. [ABSTRACT FROM AUTHOR]

The Transition Zone Chlorophyll Front (TZCF) is a dynamic region of elevated chlorophyll concentrations in the Northeast Pacific that migrates from a southern winter (February) extent of approximately 30°N to a northern summer (August) extent of approximately 40°N. The transition zone has been highlighted as important habitat for marine animals and fisheries. We re‐examine the physical and biological drivers of seasonal TZCF variability using a variety of remote sensing, reanalysis, and in situ data sets. Satellite‐based remote sensing estimates of chlorophyll and carbon concentrations show that seasonal TZCF migration primarily reflects a seasonal increase in the chlorophyll to carbon ratio, rather than changes in phytoplankton carbon. We use our data compilation to demonstrate how the seasonality of light and nutrient fluxes decouple chlorophyll and carbon seasonality at the transition zone latitudes. Seasonal mixed‐layer‐averaged light availability is positively correlated with carbon and negatively correlated with chlorophyll through the transition zone, while climatological nitrate profiles show that chlorophyll to carbon ratios are facilitated by wintertime nitrate entrainment. These empirical results are consistent with physiological data and models describing elevated chlorophyll to carbon ratios in low light, nutrient‐replete environments, demonstrating the importance of latitudinal structure in interpreting seasonal chlorophyll dynamics at the basin scale. Plain Language Summary: Satellite‐observed marine chlorophyll concentrations are regularly interpreted as phytoplankton carbon. However, the chlorophyll content of cells can vary due to several environmental factors, thus complicating the interpretation of satellite‐observed chlorophyll variability. In this study, we examine the relationship between chlorophyll and phytoplankton carbon in the Northeast Pacific—a region that has been highlighted as important habitat for marine animals. We find that satellite‐observed chlorophyll seasonality is strongly correlated with light and nutrient availability but relatively uncorrelated with phytoplankton carbon due to changes in the chlorophyll to carbon ratio. Deep winter mixed layers are the primary physical factor driving the seasonal cycle in light and nutrient availability. These results provide a new perspective on marine primary productivity in the Northeast Pacific and demonstrate how latitudinal differences in the seasonality of light and nutrient fluxes connect chlorophyll and carbon dynamics at the basin scale. Key Points: Seasonal variations in North Pacific chlorophyll show a distinct latitudinal structure in phase and magnitudeChlorophyll concentrations are negatively correlated with carbon concentrations in the transition zone (30°–40°N)Latitudinally varying seasonal nutrient and light supply drive chlorophyll and carbon covariation via photoacclimation [ABSTRACT FROM AUTHOR]

Se aisló a la cianobacteria marina Spirulina subsalsa con una coloración poco usual de color vino-rojiza, rica en ficoeritrina, lo que es el primer registro en costas mexicanas (especialmente para Oaxaca). Se caracterizó su crecimiento con base en el peso húmedo, la concentración de clorofila a, extraída con acetona al 90%, y la determinación de su espectro de absorción a los 36 días en medio ASNIII bajo diferente intensidad y calidad de luz, empleando papel celofán como filtro. El incremento por peso húmedo fue apenas perceptible en todas las condiciones evaluadas, observándose mayor variación al inicio que al final del ensayo. La clorofila a y su espectro mostraron un incremento, mayor con los filtros rojo y azul, menor con amarillo, verde y blanca (control), en donde los espectros de absorción de la clorofila a incluyeron longitudes de onda de luz azul y rojo anaranjado, con picos en aproximadamente 450-475 nm y 650-675 nm y un hombro característico de los carotenoides a 450-500 nm. El color de la biomasa varió según el filtro, por lo que la luz en cuanto a cantidad (intensidad) y calidad (longitud de onda) inciden en la calidad de la biomasa producida por dicha cianobacteria. Además, su cambio de coloración se mostró con base en el filtro empleado y evidencia su potencial para obtención de pigmentos para las industrias acuícola y biotecnológica.

The sacoglossan sea slug Plakobranchus ocellatus is a pantropical gastropod that pilfers and incorporates algal chloroplasts (kleptoplasts) into its digestive cells and benefits from the production of photosynthate. It is a mobile, reef forager with mottled, wing-like parapodia that provide good camouflage in sand and are typically observed closed over the kleptoplast-filled digestive tubules. Functional kleptoplasts continue to photosynthesize but are separated from the algal nuclei and are unable to divide within host cells. The mechanisms that enable kleptoplasts to endure are poorly understood and the extent and limitations of functionality have not yet been fully characterized. We investigated kleptoplasts in three tropical sacoglossan species, Elysia ornata, Thuridilla gracilis, and P. ocellatus, collected from different depths and light fields to identify pigments, quantify retention times, and determine photosynthetic parameters. We found that P. ocellatus had the highest estimated retention time and maintained the highest ratio of photoprotective to photosynthetic pigments. A subsequent manipulative experiment on P. ocellatus specimens collected at the same site, depth, and time involved exposure to three different irradiances and showed that kleptoplasts did not photoacclimate over the course of 7 d. No significant changes in in vivo kleptoplast photosynthetic parameters or corresponding spectral reflectance occurred when measuring kleptoplasts directly with open parapodia. Reflectance of closed parapodia, however, showed significant increases in the medium- and high-light treatments on day seven indicating localized kleptoplast degradation. Our results suggest that closed parapodia play an important role in kleptoplast protection by shielding internal kleptoplasts while permitting filtered light energy to reach kleptoplasts on the parapodial undersides. The cryptically patterned parapodia assume the role of photoprotectant, compensating for kleptoplast inability to photoacclimate. This allows P. ocellatus to forage in high-light, exposed, sandy areas and exploit algal food resources that may be unavailable or too risky for other sacoglossans. [ABSTRACT FROM AUTHOR]

Xanthophyll cycles (XC) have proven to be major contributors to photoacclimation for many organisms. This work describes a light-driven XC operating in the chlorophyte Chlamydomonas reinhardtii and involving the xanthophylls Lutein (L) and Loroxanthin (Lo). Pigments were quantified during a switch from high to low light (LL) and at different time points from cells grown in Day/Night cycle. Trimeric LHCII was purified from cells acclimated to high or LL and their pigment content and spectroscopic properties were characterized. The Lo/(L + Lo) ratio in the cells varies by a factor of 10 between cells grown in low or high light (HL) leading to a change in the Lo/(L + Lo) ratio in trimeric LHCII from.5 in low light to.07 in HL. Trimeric LhcbMs binding Loroxanthin have 5 ± 1% higher excitation energy (EE) transfer (EET) from carotenoid to Chlorophyll as well as higher thermo- and photostability than trimeric LhcbMs that only bind Lutein. The Loroxanthin cycle operates on long time scales (hours to days) and likely evolved as a shade adaptation. It has many similarities with the Lutein-epoxide – Lutein cycle (LLx) of plants. [ABSTRACT FROM AUTHOR]

The recent advancement in LED technology led many incubator manufacturers and research labs to switch to these more efficient, yet spectrally restricted, light sources. Potential effects of commercially available broad range "white" light systems on phytoplankton growth, productivity, light absorption spectra, and cellular composition have not yet been characterized but could affect our interpretation of lab-based projections on responses to environmental changes. In this study, we investigated such effects using cultures of Prochlorococcus marinus, Synechococcus sp., and Thalassiosira weissflogii grown under three different commercially available LED lights as well as one fluorescent growth light. Photosynthetically active radiation was equal for each species, while photosynthetically usable radiation differed among the combinations of species and treatments. Growth rate was unaffected across species, yet all species displayed changes in cellular carbon, nitrogen, and chlorophyll a quotas as a direct response to the different light spectra.14C-based primary productivity was also affected in P. marinus and T. weissflogii. Analysis of pigment ratios and photophysiological data indicated changes in the photoacclimation state between different light environments. The results of this study show that these species undergo changes in underlying cellular metabolism which in turn affect cellular composition while keeping specific growth constant. The data presented here illustrate ecophysiological responses of differently pigmented species when grown under different artificial growth light spectra. These cellular acclimation responses should be considered when designing laboratory-based incubation experiments, especially when comparing responses to specific changes in environmental conditions, or when implementing physiological parameters, derived from laboratory experiments, into numerical models. [ABSTRACT FROM AUTHOR]

In lichens, each symbiotic partner cooperates for the survival of the symbiotic association. The protection of the susceptible photosynthetic apparatus is essential for both participants. The mycobiont and photobiont contribute to the protection against the damaging effect of excess light by various mechanisms. The present study investigated the effect of seasonality and microhabitat exposure on photoprotection and photoacclimation in the photo- and the mycobiont of six lichen species with different thallus morphology in inland dune system in the Kiskunság region (Hungary) with shaded, more humid and exposed, drier dune sides. High-Performance Liquid Chromatography, spectrophotometry, chlorophyll a fluorescence kinetic technique were used, and micrometeorological data were collected. The four years data series revealed that the north-east-facing side was characterized by higher relative humidity and lower light intensities compared to the south-west-facing drier and more exposed sides. The south-west facing side was exposed to direct illumination 3–4 hours longer in winter and 1–2 hours shorter in summer than the north-east facing side of the dune, influencing the metabolism of sun and shade populations of various species. Because rapid desiccation caused short active periods of lichens during bright and drier seasons and on exposed microhabitats, the rapid, non-regulated non-photochemical quenching mechanisms in the photobiont had a significant role in protecting the photosynthetic system in the hydrated state. In dehydrated conditions, thalli were mainly defended by the solar screening metabolites produced by the mycobiont and curling during desiccation (also caused by the mycobiont). Furthermore, the efficacy of light use (higher chlorophyll and carotenoid concentration) increased because of short hydrated periods. Still, a lower level of received irradiation was appropriate for photosynthesis in dry seasons and on sun exposed habitats. In humid seasons and microhabitats, more extended active periods lead to increased photosynthesis and production of solar radiation protectant fungal metabolites, allowing a lower level of photoprotection in the form of regulated non-photochemical quenching by the photobiont. Interspecific differences were more pronounced than the intraspecific ones among seasons and microhabitat types. [ABSTRACT FROM AUTHOR]

Brown macroalgae, being important components of benthic communities in temperate regions, are frequently subjected to light limitation. To extend our understanding of their low light acclimation strategies to the regulation of membrane lipid environment, photosynthetic characteristics, lipid class, fatty acid profiles and chloroplast ultrastructure were compared in Undaria pinnatifida (Phaeophyceae, Ochrophyta) after long‐term exposure to low and moderate light intensities (LL, 100 and ML, 280 µmol photons · m−2 · s−1). We show that light limitation significantly increased PSII quantum efficiency and photosynthetic electron transport rate, enhanced pigment contents and concentration of thylakoid membranes in chloroplasts but decreased the distance between the thylakoid stacks. These physiological alterations at LL were accompanied by a selective remodeling of thylakoid membrane lipids driven by increases in monogalactosyldiacylglycerol (MGDG) and phosphatidylglycerol (PG) contents. Light limitation also induced active production of PG specific trans‐Δ3‐hexadecenoic acid and accumulation of n−3 polyunsaturated fatty acids (PUFA) mostly in PG and MGDG at the expense of the rise in 18:3n−3 and 20:5n−3, 18:4n−3, respectively. These changes in lipid and FA profiles are apparently responsible for supporting thylakoid biogenesis and efficient photosynthesis at light limitation, thus contributing to photoacclimation strategies in brown algae. The content of triacylglycerols (TAG) and the level of their PUFA were decreased at LL, suggesting the consumption of TAG as a source of PUFA and energy reserves. Thus, U. pinnatifida is able to successfully overcome periods of low irradiance through the effective light harvesting and utilization that are provided by high flexibility of lipid biosynthesis. [ABSTRACT FROM AUTHOR]

An efficient use of light is essential to achieve good performances in microalgae cultivation systems. This can be challenging particularly under solar conditions where light is highly dynamic (e.g., day/night cycles, rapid changes in wind and weather conditions). Microalgae display different mechanisms to optimize light use efficiency. In the short term, when high light is encountered, several processes of photoprotection can be involved to avoid cell damages (e.g., xanthophyll cycle). In the long term, when cells are exposed to a different light intensity, pigment content changes, i.e., photoacclimation. The purpose of this study is to investigate the photosynthetic response of Chlorella vulgaris cultures grown in closed lab-scale, torus-shape photobioreactor under well-controlled light conditions, namely, constant and dynamic light transitions. Experiments were conducted in continuous mode with detailed characterization of the light attenuation conditions for each condition, as represented by the mean rate of photon absorption (MRPA), so as to characterize the time responses of the photosynthetic cells toward light changes. This enables to observe short-term and long-term responses with their own characteristic times. The mechanisms involved were found to be different between increasing and decreasing light transitions. Furthermore the MRPA was found a valuable parameter to relate the effect of light to biological responses (i.e., pigment changes) under constant light and dynamic light conditions. Key points • MRPA proved valuable to relate C. vulgaris responses to light changes. • A linear evolution was found between pigment content and MRPA in continuous light. • A rising PFD step induced fast protection and acclimation mechanisms. [ABSTRACT FROM AUTHOR]

Studying how natural phytoplankton adjust their photosynthetic properties to the quantity and quality of underwater light (i.e. light climate) is essential to understand primary production. A wavelength-dependent photoacclimation strategy was assessed using a multi-color pulse-amplitude-modulation chlorophyll fluorometer for phytoplankton samples collected in the spring at 19 locations across the English Channel. The functional absorption cross section of photosystem II, photosynthetic electron transport (PETλ) parameters and non-photochemical quenching were analyzed using an original approach with a sequence of three statistical analyses. Linear mixed-effects models using wavelength as a longitudinal variable were first applied to distinguish the fixed effect of the population from the random effect of individuals. Population and individual trends of wavelength-dependent PETλ parameters were consistent with photosynthesis and photoacclimation theories. The natural phytoplankton communities studied were in a photoprotective state for blue wavelengths (440 and 480 nm), but not for other wavelengths (green (540 nm), amber (590 nm) and light red (625 nm)). Population-detrended PETλ values were then used in multivariate analyses (partial triadic analysis and redundancy analysis) to study ecological implications of PETλ dynamics among water masses. Two wavelength ratios based on the microalgae saturation parameter Ek (in relative and absolute units), related to the hydrodynamic regime and underwater light climate, clearly confirmed the physiological state of microalgae. They also illustrate more accurately that natural phytoplankton communities can implement photoacclimation processes that are influenced by in situ light quality during the daylight cycle in temporarily and weakly stratified water. Ecological implications and consequences of PETλ are discussed in the context of turbulent coastal ecosystems.

Marine debris has become a global problem affecting coral health around the globe. However, the photophysiological responses of corals to marine debris stress remain unclear. Therefore, this study firstly investigated transparent and opaque plastic bag shading and fishing nets directly contacting the coral. Photosynthetic performance, pigment content, symbiont density, and calcification rate of a branching coral Pocillopora acuta and a massive coral Porites lutea were investigated after 4 weeks of exposure to marine debris. The results show that the maximum quantum yield of PSII significantly decreased in P. lutea with all treatments, while P. acuta showed no effect on the maximum quantum yield of PSII from any treatments. Transparent plastic bag shading does not affect P. acuta, but significantly affected the maximum photochemical efficiency of P. lutea. Photoacclimation of cellular pigment content was also observed under opaque plastic bag shading for both species at week 2. Fishing nets had the strongest effect and resulted in P. acuta bleaching and P. lutea partial mortality as well as a decline in zooxanthellae density. Calcification rate of P. acuta significantly decreased with treatments using opaque plastic bag and fishing net, but for P. lutea only the treatment with fishing net gave any observable effects. This study suggests that the sensitivities of corals to marine debris differ strongly by species and morphology of the coral.

Photoacclimation is a photo‐physiological mechanism of phytoplankton associated with the modulation of the ratio of phytoplankton chlorophyll (Chl) and carbon (C), yet little is known on how it modulates Chl dynamics and the Chl‐C coupling relationship in the mid‐latitude oceans. In this study, Chl and C estimated from optical sensors on three BGC‐Argo floats, provided a record of the biomass and physiological variability at high temporal and vertical resolutions in the mid‐latitude northwest Pacific. It is found that, despite the Chl seasonal cycle being mainly dominated by variability in biomass, Chl and C became decoupled and inversely correlated in winter, due to the dominant effect of photoacclimation. Although Chl and C co‐varied in spring and autumn, photoacclimation modulated the Chl‐C coupling relationship as well, leading to a faster increase in C and Chl, respectively, in spring and late autumn. On daily and synoptic scales, Chl dynamics were also driven by photoacclimation to a large extent. Particularly, during a typhoon event, both cloud cover and mixed‐layer dynamics introduced fast changes in mixed‐layer light level, resulting in quick photo‐physiological responses of phytoplankton in modulating the variances of Chl. Similarly, during wintertime mixing‐restratification events associated with abrupt mixed‐layer changes, photoacclimation muted the Chl response, leading to a faster increase in C than Chl. Plain Language Summary: Although chlorophyll concentration (Chl) has been widely used as a proxy for phytoplankton biomass abundance, it is co‐determined by biomass (in unit of carbon concentration, C) and photoacclimation, the physiological mechanism of phytoplankton cells which adjusts intracellular pigment density (Chl:C) in response to ambient light and nutrients. In this study, based on three BGC‐Argo floats, we investigated how biomass and photoacclimation modulated the Chl dynamics in the mid‐latitude northwest Pacific. It was found that biomass dynamics dominated Chl dynamics in spring and summer, while photoacclimation determined Chl variations in winter leading to an inverse correlation between Chl and C. Even during the biomass‐dominated seasons, photoacclimation played an important role resulting in a slower Chl accumulation than C in spring, but a faster Chl accumulation in late autumn. Remarkably, the Chl dynamics associated with short time‐scale events such as a typhoon and wintertime mixing‐restratification events were also highly affected by photoacclimation. Key Points: Phytoplankton chlorophyll and biomass dynamics were observed in the mid‐latitude northwest Pacific using BGC‐Argo floatsDuring biomass accumulation periods, pigment density was down‐regulated, associated with faster carbon increase than chlorophyllDuring late autumn, winter and typhoon events, photoacclimation, rather than biomass, dominated the mixed‐layer chlorophyll dynamics [ABSTRACT FROM AUTHOR]