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Here, we explore the responses of photosynthesis and related cellular processes in the thermotolerant microalga Micractinium sp. acclimated to limiting and saturating irradiances combined with elevated temperatures, using a novel computer-controlled multi-sensor system. This system allows for the monitoring of online values of oxygen exchange during photosynthesis and respiration with high accuracy. Micractinium sp. cells showed maximum growth and net oxygen production rates under the optimal temperature of 25°C regardless of the light acclimation conditions. Our results show that the upper thermal threshold for Micractinium sp. photosynthesis and growth ranges between 35°C and 40°C. This microalga exhibited stable photosynthetic efficiency and effective non-photochemical quenching (NPQ) under saturating light, and was more susceptible to temperature change when acclimated to limiting light levels. These results demonstrate that the acclimation of thermotolerant microalgae to saturating light helps to enhance the thermal tolerance of PSII. This feature results from enhanced heat stability of PSII photochemistry and oxygen evolution., (© 2020 Phycological Society of America.)

Oxygenic photosynthesis has historically been considered limited to be driven by the wavelengths of visible light. However, in the last few decades, various adaptations have been discovered that allow algae, cyanobacteria, and even plants to utilize longer wavelength light in the far-red spectral range. These adaptations provide distinct advantages to the species possessing them, allowing the effective utilization of shade light under highly filtered light environments. In prokaryotes, these adaptations include the production of far-red-absorbing chlorophylls d and f and the remodeling of phycobilisome antennas and reaction centers. Eukaryotes express specialized light-harvesting pigment-protein complexes that use interactions between pigments and their protein environment to spectrally tune the absorption of chlorophyll a. If these adaptations could be applied to crop plants, a potentially significant increase in photon utilization in lower shaded leaves could be realized, improving crop yields.

This work introduces a new experimental method for the comprehensive description of the physiological responses to light of photosynthetic organisms. It allows the integration in a single experiment of the main established manipulative chlorophyll fluorescence-based protocols. It enables the integrated characterization of the photophysiology of samples regarding photoacclimation state (generating non-sequential light-response curves of effective PSII quantum yield, electron transport rate or non-photochemical quenching), photoprotection capacity (running light stress-recovery experiments, quantifying non-photochemical quenching components) and the operation of photoinactivation and photorepair processes (measuring rate constants of photoinactivation and repair for different light levels and the relative quantum yield of photoinactivation). The new method is based on a previously introduced technique, combining the illumination of a set of replicated samples with spatially separated actinic light beams of different intensity, and the simultaneous measurement of the fluorescence emitted by all samples using an imaging fluorometer. The main novelty described here is the independent manipulation of light intensity and duration of exposure for each sample, and the control of the cumulative light dose applied. The results demonstrate the proof of concept for the method, by comparing the responses of cultures of Chlorella vulgaris acclimated to low and high light regimes, highlighting the mapping of light stress responses over a wide range of light intensity and exposure conditions, and the rapid generation of paired light-response curves of photoinactivation and repair rate constants. This approach represents a chlorophyll fluorescence 'protocol of everything', contributing towards the high throughput characterization of the photophysiology of photosynthetic organisms., (© The Author 2017. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

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]

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]

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]

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]

• A trapezoidal" relationship between chlorophyll and surface PAR at centers of subtropical gyres is reported. • Multiple processes co-regulated chlorophyll dynamics at the center of South Pacific Gyre. • The "trapezoidal" spiral relationship indicated a dynamic equilibrium influenced by the alternating co-regulation mechanisms. Understanding the driving mechanism of phytoplankton dynamics is key to forecasting future changes in the ocean. Here, we report an apparent "trapezoidal" relationship between chlorophyll concentrations (Chl) and surface photosynthetically available radiation (PAR(0)) at the center of the South Pacific Gyre (cSPG) based on 18 years of MODIS Aqua measurements. A comparison of Chl with a photoacclimation model revealed that photoacclimation alone could not explain the temporal dynamics of Chl. Instead, the Chl dynamics were explained by a combination of photoacclimation, nutrient limitation, and the grazing pressure of zooplankton at different times throughout the year. An annual "trapezoidal" spiral relationship between Chl and PAR(0) suggested that the steady state of phytoplankton populations at the cSPG could be influenced by the alternation of co-regulation mechanisms during a year. Because this same pattern occurs in other subtropical gyres, this understanding of the underlying mechanisms not only facilitates simulating and forecasting phytoplankton dynamics but also provides a new perspective on how multiple stressors may impact phytoplankton communities in a warmer climate. [ABSTRACT FROM AUTHOR]

Satellite observations show higher near‐surface phytoplankton chlorophyll (Chl) in anticyclonic eddies (AEs) than in cyclonic eddies (CEs) in the subtropical gyres, which is opposite to the prevailing features in midlatitude oceans. Recent studies have attributed this anomalous effect solely to changes in phytoplankton biomass, therefore reached a conclusion that AEs are more productive than CEs, contrary to the conventional eddy pumping paradigm. Here, we show that the Chl variations are due mainly to physiological adjustments instead. The high Chl in AEs is mostly explained by photoacclimation to low‐light environment caused by deepened mixing and more rapid light attenuation, and vice versa for CEs. Such a scenario indicates that phytoplankton are likely more cellular pigmented, rather than more abundant, in the AEs than in the CEs. These findings invite us to rethink the effects of eddies on oceanic productivity and carbon cycling in the subtropical gyres. Plain Language Summary: Oceanic mesoscale eddies occupy a quarter of the global ocean surface. They have been widely documented to form and maintain high near‐surface phytoplankton chlorophyll (Chl) patches with horizontal scales of O (100 km) for months. This signal has long been interpreted as changes in phytoplankton abundance and primary production arising from changes in nutrient supply driven by eddy dynamics. However, Chl is not simply a measurement of phytoplankton abundance. It also reflects changes in intracellular pigmentation originate from light‐driven and/or nutrient‐driven physiological adjustments. Here, we show that physiological response is an important component of the Chl variability induced by eddies across the global ocean. Especially, in the subtropical gyres, the eddy‐driven Chl variability is more dominated by physiological adjustments than by the changes in phytoplankton biomass. For anticyclonic eddies in these regions, deepened mixing and more rapid light attenuation reduce the exposure of phytoplankton to light, upregulate the synthesis of intracellular Chl, thus resulting in higher Chl than in cyclonic eddies. In contrast, no apparent difference in phytoplankton carbon was detected between these two types of eddies. Such a decoupling response of phytoplankton Chl from biomass invites us to rethink the effects of eddies on oceanic productivity and carbon cycling. Key Points: Physiological adjustments make considerable contributions to the response of phytoplankton chlorophyll to eddies in the global oceanEddy‐driven chlorophyll variability in the subtropical gyres is more dominated by changes in phytoplankton physiology than in biomassPhotoacclimation plays an important role in the processes of physiological adjustments in eddies in the subtropical gyres [ABSTRACT FROM AUTHOR]

Microalgae can respond to long-term increases in light intensity by altering the concentration of photosynthetic complexes. Under active growth, the ability of Chlamydomonas reinhardtii to acclimate to excess light is dependent on cell division to reduce the concentration of photosynthetic complexes. But, in batch culture, cells eventually reach stationary phase where their ability to divide is limited; this should impact their capacity to undergo photoacclimation. Our goal is to dissect excess-light responses as cells approach stationary phase and to determine how the strategies of photoacclimation differ compared to cells in the exponential-growth phase. In this study, cultures exited exponential growth and transitioned into a declining growth phase (DGP), where cells continued a slow rate of growth for the next seven days in both low (LL) and high-light (HL). During this period, both cultures experience a conditional senescence-related decline in chlorophyll levels. Under HL, however, the senescing cultures have a rapid decline in PSII reaction centres, maintain a stable concentration of LHCII antenna, rapidly increase LHCSR levels, and have a sustained increase in Fo/Fm. Collectively this implies that the remaining antenna act as pH-dependent, quenching centres, presumably to protect the senescing chloroplast against HL. We discovered that acclimating to HL post-exponential phase involves active degradation that is intertwined with the normal senescence process that allowed for a limited rate of cell division. Image 1 • There is an age-dependent decline in chlorophyll. • Senescing cultures with gas exchange maintain a limited growth that facilities acclimation. • Senescing cultures react to HL by degrading PSII and maintaining LHCII antenna. • Antennae, possibly detached from PSII, form pH-dependent quenching centres under HL. • Autophagy has a role in acclimating to HL. [ABSTRACT FROM AUTHOR]

The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligotrophic oceans, the foundation of marine food webs, and an important component of global biogeochemical cycles. The prominence of Prochlorococcus across the environmental gradients of the open ocean is attributed to its extensive genetic diversity and flexible chlorophyll physiology, enabling light capture over a wide range of intensities. What remains unknown is the balance between temporal dynamics of genetic diversity and chlorophyll physiology in the ability of Prochlorococcus to respond to a variety of short (approximately 1 day) and longer (months to year) changes in the environment. Previous field research established depth-dependent Prochlorococcus single cell chlorophyll distributions in the North Pacific Subtropical Gyre. Here, we examined whether the shifts in chlorophyll distributions correspond to changes in Prochlorococcus genetic diversity (i.e., ecotype-level community structure) or photoacclimation of stable communities over short time intervals. We report that community structure was relatively stable despite abrupt shifts in Prochlorococcus chlorophyll physiology, due to unexpected physiological plasticity of high-light adapted Prochlorococcus ecotypes. Through comparison with seasonal-scale changes, our data suggest that variability on daily scales triggers shifts in Prochlorococcus photoacclimation, while seasonal-scale dynamics trigger shifts in community structure. Together, these data highlight the importance of incorporating the process of photoacclimation and chlorophyll dynamics into interpretations of phytoplankton population dynamics from chlorophyll data as well as the importance of daily-scale variability to Prochlorococcus ecology.

Under-ice blooms of phytoplankton in the Chukchi Sea have been observed, with strong implications for our understanding of the production regimes in the Arctic Ocean. Using a combination of satellite remote sensing of phytoplankton biomass, in situ observations under sea ice from an autonomous underwater vehicle (AUV), and in vivo photophysiology, we examined the composition, magnitude and origin of a bloom detected beneath the sea ice Northwest of Svalbard (Southern Yermak Plateau) in May 2010. In situ concentration of up to 20 mg chlorophyll a [Chl a] m−3, were dominated by the northern planktonic spring species of diatoms, Thalassiosira nordenskioeldii, T. antarctica var. borealis, Chaetoceros socialisspecies complex and Fragilariopsis oceanica. These species were also found south of the marginal ice zone (MIZ). Cells in the water column under the sea ice were typically high-light acclimated, with a mean light saturation index (Ek) of 138 μmol photons m−2 s−1 and a ratio between photoprotective carotenoids (PPC) and Chl a (w:w) of 0.2. Remotely sensed data of [Chl a] showed a 32,000 km2 bloom developing south of the MIZ. In effect, our data suggest that the observed under-ice bloom was in fact a bloom developed in open waters south of the ice edge, and that a combination of northward-flowing water masses and southward drifting sea ice effectively positioned the bloom under the sea ice. This have implications for our general understanding of under-ice blooms, suggesting that their origin and connection with open water may be different in different regions of the Arctic. [ABSTRACT FROM AUTHOR]

Cryoconite is a matrix of sediment, biogenic polymer and a microbial community that resides on glacier surfaces. The phototrophic component of this community is well adapted to this extreme environment, including high light stress. Photoacclimation of the cryoconite phototrophic community on Longyearbreen, Svalbard, was investigated using in situ variable chlorophyll fluorescence. Rapid light curves (RLCs) and induction-recovery curves were used to analyse photosystem II quantum efficiency, relative electron transport rate and forms of downregulation including non-photochemical quenching (NPQ) and state transitions in cyanobacteria. Phototrophs used a combination of behavioural and physiological photochemical downregulation. Behavioural downregulation is hypothesised to incorporate chloroplast movement and cell or filament positioning within the sediment matrix in order to shade from high light, which resulted in a lack of saturation of RLCs and hence overestimation of productivity. Physiological downregulation likely consisted of biphasic NPQ, comprising a steadily induced light-dependent form and a light-independent NPQ that was not reversed with decreasing light intensity. State transitions by cyanobacteria were the most likely physiological downregulation employed by cyanobacteria within the mixed phototroph community. These findings demonstrate that cryoconite phototrophs combine multiple forms of physiological and behavioural downregulation to optimise light exposure and maximise photosynthetic productivity. This plasticity of photoacclimation enables them to survive productively in the high-light stress environment on the ice surface. [ABSTRACT FROM AUTHOR]

Photoacclimation is a cellular process that allows phytoplankton to change the intracellular chlorophyll- a concentration ( Chl ) in relation to environmental factors such as light and nutrients. This process is currently overlooked by standard operational ocean colour algorithms used to retrieve information about both the phytoplankton standing stock and production. Here, we describe the photoacclimation effect on the phytoplankton seasonal cycle through the chlorophyll to carbon ratio ( Chl : C ) over the Mediterranean Sea, using SeaWiFS (1998–2007) monthly data. Chlorophyll was calculated through a regionalized empirical algorithm and Carbon was estimated from the particle backscattering, after subtraction of the non-algal contribution (b bp NAP ). Historically, b bp NAP has been neglected or assumed constant, and only recently its space-time variability started to be taken into account. Here, bootstrapping simulations are used to compute the monthly b bp NAP s over a former bio-regionalization map of the Mediterranean Sea, all highlighting significant space-time variability. Resulting Chl : C varies within a factor of three, demonstrating the important role that photoacclimation plays at seasonal and basin scales. Lowest Chl : C values (~ 0.0013) are observed during summer; this is due to the combined effect of high irradiation, shallow mixing and low nutrient concentration. Maximum values (~ 0.031) are observed in winter and spring dominated by both low solar radiation, deeper mixing and high nutrient availability. It emerges that a better view of the phytoplankton space and time variability can be achieved through the joint use of Chl and C . [ABSTRACT FROM AUTHOR]

Photoacclimation ( PA) and freezing tolerance ( FT) have been identified as closely related traits, due to common mechanisms of environmental control. In this study, diversity array technology ( DArT) was used for identification of the quantitative trait loci ( QTL) of FT and PA in winter barley. Simple sequence repeat ( SSR) and single nucleotide polymorphism ( SNP) markers were subsequently used to saturate QTL regions. Two F2 mapping populations were created, for two-rowed (P44) and six-rowed barley (CaP). Different regions of the genome were responsible for differences in traits between parents in these two populations. Eleven QTLs were identified in the P44 population, including five typical for FT and PA, on chromosomes 2H, 3H and 7H. In the CaP population, only one QTL connected with PA and 10 connected with FT were found on all chromosomes except 2H. Our results demonstrate that different sets of markers should be applied in marker-assisted selection for FT in two- and six-rowed barley, as several loci determine FT at the level of biparental crosses. [ABSTRACT FROM AUTHOR]

The photoresponse of chlorophyll a (Chl a) fluorescence and xanthophyll pigments to light was studied in the diatom Thalassiosira weissflogii with respect to various durations of dark storage. The light-limited slope (a) obtained from the electron transport rate (ETR) versus the irradiance curve remained steady, whereas the light-saturated rate (ETRmax) decreased gradually during dark storage. Consequently the light-saturation index (Ek=ETRmax/a) decreased with time, suggesting that cells acclimate to dark conditions. Dark-acclimated cells were exposed to light-dark conditions with varied silicate availability. The maximum quantum yield of PSII (Fv/Fm), as well as the changes in the a and the ETRmax, increased immediately to the maximum on the first day and decreased for the second half of the light exposure period. The ratio of diatoxanthin (DT) to the xanthophyll pigment diadinoxanthin (DD) and DT (DT/ [DD+DT]) represents thermal dissipation behavior through non-photochemical quenching (NPQ). This study indicates that Fv/Fm and Ek are a good index to follow photoacclimation during a transition from continuous dark conditions to the L-D cycle of saturated light. This study also indicates that dark-acclimated T. weissflogii can acclimate to the growth irradiance by the enhancement of photosynthetic activity with changes in xanthophyll pigments, in relation to the dark duration and silicate availability. Such a response to light suggests that the vegetative cells of T. weissflogii could possibly proliferate at ports in the coastal zone on a global scale after ejection into high light environments from the darkness of a ship's ballast water tank. [ABSTRACT FROM AUTHOR]

In diatoms, light-harvesting processes take place in a specific group of proteins, called fucoxanthin chlorophyll a /c proteins (FCP). This group includes many members and represents the major characteristic of the diatom photosynthetic apparatus, with specific pigments bound (chlorophyll c , fucoxanthin, diadino- and diatoxanthin besides chlorophyll a). In thylakoids, FCP and photosystems (PS) form multimeric supercomplexes. In this study, we compared the biochemical properties of PS supercomplexes isolated from Thalassiosira pseudonana cells grown under low light or high light conditions, respectively. High light acclimation changed the molecular features of the PS and their ratio in thylakoids. In PSII, no obvious changes in polypeptide composition were observed, whereas for PSI changes in one specific group of FCP proteins were detected. As reported before, the amount of xanthophyll cycle pigments and their de-epoxidation ratio was increased in PSI under HL. In PSII, however, no additional xanthophyll cycle pigments occurred, but the de-epoxidation ratio was increased as well. This comparison suggests how mechanisms of photoprotection might take place within and in the proximity of the PS, which gives new insights into the capacity of diatoms to adapt to different conditions and in different environments. • Diatom photosystem I alters its light-harvesting system upon high-light acclimation. • Photosystem II supercomplexes do not change in protein composition. • In Photosystem I, more xanthophyll cycle pigments are bound under high-light. • In Photosystem II, only the de-epoxidation ratio is increased under high-light. [ABSTRACT FROM AUTHOR]

Phalaenopsis ( Phalaenopsis amabilis ‘KC1410’) grow slowly and experience photoinhibition during the daytime. Thus, this study used orchid mycorrhizal (OM) fungus to promote orchid growth hypothetically via increasing the photoacclimation capacity of Phalaenopsis in response to high light (HL). By combining chlorophyll fluorometer and spectral reflectance techniques, the influences of OM colonization on the growth potentials and the photosynthetic performance of HL-grown Phalaenopsis were investigated. OM symbiosis significantly promoted the growth of HL-grown orchids, such as leaf span, fresh weight, and dry weight. Consistent with these observations, OM symbiosis obtained higher value of normalized difference vegetation index (NDVI) and enhanced CO 2 uptake rate. This could be attributed to an increase in photosynthetic performance in OM plants. Notably, maximum quantum efficiency of PSII photochemistry (Fv/Fm) was not altered by OM symbiosis, however, the light-adapted fluorescence measurements of PSII operating efficiency (ФPSII) and electron transport rate (ETR) were increased. Nevertheless, OM orchids had higher photochemical quenching (qP) and lower non-photochemical quenching (NPQ). This indicates a relative higher photosynthetic performance and radiation-use efficiency (RUE) in OM plants. The photochemical reflectance index (PRI) as a photoprotection indicator was also enhanced. Based on these results, we proposed that OM symbiosis increases both the photosynthetic performance and the RUE in the HL-acclimated Phalaenopsis , resulting in an advantage to the OM plants to survive the HL stress. [ABSTRACT FROM AUTHOR]

Chrococcoid cyanobacteria of the genus Synechococcus are the important component of marine and freshwater ecosystems. Picocyanobacteria comprise even 80% of total cyanobacterial biomass and contribute to 50% of total primary cyanobacterial bloom production. Chlorophyll (Chl) fluorescence and photosynthetic light response ( P-I) curves are commonly used to characterize photoacclimation of Synechococcus strains. Three brackish, picocyanobacterial strains of Synechococcus (BA-132, BA-124, BA-120) were studied. They were grown under 4 irradiances [10, 55, 100, and 145 μmol(photon) m s] and at 3 temperatures (15, 22.5, and 30°C). Photosynthetic rate was measured by Clark oxygen electrode, whereas the Chl fluorescence was measured using Pulse Amplitude Modulation fluorometer. Based on P-I, two mechanisms of photoacclimation were recognized in Synechococcus. The maximum value of maximum rate of photosynthesis ( P) expressed per biomass unit at 10 μmol(photon) m s indicated a change in the number of photosynthetic units (PSU). The constant values of initial slope of photosynthetic light response curve (α) and the maximum value of P expressed per Chl unit at 145 μmol(photon) m s indicated another mechanism, i.e. a change in PSU size. These two mechanisms caused changes in photosynthetic rate and its parameters (compensation point, α, saturation irradiance, dark respiration, P) upon the influence of different irradiance and temperature. High irradiance had a negative effect on fluorescence parameters, such as the maximum quantum yield and effective quantum yield of PSII photochemistry (φ), but it was higher in case of φ. [ABSTRACT FROM AUTHOR]

The cyanobacterium Microcystis aeruginosa forms blooms that can consist of colonies. We have investigated how M. aeruginosa acclimatizes to changing light conditions such as can occur during blooms. Three different strains were exposed to two irradiance levels: lower ( LL) and higher ( HL) than the irradiance-onset saturation parameter. We measured the photosynthetic pigment concentrations, PSII photochemical efficiency, electron transport rate ( ETR), irradiance-saturated ETR and ETR efficiency. The relationship between ETR and photosynthetic oxygen production and the excess in PSII capacity were also studied for one strain. Higher values of chlorophyll a and phycocyanin and lower values of total carotenoids were found under LL conditions in the three strains. The strains showed clear differences in the irradiance-saturated ETR and in ETR efficiency under both LL and HL treatments. No differences were found in the linear relationship between ETR and photosynthetic oxygen production under both irradiance treatments. LL-acclimated cells showed higher PSII excess capacity than HL ones, possibly because their higher pigment content could result in a higher light stress than HL cells when forming surface blooms. The fact that the genetically different strains show different photosynthetic physiologies suggests that the very dynamic light climate observed in lakes may allow their coexistence. [ABSTRACT FROM AUTHOR]

Abstract: Growth and photosynthetic responses of Zostera marina transplants along a depth gradient were examined to determine appropriate transplanting areas for seagrass restoration. Seagrass Z. marina was once widely distributed in the Taehwa River estuary in southeastern Korea, but has disappeared since the 1960s due to port construction and large scale pollutant inputs from upstream industrial areas. Recently, water quality has been considerably improved as a result of effective sewage treatment, and the local government is attempting to restore Z. marina to the estuary. For seagrass restoration in this estuary, a pilot transplantation trial of Z. marina at three water depths (shallow: 0.5 m; intermediate: 1.5 m; deep: 2.5 m relative to MLLW) was conducted in November 2008. The transplant shoot density increased gradually at the intermediate and deep sites, whereas the transplants at the shallow site disappeared after 3 months. To find the optimal transplantation locations in this estuary, the growth and photosynthetic responses of the transplants along a depth gradient were examined for approximately 4 months following transplantation in March 2009. In the 2009 experimental transplantation trial, shoot density of transplants at the shallow site was significantly higher than those at the intermediate and deep sites during the first 3 months following transplantation, but rapidly decreased approximately 4 months after transplantation. The chlorophyll content, photosynthetic efficiency (α), and maximum quantum yield (F v/F m) of the transplants were significantly higher at the deep site than at the shallow site. Shoot size, biomass and leaf productivity were also significantly higher at the deep site than at the shallow site. Although underwater irradiance was significantly lower at the deep site than at the shallow site, transplants at the deep site were morphologically and physiologically acclimated to the low light. Transplants at the shallow site exhibited high mortality during the early period of transplantation perhaps due to high physical disturbances at the site, but transplants at the intermediate and deep sites showed higher growth through more efficient photosynthesis and morphological adaptation. Thus, the intermediate and deep sites (1.5–2.5 m relative to MLLW) appeared to be more appropriate seagrass transplantation sites in this estuary. [Copyright &y& Elsevier]

The article presents a molecular study of Symbiodinium diversity in the Mediterranean sea whip Eunicella singularis. It considers Eunicella singularis as the only symbiotic gorgonian in the Mediterranean. It explores the role of the lack of capacity to acquire new symbionts in limiting the bathymetric diversity in Eunicella singularis. It reveals host-symbiont specificity among E. singularis and other Mediterranean cnidrian species. It conducts transplantation experiments to determine a limit of plasticity for symbiont population density and chlorophyll content.

The article focuses on a study about the accuracy of chlorophyll a (Chl)-based methods in determining in situ phytoplankton growth rate. It notes that a common approach to measure the growth rate of phytoplanktons involves the incubation of a natural population inside a closed container and the measurement of Chl change. It cites two problems associated with the incubation approach, one is the differences between growth rate estimates taken from chlorophyll concentration and carbon changes. Findings of the study are given, one of which is the assertion that errors in turbulent and turbid mixing layers can be reduced by adopting the so-called yo-yo method.

We have studied the plasticity of the photosynthetic apparatus in the endangered aquatic macrophyte Althenia orientalis to the gradient of light availability within its meadow canopy. We determined diurnal change in situ irradiance, light quality, in vivo chlorophyll a fluorescence, ex situ oxygen evolution rates, respiration rate and pigment concentration. The levels of photosynthetic photon flux density (PFD) and ultraviolet radiation (UVR) and the red/far-red ratio decreased with depth within the canopies of A. orientalis. Apical leaves had a greater decrease of the maximal quantum yield ( F/ F) in the morning and a faster recovery rate in the afternoon than those in the basal ones. The relative electron transport rate (ETRr) was not saturated at any time of the day, even in the apical leaves that received the highest light. The maximum light-saturated rate of gross photosynthesis (GP) took place in apical leaves around noon. The chlorophyll a/ b ratio values were higher, and the chlorophyll/carotenoid ratio values lower, in apical leaves than basal ones. The highest concentrations in total carotenoids were reached in the apical leaves around noon. A. orientalis has a high capacity to acclimatize to the changes in the light environment, both in quality and quantity, presenting sun and shade leaves in the same stem through the vertical gradient in the canopy. [ABSTRACT FROM AUTHOR]

The article discusses a study regarding non-photochemical chlorophyll fluorescence quenching (NPQ) and xanthophyll cycle responses resulting from photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) in certain coastal phytoplanktons from Potter Cove on King George Island in Antarctica. The study involving Chaetoceros brevis and Phaeocystis antarctica cultures aims to determine whether photoinhibition plays a role in taxon dominance and moderate phytoplankton biomass in the region during summer. Results show that high light acclimation has led to significant NPQs and a decrease in excess PAR and UVR effects on photosynthesis. Another finding indicates that P. antarctica was more dependent on chloroplast protein synthesis under protein synthesis inhibition.

The relationships between changes in cell suspension absorbance, pigment composition, and resistance to photodamage were investigated in the microalga Haematococcus pluvialis Flotow em. Wille (Chlorophyta) IPPAS H-239 cultivated under high level of photosynthetically active radiation (PAR, 50 W/m). When the green flagellated cells of H. pluvialis lacking astaxanthin (Ast) and possessing low (<0.5) carotenoid/chlorophyll ratio were irradiated by intense light (2500 W/m PAR), rapid and synchronous photobleaching of 70-80% of chlorophyll (Chl) and carotenoids (Car) was observed. By contrast, the rate of pigment photobleaching in cells with Car/Chl > 1, which retained high Chl content (> 0.6 fmol/cell) and accumulated significant amounts of Ast, was two times lower than in the green cells. Red aplanospores, with Car/Chl > 10, containing high amounts of Ast and low amounts of Chl (> 0.8 and < 0.1 fmol/cell, respectively) were resistant to photodestruction. The extent of cell resistance to photobleaching correlated closely with an increase in contribution of Car to light absorption by H. pluvialis cell suspensions. The build up of Ast during acclimation to high light was accompanied by a gradual increase in the optical density ratio OD/OD, whereas synchronous (OD/OD ≈ const; r > 0.99) and profound (>20%) bleaching of Car and Chl absorption bands was characteristic of photodamage. The spectral features of photoacclimation and photodamage revealed in this work can be used for nondestructive diagnostics of photodamage in H. pluvialis cultures and for on-line assessment of cell resistance to photooxidative death. The results are discussed with respect to the nondestructive monitoring of laboratory and production cultures of H. pluvialis and their protection from photooxidative death. [ABSTRACT FROM AUTHOR]

Hizikia fusiformis thalli experience dynamic incident light conditions during the period of growth. The present study was designed to examine how changing photon irradiance affects the photosynthesis both in the short and long terms by culturing H. fusiformis under three different light levels: 35 μmol photons m-2 s-1 (low light, LL), 85 μmol photons m-2 s-1 (intermediate light, IL), and 165 μmol photons m-2 s-1 (high light, HL). A similar relative growth rate was observed between IL- and HL-grown algae, but the growth rate was significantly reduced in LL-grown algae. The photosynthetic rates ( Pn) measured at their respective growth light levels were found to be lowest in the thalli grown at LL and highest at HL. However, LL-grown algae exhibited much higher Pn in comparison with IL- and the HL-grown thalli at the same measuring photosynthetic photon flux density, indicating the photosynthetic acclimation to low growth light in H. fusiformis. The photosynthesis–light curves showed that LL-grown algae had a highest light-saturating maximum Pn ( Pmax) in comparison with IL- or HL-grown algae when the photosynthetic rates were expressed on the biomass basis. However, Pmax was highest in HL-grown algae compared to IL- or LL-grown algae when the rates were normalized to chlorophyll a. The photosynthesis–inorganic carbon (Ci) response curves were also significantly affected by the growth light conditions. The highest value of apparent photosynthetic conductance occurred in LL-grown algae while the lowest value in HL-grown algae. Additionally, the activity of external carbonic anhydrase (CA) tended to increase while the total CA activity inclined to decrease in H. fusiformis thalli when the growth light level altered from 35 to 165 μmol photons per square meter per second. The external CA inhibitors showed a higher inhibition in HL-grown algae compared with LL-grown algae. It was proposed that photosynthetic acclimation to low light condition in H. fusiformis was achieved through an increase in the number of reaction centers and increased capacities of electron transport and of Ci transport within cells. The ability of photosynthetic acclimation to low light confers H. fusiformis thalli to overcome the environmental low light condition as a result of the attenuation of seawater or self-shading through enhancing its photosynthetic performance and carbon assimilation necessary for growth. [ABSTRACT FROM AUTHOR]

Abstract: We characterized the planular-zooxanthellae symbiosis of the coral Pocillopora damicornis using criteria that are familiar in studies on corals. Similar to adult corals, planulae exhibited photoacclimation, as changes in symbiont chlorophyll a (chl a); changes in the light-saturation constant for photosynthesis (I k); and, at insufficient light, fewer zooxanthellae, decreased respiration, increased weight loss, and increased sensitivity to photoinhibition. Numbers of zooxanthellae in newly-released planulae varied by at least three-fold within broods. Planulae with low versus high numbers of zooxanthellae (termed pale versus dark planulae, respectively) did not differ in symbiont chl a content, I k, or biomass-specific rate of dark respiration. Pale planulae had lower rates of photosynthesis, but this difference vanished after three weeks, when zooxanthellar numbers increased by 225% in pale planulae and by 31% in dark planulae. Numbers of zooxanthellae also increased significantly in planulae cultured in ammonium-enriched seawater; ammonium also apparently prevented weight loss and induced settlement. Approximately 70% of photosynthetically-fixed carbon (labeled using 14C) apparently was translocated from the zooxanthellae to their host. A comparison of planulae cultured at 0.3% versus 11% sunlight suggested that photosynthesis provided ∼31% of the energy utilized by the latter. Overall, we conclude that the physiology of symbiosis in planulae of P. damicornis is broadly similar to symbiosis physiology in adult corals. [Copyright &y& Elsevier]

Abstract: A mechanistic model of microalgae is used to explore the implications of modifying microalgal chlorophyll content and photosynthetic efficiency with an aim to optimising commercial biomass production. The models show the potential for a 10 fold increase in microalgae productivity in genetically modified versus unmodified configurations, while also enabling the use of bioreactors of greater optical depth operating at lower dilution rates. Analysis suggests that natural selection of a trait benefiting the individual (high Chl:Cmax, i.e., high antennae size) conflicts with artificial selection of a trait (low Chl:Cmax) of most benefit to production at the population level. The implication is that GM strains rather than strains selected from nature will be most beneficial for commercial algal biofuels production. Further, escaped GM algae populations may, depending on the specific nature of the modification, be quickly out-competed by the natural forms because individually a high Chl:C is beneficial in low light environments. However, it remains possible that changes in biochemical composition associated with genetic modification of photosystem competence, or with other selection processes to enhance commercial gain, may adversely affect the value of such organisms as prey for zooplankton, leading to the unwanted generation of future harmful algae. [Copyright &y& Elsevier]

Simple Summary: Coral aquaculture is a new industry, which is of great importance to the sustainable development of coral reefs and meeting commercial needs. Light sources are crucial for the growth of corals because zooxanthellae provide them with essential nutrients through photosynthesis. Different corals and zooxanthellae have different photoacclimation characteristics; therefore, selecting a suitable light wavelength remains the critical inhibitor of coral maintenance in marine aquariums. Accordingly, this study investigated the effects of different light wavelengths and feeding of G. columna. The results showed that blue light (440–470 nm) and purple light (400–430 nm) increased the protease and body protein in corals, and the growth and survival rate also increased. In summary, G. columna's efficient cultivation can use 400–470 nm wavelengths as the primary source of illumination. Goniopora columna is a stony coral valued for its reef-building potential and its unique appearance. Thus, identifying the optimal culture conditions for G. columna would enable efficient cultivation and prevent the illegal exploitation of marine resources. Light sources are crucial for the growth of corals because zooxanthellae provide them with basic nutrients through photosynthesis. Different corals and zooxanthellae have different photoacclimation characteristics; therefore, selecting a suitable light wavelength remains the key inhibitor of coral maintenance in marine aquariums. Accordingly, this study investigated the effects of different light wavelengths on G. columna. It was illuminated for 6 or 12 h a day under white light, yellow light, red light (LR), green light (LG), blue light (LB), or purple light (LP) for 8 weeks. During the experiment, R(R; i.e., a formula feed that combines sodium alginate, protein and probiotics) of 5% (w/v) of G. columna tissue and skeletal dry weight was fed every day. Coral polyps were counted, zooxanthellae density, chlorophyll a concentration, specific growth rates, and survival rates were calculated; polyp stretching and contractile behaviors were observed; and body composition and digestive enzyme activity were analyzed. LB or LP (but not LG or LR) illumination for at least 6 h per day significantly promoted the growth, survival, protein content, and protease activity of the G. columna specimens. Furthermore, coral polyp extension reached 100% after 30 min of LP and LB light irradiation. Although no significant differences in the zooxanthellae density or chlorophyll a concentration were noted under various light wavelengths, significant reductions were detected in the absence of light. To achieve energy-efficient coral aquaculture with regard to G. columna cultivation, 6 h of LB or LP illumination per day can improve the growth. [ABSTRACT FROM AUTHOR]

Abstract: We collected biological and physical oceanographic data simultaneously from shipboard observations and mooring buoys in the Pacific equatorial warm pool during the 2002–2003 El Niño event and found that the vertical profiles of cyanobacterial pigments reflected the turbulent kinetic energy (TKE) better than did analyses of the mixed layer by temperature and salinity profiling. Zeaxanthin, an accessory pigment of cyanobacteria, was abundant and almost homogeneous in the warm pool within the surface mixed layer, although chlorophyll a concentrations were low. The intracellular content of chlorophyll a increases with increasing depth and decreasing light in a photoprotective response, but the zeaxanthin content does not change with depth. Hence, we hypothesized that the profile of the ratio of zeaxanthin to chlorophyll a would decrease with increasing depth if the water column were stable, without vertical mixing. On the contrary, vertically constant ratios would indicate vertical mixing. Our analysis using a numerical model showed a good agreement between profiles of these ratios and the profiles of TKE and supported the hypothesis. However, a comparative analysis of the zeaxanthin:chlorophyll a ratio profiles with mixed layer depth based on temperature and salinity data revealed two distinct patterns. In the first pattern, the ratio was uniform in the mixed layer when TKE was strong. In the second, the ratio decreased with increasing depth, even in the mixed layer, because of reduced TKE. [Copyright &y& Elsevier]

Reef-building corals occur as a range of colour morphs because of varying types and concentrations of pigments within the host tissues, but little is known about their physiological or ecological significance. Here, we examined whether specific host pigments act as an alternative mechanism for photoacclimation in the coral holobiont. We used the coral Montipora monasteriata (Forskål 1775) as a case study because it occurs in multiple colour morphs (tan, blue, brown, green and red) within varying light-habitat distributions. We demonstrated that two of the non-fluorescent host pigments are responsive to changes in external irradiance, with some host pigments up-regulating in response to elevated irradiance. This appeared to facilitate the retention of antennal chlorophyll by endosymbionts and hence, photosynthetic capacity. Specifically, net Pmax Chl a−1 correlated strongly with the concentration of an orange-absorbing non-fluorescent pigment (CP-580). This had major implications for the energetics of bleached blue-pigmented (CP-580) colonies that maintained net Pmax cm−2 by increasing Pmax Chl a−1. The data suggested that blue morphs can bleach, decreasing their symbiont populations by an order of magnitude without compromising symbiont or coral health. [ABSTRACT FROM AUTHOR]

Dynamics and possible function of the lutein epoxide (Lx) cycle, that is, the reversible conversion of Lx to lutein (L) in the light-harvesting antennae, were investigated in leaves of tropical tree species. Photosynthetic pigments were quantified in nine Inga species and species from three other genera. In Inga, Lx levels were high in shade leaves (mostly above 20 mmol mol−1 chlorophyll) and low in sun leaves. In Virola surinamensis, both sun and shade leaves exhibited very high Lx contents (about 60 mmol mol−1 chlorophyll). In Inga marginata grown under high irradiance, Lx slowly accumulated within several days upon transfer to deep shade. When shade leaves of I. marginata were briefly exposed to the sunlight, both violaxanthin and Lx were quickly de-epoxidized. Subsequently, overnight recovery occurred only for violaxanthin, not for Lx. In such leaves, containing reduced levels of Lx and increased levels of L, chlorophyll fluorescence induction showed significantly slower reduction of the photosystem II electron acceptor, QA, and faster formation as well as a higher level of non-photochemical quenching. The results indicate that slow Lx accumulation in Inga leaves may improve light harvesting under limiting light, while quick de-epoxidation of Lx to L in response to excess light may enhance photoprotection. [ABSTRACT FROM AUTHOR]

Photoacclimation models for marine phytoplankton describe the changes in their composition (typcially C, N and chlorophyll) and growth in response to changing light and nutrient environment. We compared two such models: that of Geider et al. (Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplankton acclimation to light, nutrients, and temperature. Limnol. Oceanogr. 43, 679–694), hereafter the G model, and that of Pahlow (Pahlow, M., 2005. Linking chlorophyll-nutrient dynamics to the redfield N:C ratio with a model of optimal phytoplankton growth. Mar. Ecol. Prog. Ser. 287, 33–43), hereafter the P model. Using the Monte Carlo Markov Chain method, we fitted both models to the data set from an incubation experiment by Flynn et al. (Flynn, K.J., Davidson, K., Leftley, J.W., 1994. Carbon–nitrogen relations at whole-cell and free amino-acid levels, during batch growth of isochrysis galbanna (prymnesiophyceae) under conditions of alternating light and dark. Mar. Biol. 118, 229–237). Data consisted of measured concentrations for particulate organic N, particulate organic C, chlorophyll and ammonium. The authors of the G model began their simulation from day 5 for this experiment, claiming that their model could not reproduce the initial lag phase (slow growth during the first few days of the experiment). The author of the P model claimed that its ability to reproduce this initial lag phase (starting from the beginning of the experiment) was a significant improvement over previous models. Our fitting revealed that the G model can reproduce this initial lag phase as well as the P model, and that both models simulate the data set well. In the best-fits of both models, chlorophyll synthesis during the initial lag phase was limited by the rate of carbon assimilation, even though the G model was designed to have chlorophyll synthesis limited only by the rate of N assimilation. The requirement of organic carbon for energy to assimilate nitrogen results in this indirect limitation of chlorophyll synthesis by C assimilation in the G model, whereas chlorophyll synthesis is explicitly limited by the rate of C assimilation in the P model. This suggests that chlorophyll synthesis is in fact limited by the rate of C assimilation during the initial lag phase. As in the hand-tuned simulations previously published, the P model simulated the initial decrease in Chl:N ratio (as observed) whereas the G model simulated an initial increase in this ratio (contrary to the observations). We also discuss the relative merits of the two models for applications at large scales. Although data assimilation is not perfectly objective, because it requires certain choices such as weights for various data types and which data to include when fitting, our results show the advantage of mathematically rigorous fitting as opposed to hand-tuning of models. Our best-fits were significantly better than the hand-tuned fits originally published, especially for the G model, and this yielded insight into the mechanism responsible for the initial lag in phytoplankton growth. [Copyright &y& Elsevier]

A coupled 1D physical-biological model of Crater Lake is presented. The model simulates the seasonal evolution of two functional phytoplankton groups, total chlorophyll, and zooplankton in good quantitative agreement with observations from a 10-year monitoring study. During the stratified period in summer and early fall the model displays a marked vertical structure: the phytoplankton biomass of the functional group 1, which represents diatoms and dinoflagellates, has its highest concentration in the upper 40 m; the phytoplankton biomass of group 2, which represents chlorophyta, chrysophyta, cryptomonads and cyanobacteria, has its highest concentrations between 50 and 80 m, and phytoplankton chlorophyll has its maximum at 120 m depth. A similar vertical structure is a reoccurring feature in the available data. In the model the key process allowing a vertical separation between biomass and chlorophyll is photoacclimation. Vertical light attenuation (i.e., water clarity) and the physiological ability of phytoplankton to increase their cellular chlorophyll-to-biomass ratio are ultimately determining the location of the chlorophyll maximum. The location of the particle maxima on the other hand is determined by the balance between growth and losses and occurs where growth and losses equal. The vertical particle flux simulated by our model agrees well with flux measurements from a sediment trap. This motivated us to revisit a previously published study by Dymond et al. (). Dymond et al. used a box model to estimate the vertical particle flux and found a discrepancy by a factor 2.5–10 between their model-derived flux and measured fluxes from a sediment trap. Their box model neglected the exchange flux of dissolved and suspended organic matter, which, as our model and available data suggests is significant for the vertical exchange of nitrogen. Adjustment of Dymond et al.’s assumptions to account for dissolved and suspended nitrogen yields a flux estimate that is consistent with sediment trap measurements and our model. [ABSTRACT FROM AUTHOR]

Summary: This study investigated carotenoid and chlorophyll a (Chl-a) contents under two different growth irradiances in four freshwater cyanobacterial strains. We found an increased weight ratio of zeaxanthin to Chl-a after exposure to high irradiances over several days. Two out of four strains showed higher zeaxanthin amounts on a biomass basis as well. It appears that cyanobacteria enhance their carotenoid pool in response to high light conditions, as increased production of other carotenoids with photoprotective abilities has also been observed under high irradiance levels. Cyanobacteria do not possess the violaxanthin cycle, which enables a rapid reversible conversion from violaxanthin into zeaxanthin and functioning as a quencher of excessive energy, and elevated zeaxanthin concentrations could therefore be seen as an adaptive strategy against excess light energy. Some differences in the acclimation pattern were revealed between different cyanobacteria. Anabaena torulosa contained higher amounts of every carotenoid, while Nostoc sp. mainly increased zeaxanthin, and myxoxanthophyll. Anabaenopsis elenkinii produced exceptionally high amounts of myxoxanthophyll and β-carotene under higher irradiances. Anabaena cylindrica generally showed less variation of carotenoids under different irradiances. [Copyright &y& Elsevier]

Abstract: A north–south transect crossing the quasi-permanent upwelling area and the front which usually occupies the north-western part of the Alboran Sea (Western Mediterranean) was sampled 16 times during four cruises, covering an annual cycle in order to study the coupling between hydrodynamics and phytoplankton spatial distribution. On a spatial scale, considerable changes in biomass, size-structure and composition of the phytoplankton community were found at both sides of the front. The upwelling area was characterized by higher cell abundance, higher mean integrated chlorophyll values, intense subsurface chlorophyll maximum (SSCM) dominated by the size fraction >10μm, as well as significantly less negative slopes of the size-spectra. Prokaryotic phytoplankton cells were more abundant in the oligotrophic waters off the front. These qualitative and quantitative differences of the phytoplanktonic spatial distribution were maintained in spite of the north–south displacement of the front that might occur even within days, in association with changes in position and size of the Western Alboran Gyre. On a temporal scale, the mean chlorophyll concentrations at the SSCM were similar in July and December (3.0±0.4 and 3.5±1.0mgl−1 respectively), while abundance and biomass decreased in winter. The resulting lower C:Chl a and higher fluorescence cell−1 ratio indicate photoacclimation in winter. Implications of the different SAS at both sides of the front on the fate of phytoplankton biomass and importance of field measurements for satellite image interpretation are discussed. [Copyright &y& Elsevier]

The kinetics of response to strong light have been examined in deeply shaded leaves of the tropical tree legume (Inga sp.) which have extraordinarily high levels of the α-xanthophyll lutein-epoxide that are co-located in pigment–protein complexes of the photosynthetic apparatus with the β-xanthophyll violaxanthin. As in other species, rapidly reversible photoprotection (measured as non-photochemical chlorophyll fluorescence quenching) is initiated within the time frame of sun-flecks (minutes), before detectable conversion of violaxanthin to antheraxanthin or zeaxanthin. Photoprotection is stabilized within hours of exposure to strong light by simultaneously engaging the reversible violaxanthin cycle and a slowly reversible conversion of lutein-epoxide to lutein. It is proposed that this lutein ‘locks in’ a primary mechanism of photoprotection during photoacclimation in this species, converting efficient light-harvesting antennae of the shade plant into potential excitation dissipating centres. It is hypothesized that lutein occupies sites L2 and V1 in light-harvesting chlorophyll protein complexes of photosystem II, facilitating enhanced photoprotection through the superior singlet and/or triplet chlorophyll quenching capacity of lutein. [ABSTRACT FROM PUBLISHER]

An individual-based model is presented which describes the spatial and temporal evolution of phytoplankton growth in terms of a Lagrangian ensemble of cells affected by various physical and biological forcing factors. The motion of cells develops according to a turbulent velocity field which simulates the Antarctic mixed layer during the summer. The cell growth is a function of the irradiance regime, nutrient availability and the vertical position of the individual with respect to the other cells (in order to evaluate the self-shading effect). The values of photosynthetic parameters used to simulate the photophysiological response of the organisms are derived from measurements collected in the Ross Sea. In contrast to previous individual-based descriptions, all the physical and biological processes involved are explicitly reproduced in their dynamical features. Coupling different mixing levels with photoacclimation strategies leads to a wide range of photophysiological responses which underline the role of individual physiological histories in determining the growth of the population as a whole. Simulated photosynthetic parameters, chlorophyll a concentrations and integrated primary production correspond closely to in situ data and confirm that photoacclimation to low irradiance and strong mixing regimes may be considered as crucial factors in the photosynthetic performance of Antarctic phytoplankton. [ABSTRACT FROM AUTHOR]

Although seagrasses possess numerous adaptations for life underwater, they lack the specialized accessory pigments for the efficient harvesting of green light that dominates many aquatic environments. Without these specialized pigments, photoacclimation by seagrasses is likely to result in a severe package effect, i.e. the loss of linearity between light harvesting efficiency and pigment loading. Here, we investigated the optical constraints imposed by the package effect on photoacclimation in seagrass leaves. Pigment concentrations and optical properties (absorptances, absorption coefficients and optical cross-sections) of turtlegrass (Thalassia testudium) leaves from Lee Stocking Island (LSI; Bahamas) and eelgrass (Zostera marina) leaves from Monterey Bay, CL, USA were measured at different times of the year. Chlorophyll concentrations and optical cross-sections differed by a factor of five across sampling dates and populations. Increases in leaf-specific absorption among seagrass leaves were greatest in the green (500–600 nm), while the package effect, as measured by a decrease in leaf optical cross-section, was most severe in the blue (400–500 nm) and red (600–700 nm). Consequently, the five-fold range in pigment concentration resulted in similar photosynthetic light harvest efficiencies (φL&ap;50% of incident PAR) for intact seagrass leaves in their native light environments. Although the package effect has significant impacts on the optical properties of seagrass leaves, chlorophyll use efficiency does not appear to play a strong role in the ecology or evolution of seagrasses. [Copyright &y& Elsevier]

Pigment concentration, in vivo absorption, and photosynthetic parameters of the coralline alga Hydrolithon onkodes (Heydrich) Penrose and Woelkerling were compared among samples from a lagoon and from a reef crest of Tahiti Island. Four groups of specimens were considered, differing in their natural exposure to PAR. For specimens collected from the lagoon, the tissues from low-light samples had significantly higher pigment concentration, particularly chl a and phycobilins, compared with the high-light exposed plants that contained more total carotenoids. The in vivo absorption spectra normalized to chl a (called a* values) also revealed differences. The low-light samples had a reduced absorption capacity and a well-marked phycobilin absorption signature, whereas sunlit samples showed a greater absorption at wavelengths absorbed mainly by chl a and carotenoids. The decrease of a* when pigment concentration increased is interpreted as a consequence of the pigment packaging. Significantly lower α (chlorophyll basis) and higher Ek values were found in the shaded plants. The values of Pmax for the four groups of specimens were not significantly different. The samples showed various degrees of photoinhibition depending on the light exposure during growth, and this effect was more pronounced in the shaded plants. The specimens from the reef crest deviate from the general model presented for the lagoon samples and show a mix of sun- and shade-exposed characteristics. We have shown that the coralline alga H. onkodes responds to its light environment, probably by acclimation rather than ecotypic genetic variation, by adjusting its physiology, but some morphological differences are also involved. Photoacclimation can explain partly the wide distribution of this species over the reef ecosystem and its major contribution to the building of the reef. [ABSTRACT FROM AUTHOR]

Photoacclimation in the marine eustigmatophyte Nannochlropsis sp., used extensively as a food chaincomponent in aquaculture, was studied both in thelaboratory and outdoors. Cell-chlorophyll andcarotenoids were used as markers to assessphotoacclimation to strong light, as well as todecreasing growth irradiance due to cellproliferation. Focusing on practical aspects involvedin mass cultivation, three different approaches wereused as follows: (a) cultures initially exposed to lowlight (150 μmol photon m-2 s-1) thentransferred to strong light (1000 to 3000 μmolphoton m-2 s-1); (b) initially low celldensity cultures grown in reactors of differentlight-paths, exposed to strong PFD, in the laboratoryand outdoors; (c) initially low or high cell densitycultures exposed to strong light. As has already beenestablished in many reports, cell-chlorophyllrepresented a sensitive parameter in assessing cellresponse to changes in the intensity of the lightsource as well as to modifications in the light regimeto which the cells were exposed. Cell-chlorophyllconcentration sharply decreased initially upontransferring the culture from low PFD cell-1 tohigh PFD cell-1 due to either culture dilution(i.e. decrease in cell density and mutual shading) orto an increase in PFD. After some 7 days ofphotoacclimating to 2000 and 3000 μmol photonm-2 s-1, chlorophyll a content began to riseat a much faster rate than cell number, which alsoincreased in response to the higher irradiance.Cell-chlorophyll in the culture exposed to 2000μmol photon m-2 s-1 increased afteracclimation earlier and at a faster rate than in theculture exposed to 3000 μmol photon m-2s-1, indicating the later irradiance affected astronger stress. The length of the reactor's lightpath exerted a decisive effect on cell response tostrong light through its influence on the light regimein the culture. Upon a sharp increase in PFD,carotenoids in the 1-cm reactor increased in muchhigher rate than chlorophyll, compared with the 3-cmlight path reactors. This marked difference in cellresponse to a shift-up in light was attributed to thevast variations in the light regime associated withdifferences in the length of the light path and areal density. Growth oflow cell density cultures ceased temporarily upontransfer to strong light, in contrast with high celldensity cultures transferred to strong light, whichcontinued growth without a lag. [ABSTRACT FROM AUTHOR]

The Variability of chlorophyll-specific phytoplankton light absorption a * ph (λ)] was examined over depth and time in stratified offshore waters of the North-Western Mediterranean Sea. Coherent water patches were tracked with Lagrangian drifters during two oceanographic cruises in September (late summer) and May (post-spring bloom phase). By simultaneously analysing the phytoplankton absorption and pigment measurements, we explicitly separated the impact of pigment composition from that of pigment packaging on a * ph (λ). We further partitioned the packaging effect by comparing the variation of the packaging index Q a *(440)], the phytoplankton community size structure (derived from diagnostic pigment analysis), and the chlorophyll-specific beam attenuation by particles c p * (660)] as an optical index of phytoplankton photophysiology. In the ensemble of cruises, around 50% of the a * ph (440) vertical variation was explained by changes in the pigment composition (ruled by the decrease of photoprotective pigments with depth). The remaining vertical and inter-cruise variation of a * ph (440) was attributed to the packaging effect. We found that differences in the c p *(660) index (most likely indicating changes in the intracellular pigment concentration due to photoacclimation) mainly explained the observed variation in the packaging effect. Differences in c p * (660) were coincident with either the vertical gradient of light availability or the lower mean daily PAR irradiance in the euphotic layer of the September cruise. These explained the stronger packaging and lower a * ph (440) values observed with increasing depth in both cruises, and in September relative to May. On the other hand, differences in the phytoplankton community size structure did not explain the observed patterns in the packing effect. Our results highlight the importance of phytoplankton short-term acclimation to the prevailing light conditions, determining the vertical and temporal variability of a * ph (λ). A better understanding of the a * ph (λ) variability and its main drivers are key to improve different bio-optical applications. • Changes in pigment composition, ruled by PPCs, explained 50% of a* ph vertical variation. • The remaining spatiotemporal variation of a * ph was attributed to packaging effect. • Variation in c p * (660) was coincident with vertical and seasonal differences in irradiance. • Changes in intracellular pigment content explained differences in packaging effect. • Photoacclimation shaped the vertical and inter-cruise variability of a* ph. [ABSTRACT FROM AUTHOR]