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To investigate effects of UV radiation (UVR, 280-400 nm) on coccolithophorids under nutrient-limited conditions, we grew Gephyrocapsa oceanica to determine its resilience to consecutive daily short-term exposures to +UVR (irradiances >295 nm) under a range of nitrate availabilities (100, 24, 12, 6 and 3 μM). +UVR alone significantly hampered the growth of G. oceanica, with the synergistic negative effects of +UVR and N-limitation being about 58% and 22% greater than under UVR or N-limitation alone, respectively. Most 3 μM nitrate cultures died, but those exposed to UVR succumbed sooner. This was due to a failure of photoprotection and repair mechanisms under low N-availability with exposures to UVR. Additionally, the UVR-induced inhibition of the effective quantum yield of photosystem II (PSII) was significantly higher and was further aggravated by N limitation. The algal cells increased photoprotective pigments and UV-absorbing compounds as a priority rather than using calcification for defense against UVR, indicating a trade-off in energy and resource allocation. Our results indicate the negative effects of UVR on coccolithophorid growth and photosynthesis, and highlight the important role of N availability in defense against UVR as well as high PAR. We predict that enhanced N-limitation in future surface oceans due to warming-induced stratification will exacerbate the sensitivity of G. oceanica to UVR, while coccolithophores can be potentially more susceptible to other environmental stresses due to increased levels of nutrient limitation., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Trichodesmium species, as a group of photosynthetic N2 fixers (diazotrophs), play an important role in the marine biogeochemical cycles of nitrogen and carbon, especially in oligotrophic waters. How ongoing ocean warming may interact with light availability to affect Trichodesmium is not yet clear. We grew Trichodesmium erythraeum IMS 101 at three temperature levels of 23, 27, and 31 ∘ C under growth-limiting and growth-saturating light levels of 50 and 160 µ mol quanta m -2 s -1 , respectively, for at least 10 generations and then measured physiological performance, including the specific growth rate, N2 fixation rate, and photosynthesis. Light availability significantly modulated the growth response of Trichodesmium to temperature, with the specific growth rate peaking at ∼27 ∘ C under the light-saturating conditions, while growth of light-limited cultures was non-responsive across the tested temperatures (23, 27, and 31 ∘ C). Short-term thermal responses for N2 fixation indicated that both high growth temperature and light intensity increased the optimum temperature (Topt) for N2 fixation and decreased its susceptibility to supra-optimal temperatures (deactivation energy – Eh). Simultaneously, all light-limited cultures with low Topt and high Eh were unable to sustain N2 fixation during short-term exposure to high temperatures (33–34 ∘ C) that are not lethal for the cells grown under light-saturating conditions. Our results imply that Trichodesmium spp. growing under low light levels while distributed deep in the euphotic zone or under cloudy weather conditions might be less sensitive to long-term temperature changes that occur on the timescale of multiple generations but are more susceptible to abrupt (less than one generation time span) temperature changes, such as those induced by cyclones and heat waves. [ABSTRACT FROM AUTHOR]

Kelp are main iodine accumulators in the ocean, and their growth and photosynthesis are likely to benefit from elevated seawater CO2 levels due to ocean acidification. However, there are currently no data on the effects of ocean acidification on iodine metabolism in kelp. As key primary producers in coastal ecosystems worldwide, any change in their iodine metabolism caused by climate change will potentially have important consequences for global geochemical cycles of iodine, including iodine levels of coastal food webs that underpin the nutrition of billions of humans around the world. Here, we found that elevated pCO2 enhanced growth and increased iodine accumulation not only in the model kelp Saccharina japonica using both short‐term laboratory experiment and long‐term in situ mesocosms, but also in several other edible and ecologically significant seaweeds using long‐term in situ mesocosms. Transcriptomic and proteomic analysis of S. japonica revealed that most vanadium‐dependent haloperoxidase genes involved in iodine efflux during oxidative stress are down‐regulated under increasing pCO2, suggesting that ocean acidification alleviates oxidative stress in kelp, which might contribute to their enhanced growth. When consumed by abalone (Haliotis discus), elevated iodine concentrations in S. japonica caused increased iodine accumulation in abalone, accompanied by reduced synthesis of thyroid hormones. Thus, our results suggest that kelp will benefit from ocean acidification by a reduction in environmental stress however; iodine levels, in kelp‐based coastal food webs will increase, with potential impacts on biogeochemical cycles of iodine in coastal ecosystems. Kelp are main iodine accumulators in the ocean, and their growth and photosynthesis are likely to benefit from elevated seawater CO2 levels due to ocean acidification. However, there are currently no data on the effects of ocean acidification on iodine metabolism in kelp. Here, we found that ocean acidification increases iodine accumulation in kelp‐based coastal food webs. There is a potential risk of iodine overconsumption in consumers of kelps if pCO-2 increases as projected for the coming decades. [ABSTRACT FROM AUTHOR]

Abstract: Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short‐term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long‐term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO2 conditions (1,000 μatm). High CO2‐selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO2 (400 μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO2‐selected cell lines. All of these trait changes persisted when high CO2‐grown populations were moved back to ambient CO2 conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short‐term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences. [ABSTRACT FROM AUTHOR]

We conducted shipboard microcosm experiments at both off-shore (SEATS) and near-shore (D001) stations in the northern South China Sea (NSCS) under three treatments, low temperature and low p CO 2 (LTLC), high temperature and low p CO 2 (HTLC), and high temperature and high p CO 2 (HTHC). Biomass of phytoplankton at both stations were enhanced by HT. HTHC did not affect phytoplankton biomass at station D001 but decreased it at station SEATS. HT alone increased net primary productivity by 234% at station SEATS and by 67% at station D001 but the stimulating effect disappeared when HC was combined. HT also increased respiration rate by 236% at station SEATS and by 87% at station D001 whereas HTHC reduced it by 61% at station SEATS and did not affect it at station D001. Overall, our findings indicate that the positive effect of ocean warming on phytoplankton assemblages in NSCS could be damped or offset by ocean acidification. [ABSTRACT FROM AUTHOR]

Biological effects of ultraviolet radiation (UVR; 280-400 nm) on marine primary producers are of general concern, as oceanic carbon fixers that contribute to the marine biological CO2 pump are being exposed to increasing UV irradiance due to global change and ozone depletion. We investigated the effects of UV-B (280-320 nm) and UV-A (320-400 nm) on the biogeochemically-critical filamentous marine N2-fixing cyanobacterium Trichodesmium (strain IMS101) using a solar simulator as well as under natural solar radiation. Short exposure to UV-B, UV-A, or integrated total UVR significantly reduced the effective quantum yield of photosystem II (PSII) and photosynthetic carbon and N2 fixation rates. Cells acclimated to low light were more sensitive to UV exposure compared to high-light grown ones, which had more UV absorbing compounds, most likely mycosporine-like amino acids (MAAs). After acclimation under natural sunlight, the specific growth rate was lower (by up to 44 %), MAAs content was higher, and average trichome length was shorter (by up to 22 %) in the full spectrum of solar radiation with UVR, than under a photosynthetically active radiation (PAR) alone treatment (400-700 nm). These results suggest that prior shipboard experiments in UV-opaque containers may have substantially overestimated in-situ nitrogen fixation rates by Trichodesmium, and that natural and anthropogenic elevation of UV radiation intensity could significantly inhibit this vital source of new nitrogen to the current and future oligotrophic oceans. [ABSTRACT FROM AUTHOR]

Gephyrocapsa oceanica is a widespread species of coccolithophore that has a significant impact on the global carbon cycle through photosynthesis and calcium carbonate precipitation. We investigated combined effects of light (50 μmol m−2 s−1, 190 μmol m−2 s−1, and 400 μmol m−2 s−1) and the nitrogen sources [ABSTRACT FROM AUTHOR]

We examined the effects of increased temperature, pCO2, and irradiance on a calcifying strain of the marine coccolithophore Emiliania huxleyi in semi-continuous laboratory cultures. Emiliania huxleyi CCMP 371 was cultured in four temperature and pCO2 treatments at both low and high irradiance (50 and 400 µmol photons m-2 s-1): (i) 20°C and 375 ppm CO2 (ambient control); (ii) 20°C and 750 ppm CO2 (high pCO2); (iii) 24°C and 375 ppm CO2 (high temperature); and (iv) 24°C and 750 ppm CO2 ('greenhouse'). The growth of E. huxleyi was greatly accelerated by elevated temperature at low irradiance. Photosynthesis was significantly promoted by increases in both pCO2 and temperature at both irradiances. Higher cellular C/P ratios were found in the higher CO2 treatments at high irradiance, indicating a reduced requirement for P. The PIC/POC (particulate inorganic to organic carbon) ratio remained constant at low light, regardless of CO2 or temperature conditions. However, both the cellular PIC content and PIC/POC ratio were greatly decreased by elevated irradiance, and were further decreased by increased pCO2 only at high light, indicating a combined effect of CO2 and light on calcification. These results suggest that future trends of CO2 enrichment, sea-surface warming and exposure to higher mean irradiances from intensified stratification will have a large influence on the growth of Emiliania huxleyi, and potentially on the PIC/POC 'rain ratio'. Our study demonstrates that it is possible to obtain a more complete picture of global change impacts on marine phytoplankton by designing experiments that consider multiple global change variables and their mutual interactions. [ABSTRACT FROM AUTHOR]

Little is known about the combined impacts of future CO2 and temperature increases on the growth and physiology of marine picocyanobacteria. We incubated Synechococcus and Prochlorococcus under present-day (380 ppm) or predicted year-2100 CO2 levels (750 ppm), and under normal versus elevated temperatures (+4°C) in semicontinuous cultures. Increased temperature stimulated the cell division rates of Synechococcus but not Prochlorococcus. Doubled CO2 combined with elevated temperature increased maximum chl a–normalized photosynthetic rates of Synechococcus four times relative to controls. Temperature also altered other photosynthetic parameters (α, Φmax, Ek, and ) in Synechococcus, but these changes were not observed for Prochlorococcus. Both increased CO2 and temperature raised the phycobilin and chl a content of Synechococcus, while only elevated temperature increased divinyl chl a in Prochlorococcus. Cellular carbon (C) and nitrogen (N) quotas, but not phosphorus (P) quotas, increased with elevated CO2 in Synechococcus, leading to ∼20% higher C:P and N:P ratios. In contrast, Prochlorococcus elemental composition remained unaffected by CO2, but cell volume and elemental quotas doubled with increasing temperature while maintaining constant stoichiometry. Synechococcus showed a much greater response to CO2 and temperature increases for most parameters measured, compared with Prochlorococcus. Our results suggest that global change could influence the dominance of Synechococcus and Prochlorococcus ecotypes, with likely effects on oligotrophic food-web structure. However, individual picocyanobacteria strains may respond quite differently to future CO2 and temperature increases, and caution is needed when generalizing their responses to global change in the ocean. [ABSTRACT FROM AUTHOR]

The harmful bloom alga Aureococcus anophagefferens Hargraves et Sieburth can survive prolonged darkness, which could facilitate overwinter survival and dispersal by anthropogenic vectors such as ballast water. Experiments were conducted to examine the biochemical and photosynthetic changes in cells during dark storage. Cells were stored in the dark for periods from 2 to 14 days and were sampled at days 0, 1, 2, 4, 7, 10, and 14. Samples from days 2, 7, and 14 were monitored during a recovery period of 4–5 days. Physiological and photosynthetic parameters were measured during the dark storage and recovery periods. Cultures resumed growth quickly when returned to light, and bacterial counts remained constant during the dark storage period but increased rapidly during recovery periods. Cellular protein, carbohydrate, and lipid concentrations declined slightly during the dark period. There were no changes in chl a per cell or in RUBISCO per cell during 14 days of darkness. The data therefore suggest that A. anophagefferens is able to maintain its photosynthetic apparatus during dark storage periods of at least 2 weeks and relies on cellular reserves until it is returned to light to resume photosynthesis. During the recovery period in the light, the cells are able to acclimate rapidly to current light levels and resume growth. [ABSTRACT FROM AUTHOR]

Abstract: Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short‐term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long‐term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO2 conditions (1,000 μatm). High CO2‐selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO2 (400 μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO2‐selected cell lines. All of these trait changes persisted when high CO2‐grown populations were moved back to ambient CO2 conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short‐term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences. [ABSTRACT FROM AUTHOR]

We examined the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera at glacial (19 Pa; 190 ppm), current (39 Pa; 380 ppm), and proj ected year 2100 (76 Pa; 750 ppm) CO2 levels. Rates of N2 and CO2 fixation and growth increased in step with increasing partial pressure of CO2 (pCO2), but only under Fe-replete conditions. N2 and carbon fixation rates at 75 Pa CO2 were 1.4-1.8-fold and 1.2-2.0-fold higher, respectively, relative to those at present day and glacial pCO2 levels. In Fe-replete cultures, cellular Fe and molybdenum quotas varied threefold and were linearly related to N2 fixation rates and to external pCO2. However, N2 fixation and trace metal quotas were decoupled from pCO2 in Fe-limited Crocosphaera. Higher CO2 and Fe concentrations both resulted in increased cellular pigment contents and affected photosynthesis vs. irradiance parameters. If these results also apply to natural Crocosphaera populations, anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation, but this effect may be tempered by Fe availability. Possible biogeochemical consequences may include elevated inputs of new nitrogen to the ocean and increased potential for Fe and/or phosphorus limitation in the future high-CO2 ocean, and feedbacks to atmospheric pCO2 in both the near future and over glacial to interglacial timescales. [ABSTRACT FROM AUTHOR]

Abstract: During two cruises to the Ross Sea, Antarctica in austral spring and summer, fast repetition rate fluorometry was used to investigate the relationship between phytoplankton photophysiology and water mass characteristics, micronutrient availability, and composition. Particulate organic matter proxies for phytoplankton biomass (chlorophyll a, particulate organic carbon and nitrogen, and biogenic silica) were all elevated in the photic zone during spring and summer. Biogenic silica concentrations were an order of magnitude higher in summer relative to spring, reflecting a shift in composition from Phaoecystis antarctica to diatoms. Quantum yields of PS II (F v /F m ) were generally higher in spring relative to summer, coincident with weaker vertical and horizontal gradients in hydrographic properties. Reduced F v /F m values (<0.4) were observed in the upper 30m in both seasons, with maximum values (ca. 0.55) observed near base and below the euphotic zone. No significant relationship between F v /F m values and dissolved Fe could be identified in the merged spring/summer data set. Functional absorption cross sections were significantly higher in spring than summer, presumably reflecting adaptations to lower irradiance in spring; little variation with depth was observed. Phytoplankton composition did not appear to be a major determinant of bulk quantum yield, although diatom-dominated waters exhibited significantly higher functional absorption cross sections when compared to waters dominated by P. antarctica. Dominance of P. antarctica appears to be related to greater photophysiological resilience and faster photoacclimation to changing light conditions, whereas diatoms were prevalent in shallow summer mixed layers, which likely reflects their enhanced photosynthetic capacity at high irradiance levels. [Copyright &y& Elsevier]

High ratios of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) have been suggested to favor the growth of the brown tide alga Aureococcus anophagefferens. DON could provide a particular advantage in low light levels, as occur when blooms induce self-shading. We examined the effects of varying DON:DIN ratios on the photosynthetic abilities of cultured Aureococcus at two light intensities, 93 and 17 μmol photons m-2 s-1. Glutamic acid and urea were used as DON sources, and the remainder of the nitrogen was added as nitrate.In experiments examining Aureococcus growth with varying ratios of DONGlu:DINNitrate at two light intensities in batch culture, higher growth rates and biomass were observed in treatments containing DIN than in those with DON only, which contrasts with the results of previous studies. In semi-continuous growth experiments with varying DONUrea:DINNitrate ratios, low light cultures with urea had higher growth rates than those without urea. Also, the effective target area for light absorption per cell and photosystem II efficiency were greater for the low light cultures of each nutrient treatment, particularly when DON:DIN mixtures (33 and 67% NUrea) were used. The same pattern was seen in the maximum photosynthetic rates per cell in the light-saturated (Pmcell) and in the initial slope (αcell) of the PE (photosynthesis versus irradiance) curve, and in PON, POC and chlorophyll a cell-1. This indicates that the ability of Aureococcus to acclimate to low light conditions may be enhanced by the presence of both organic and inorganic nitrogen sources. These results suggest that Aureococcus physiology and photosynthesis are different during growth on a mixture of urea-N and nitrate than when either nitrogen source is present alone. Results of this study suggest that Aureococcus may not respond to all DON substrates in the same way, and that mixtures of DON and DIN may provide for higher photosynthetic rates, especially at low light. Our results did not, however, support earlier suggestions that growth on DON alone provides the brown tide alga with a large advantage at low light levels. [Copyright &y& Elsevier]