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  1. Membrane permeabilities to CO2and HCO3constrain the function of CO2concentrating mechanisms that algae use to supply inorganic carbon for photosynthesis. In diatoms and green algae, plasma membranes are moderately to highly permeable to CO2but effectively impermeable to HCO3. Here, CO2and HCO3membrane permeabilities were measured using an18O‐exchange technique on two species of haptophyte algae,Emiliania huxleyiandCalcidiscus leptoporus, which showed that the plasma membranes of these species are also highly permeable to CO2(0.006–0.02 cm · s−1) but minimally permeable to HCO3. Increased temperature and CO2generally increased CO2membrane permeabilities in both species, possibly due to changes in lipid composition or CO2channel proteins. Changes in CO2membrane permeabilities showed no association with the density of calcium carbonate coccoliths surrounding the cell, which could potentially impede passage of compounds. Haptophyte plasma‐membrane permeabilities to CO2were somewhat lower than those of diatoms but generally higher than membrane permeabilities of green algae. One caveat of these measurements is that the model used to interpret18O‐exchange data assumes that carbonic anhydrase, which catalyzes18O‐exchange, is homogeneously distributed in the cell. The implications of this assumption were tested using a two‐compartment model with an inhomogeneous distribution of carbonic anhydrase to simulate18O‐exchange data and then inferring plasma‐membrane CO2permeabilities from the simulated data. This analysis showed that the inferred plasma‐membrane CO2permeabilities are minimal estimates but should be quite accurate under most conditions.

     
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  2. Temperature and nutrient supply are key factors that control phytoplankton ecophysiology, but their role is commonly investigated in isolation. Their combined effect on resource allocation, photosynthetic strategy, and metabolism remains poorly understood. To characterize the photosynthetic strategy and resource allocation under different conditions, we analyzed the responses of a marine cyanobacterium (SynechococcusPCC7002) to multiple combinations of temperature and nutrient supply. We measured the abundance of proteins involved in the dark (RuBisCO,rbcL) and light (PhotosystemII, psbA) photosynthetic reactions, the content of chlorophylla, carbon and nitrogen, and the rates of photosynthesis, respiration, and growth. We found thatrbcL and psbA abundance increased with nutrient supply, whereas a temperature‐induced increase in psbA occurred only in nutrient‐replete treatments. Low temperature and abundant nutrients caused increased RuBisCOabundance, a pattern we observed also in natural phytoplankton assemblages across a wide latitudinal range. Photosynthesis and respiration increased with temperature only under nutrient‐sufficient conditions. These results suggest that nutrient supply exerts a stronger effect than temperature upon both photosynthetic protein abundance and metabolic rates inSynechococcussp. and that the temperature effect on photosynthetic physiology and metabolism is nutrient dependent. The preferential resource allocation into the light instead of the dark reactions of photosynthesis as temperature rises is likely related to the different temperature dependence of dark‐reaction enzymatic rates versus photochemistry. These findings contribute to our understanding of the strategies for photosynthetic energy allocation in phytoplankton inhabiting contrasting environments.

     
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  3. While light limitation can inhibit bloom formation in dinoflagellates, the potential for high‐intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well‐studied. We measured the effects of high‐intensityPARon the bloom‐forming dinoflagellatesAlexandrium fundyenseandHeterocapsa rotundata. Various physiological parameters (photosynthetic efficiencyFv/Fm, cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll‐acontent) were measured before and after exposure to high‐intensity natural sunlight in short‐term light stress experiments. In addition, photosynthesis‐irradiance (P‐E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species‐specific responses to highPAR. While high light decreasedFv/Fmin both species,A. fundyenseshowed little additional evidence of light stress in short‐term experiments, although increased membrane permeability and intracellularDMSPindicated a response to handling. P‐E responses further indicated a high light‐adapted species with Chl‐ainversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per‐cell photosynthesis rates suggest a trade‐off between photoprotection and C fixation in high light‐acclimated cells.Heterocapsa rotundatacells, in contrast, swelled in response to high light and sometimes lysed in short‐term experiments, releasingDMSP. P‐E responses confirmed a low light‐adapted species with high photosynthetic efficiencies associated with trade‐offs in the form of substantial photoinhibition and a lack of plasticity in Chl‐acontent. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species‐specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.

     
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