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Ocean acidification interacts with variable light to decrease growth but increase particulate organic nitrogen production in a diatom

Li, Wei, Wang, Tifeng, Campbell, Douglas A., Gao, Kunshan
Marine environmental research 2020 v.160 pp. 104965
Bacillariophyceae, Thalassiosira pseudonana, biomass production, carbon dioxide, carbon dioxide enrichment, carbon dioxide fixation, carbon nitrogen ratio, carotenoids, chlorophyll, electron transfer, energy transfer, food quality, light intensity, mixing, ocean acidification, oceans, organic nitrogen, pH, particulate organic carbon, photons, photoperiod, phytoplankton, primary productivity, weather
Phytoplankton in the upper oceans are exposed to changing light levels due to mixing, diurnal solar cycles and weather conditions. Consequently, effects of ocean acidification are superimposed upon responses to variable light levels. We therefore grew a model diatom Thalassiosira pseudonana under either constant or variable light but at the same daily photon dose, with current low (400 μatm, LC) and future high CO₂ (1000 μatm, HC) treatments. Variable light, compared with the constant light regime, decreased the growth rate, Chl a, Chl c, and carotenoid contents under both LC and HC conditions. Cells grown under variable light appeared more tolerant of high light as indicated by higher maximum relative electron transport rate and saturation light. Light variation interacted with high CO₂/lowered pH to decrease the carbon fixation rate, but increased particulate organic carbon (POC) and particularly nitrogen (PON) per cell, which drove a decrease in C/N ratio, reflecting changes in the efficiency of energy transfer from photo-chemistry to net biomass production. Our results imply that elevated pCO₂ under varying light conditions can lead to less primary productivity but more PON per biomass of the diatom, which might improve the food quality of diatoms and thereby influence biogeochemical nitrogen cycles.