| Summary: | © The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marine Ecology Progress Series 541 (2015): 75-90, doi:10.3354/meps11541. Future shifts in phytoplankton composition and productivity are anticipated given that continuing changes are expected in environmental conditions such as temperature, the partial pressure of CO2 (pCO2) and light climate, all of which regulate phytoplankton communities and their physiology through bottom-up control. Culture experiments revealed that future (elevated) pCO2 had no effect on Thalassiosira weissflogii in the absence of environmental stressors, whereas growth rates drastically decreased under future pCO2 when cells were grown under light and temperature stress. Reduction in growth rates and a smaller decline in cellular photosynthesis under high pCO2 were associated with 2- to 3-fold increases in the production of transparent exopolymer particles (TEP) and in the cell quotas of organic carbon, as well as a similar decrease in the C:chl a ratios. Results suggest that under light- and temperature-stressed growth, elevated pCO2 led to increased energy requirements, which were fulfilled by increased light harvesting capabilities that permitted photosynthesis of acclimatized cells to remain relatively high. This was combined with the inability of these cells to acclimatize their growth rate to sub-optimal temperatures. Consequently, growth rate was low and decoupled from photosynthesis, and this decoupling led to large cell sizes and high excretion rates in future pCO2 treatments compared to ambient treatments when growth temperature and light were sub- optimal. Under optimal growth conditions, the increased energy demands required to re- equilibrate the disturbed acid-base balance in future pCO2 treatments were likely mediated by a variety of physiological acclimatization mechanisms, individually too small to show a statistically detectable response in terms of growth rate, ...
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