Shifts in phytoplankton composition and productivity are anticipated in the future, because phytoplankton are frequently bottom-up controlled, and environmental conditions, like temperature, partial pressure of CO2 (pCO2), and light climate continue to change. Culture experiments revealed that whereas future (elevated) pCO2 had no effect on T. weissflogii in the absence of environmental stressors, growth rate was drastically decreased under future pCO2 if cells grew under light and temperature stress. The 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), in the cell quotas of organic carbon, and the chl a:C 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. As a consequence, growth rate was low and decoupled from photosynthesis. This decoupling led to large cell sizes and high excretion rates in future pCO2 treatments compared to ambient treatments if 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, photosynthesis, pigment concentration, or excretion.
