The effects of pH and pCO2 on photosynthesis and respiration in the diatom Thalassiosira weissflogii

The response of marine phytoplankton to the ongoing increase in atmospheric pCO(2) reflects the consequences of both increased CO2 concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii, we explored the effects of varying pCO(2) and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H-2 O-18. In long-term experiments (similar to 6-h) at saturating light intensity, we observed no effects of pH or pCO(2) on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In shortterm experiments (similar to 3 min), we also observed no effects of pCO(2) or pH, even under limiting light intensity. We surmise that in T. weissflogii, it is the photosynthetic production of NADPH and ATP, rather than the CO2-saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H+ and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric pCO(2) predicted for the twenty-first century.