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

• Published in: Photosynthesis research 2017-04, Vol.132 (1), p.83-93
• Main Authors: Goldman, Johanna A. L., Bender, Michael L., Morel, François M. M.
• Format: Article
• Language: English
• Subjects: Biochemistry; Biomedical and Life Sciences; Carbon dioxide; Life Sciences; Original Article; Photosynthesis; Plankton; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; Respiration
• ISSN: 0166-8595; 1573-5079
• DOI: 10.1007/s11120-016-0330-2

The response of marine phytoplankton to the ongoing increase in atmospheric p CO 2 reflects the consequences of both increased CO 2 concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii , we explored the effects of varying p CO 2 and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H 2 18 O. In long-term experiments (~6-h) at saturating light intensity, we observed no effects of pH or p CO 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 short-term experiments (~3 min), we also observed no effects of p CO 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 CO 2 -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 p CO 2 predicted for the twenty-first century.