MEPS 567:41-56 (2017)  -  DOI: https://doi.org/10.3354/meps12033

ABSTRACT: Marine phytoplankton are expected to benefit from enhanced carbon dioxide (CO₂), attributable largely to down-regulation of the CO₂ concentrating mechanism (CCM) which saves energy resources for other cellular processes. However, the nitrogen (N) nutritional condition (N-replete vs. N-limiting) of phytoplankton may affect the responses of their intracellular metabolic processes to elevated CO₂. We cultured the model diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum, and Thalassiosira weissflogii at ambient and elevated CO₂ levels under N-replete and N-limiting conditions. Key metabolic processes, including light harvesting, C fixation, photorespiration, respiration, and N assimilation, were assessed systematically and then incorporated into an energy budget to compare the effects of CO₂ on the metabolic pathways and the consequent changes in photosynthesis and C fixation as a result of energy reallocation under the different N nutritional conditions. Under the N-replete condition, down-regulation of the CCM at high CO₂ was the primary contributor to increased photosynthesis rates of the diatoms. Under N-limiting conditions, elevated CO₂ significantly affected the photosynthetic photon flux and respiration, in addition to CCM down-regulation and declines in photorespiration, resulting in an increase of the C:N ratio in all 3 diatom species. In T. pseudonana and T. weissflogii, the elevated C:N ratio was driven largely by an increased cellular C quota, whereas in P. tricornutum it resulted primarily from a decreased cellular N quota. The N-limited diatoms therefore could fix more C per unit of N in response to elevated CO₂, which could potentially provide a negative feedback to the ongoing increase in atmospheric CO₂. 

KEY WORDS: Ocean acidification · N-limitation · Energy budget · C:N ratio · Diatoms