Role of extracellular carbonic anhydrase in the polar diatom Fragilariopsis cylindrus

Summary Diatoms are major primary producers in productive polar oceans, where cold temperatures and high CO 2 solubility raise questions as to whether they rely on the CO 2 ‐concentrating mechanism (CCM) to saturate photosynthesis. Knowledge of CCM function will help better predict the sensitivity of diatom primary production to warming and acidifying polar oceans. Here, we characterized the CCM in the polar diatom, Fragilariopsis cylindrus, using membrane inlet mass spectrometry (MIMS) and a numerical model. Quantification of the role of external carbonic anhydrase (eCA) was enabled through the addition of a surface boundary layer to the numerical model. We found that F. cylindrus displayed an active CCM across a wide range of inorganic carbon availability, but that the CCM was not required under air‐equilibrated CO 2 concentrations. eCA was an essential component of the CCM, facilitating HCO 3 − uptake. When eCA was inhibited, CO 2 was the primary carbon source. The eCA role in supporting HCO 3 − uptake was independent of temperature changes. eCA‐supported HCO 3 − transport is likely a mechanism for F. cylindrus to cope with fluctuating concentrations of inorganic carbon over a dynamic seasonal cycle in polar oceans. Our work challenges the hypothesis that CCMs are not required at cold temperatures.