The nature of the CO 2‐concentrating mechanisms in a marine diatom, Thalassiosira pseudonana

Diatoms are widespread in aquatic ecosystems where they may be limited by the supply of inorganic carbon. Their carbon dioxide-concentrating mechanisms (CCMs) involving transporters and carbonic anhydrases (CAs) are well known, but the contribution of a biochemical CCM involving C4 metabolism is contentious. The CCM(s) present in the marine-centric diatom, Thalassiosira pseudonana, were studied in cells exposed to high or low concentrations of CO2 , using a range of approaches. At low CO2 , cells possessed a CCM based on active uptake of CO2 (70% contribution) and bicarbonate, while at high CO2 , cells were restricted to CO2 . CA was highly and rapidly activated on transfer to low CO2 and played a key role because inhibition of external CA produced uptake kinetics similar to cells grown at high CO2 . The activities of phosphoenolpyruvate (PEP) carboxylase (PEPC) and the PEP-regenerating enzyme, pyruvate phosphate dikinase (PPDK), were lower in cells grown at low than at high CO2 . The ratios of PEPC and PPDK to ribulose bisphosphate carboxylase were substantially lower than 1, even at low CO2 . Our data suggest that the kinetic properties of this species results from a biophysical CCM and not from C4 type metabolism.