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

Summary Diatoms are widespread in aquatic ecosystems where they may be limited by the supply of inorganic carbon. Their carbon dioxide‐concentrating mechanisms ( CCM s) involving transporters and carbonic anhydrases ( CA s) are well known, but the contribution of a biochemical CCM involving C 4 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 CO 2 , using a range of approaches. At low CO 2 , cells possessed a CCM based on active uptake of CO 2 (70% contribution) and bicarbonate, while at high CO 2 , cells were restricted to CO 2 . CA was highly and rapidly activated on transfer to low CO 2 and played a key role because inhibition of external CA produced uptake kinetics similar to cells grown at high CO 2 . 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 CO 2 . The ratios of PEPC and PPDK to ribulose bisphosphate carboxylase were substantially lower than 1, even at low CO 2 . Our data suggest that the kinetic properties of this species results from a biophysical CCM and not from C 4 type metabolism.