title: The nature of the CO2 -concentrating mechanisms in a marine diatom, Thalassiosira pseudonana.
abstract: 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.
authors.affiliation: Aix-Marseille Université CNRS, BIP UMR 7281, 31 Chemin Joseph Aiguier, Marseille Cedex 20, 13402, France.
authors.forename: Romain
authors.initials: R
authors.lastname: Clement
authors.affiliation: Aix-Marseille Université CNRS, BIP UMR 7281, 31 Chemin Joseph Aiguier, Marseille Cedex 20, 13402, France.
authors.forename: Laura
authors.initials: L
authors.lastname: Dimnet
authors.affiliation: Centre for Ecology & Hydrology, Lake Ecosystems Group, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, LA1 4AP, UK.
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Maberly
authors.affiliation: Aix-Marseille Université CNRS, BIP UMR 7281, 31 Chemin Joseph Aiguier, Marseille Cedex 20, 13402, France.
authors.forename: Brigitte
authors.initials: B
authors.lastname: Gontero
mesh_terms: D000086:Acetazolamide / Q000494:pharmacology; D059001:Aquatic Organisms / Q000187:drug effects / Q000254:growth & development / Q000378:metabolism*; D001639:Bicarbonates / Q000494:pharmacology; D002245:Carbon Dioxide / Q000378:metabolism* / Q000494:pharmacology; D002256:Carbonic Anhydrases / Q000378:metabolism; D017377:Diatoms / Q000187:drug effects / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010788:Photosynthesis / Q000187:drug effects; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism; D013997:Time Factors
chemical_list: D001639:Bicarbonates; D002245:Carbon Dioxide; D012273:Ribulose-Bisphosphate Carboxylase; D002256:Carbonic Anhydrases; D000086:Acetazolamide
keywords: Bicarbonate use; CO
  2; Thalassiosira pseudonana; carbon dioxide-concentrating mechanism (CCM); diatom; photosynthesis