The regulation of photosynthetic rate and activation of extracellular carbonic anhydrase under CO₂‐limiting conditions in the marine diatom <i>Skeletonema costatum</i>

Abstract

In the marine diatom Skeletonema costatum , carbonic anhydrase activity exterior to the plasma membrane (CA ext ) was detected only when the available CO 2 concentration was less than 5·0 mmol m –3 , this activity being unaffected by the total dissolved inorganic carbon concentration. The inhibition of CA ext by dextran bound sulphonamide (DBS) demonstrated the key role of this enzyme in maintaining photosynthetic rate under CO 2 ‐limited conditions. Treatment with trypsin followed by affinity chromatography on p‐aminomethylbenzene‐sulphamide agarose and subsequent SDS‐PAGE analysis revealed a polypeptide from carbon‐replete cells of identical molecular mass to the CA ext released by trypsin from CO 2 ‐limited cells. Redox activity in the plasma membrane of intact cells was measured by following the light‐dependent reduction of ferricyanide or NADP, the greatest activity being shown by CO 2 ‐limited cells. Overall the results suggest that high rates of redox activity under conditions of CO 2 ‐limitation were required for the activation of CA ext .