Inorganic carbon limitation, exofacial carbonic anhydrase activity, and plasma membrane redox activity in marine phytoplankton species

In the marine phytoplankton species tested, a possible link between exofacial ferricyanide reduction at the plasma membrane of intact cells, inorganic carbon status of the cells, and extracellular carbonic anhydrase (CA) activity is proposed. In species with no extracellular CA activity under carbon-limited or carbon-replete conditions, barely detectable ferricyanide reduction was observed. Species in which extracellular CA was only detected under carbon-limited conditions showed high rates of exofacial ferricyanide reduction, as shown previously for Skeletonema costatum. Immunological analysis has demonstrated that the CA protein was present in both carbon-limited and carbon-replete cells, even though the CA activity could only be detected when inorganic carbon was limiting. Incubation of the inactive extracellular CA protein from carbon-replete cells with DTT caused activation of the enzyme. It is proposed that CA limitation in the light promotes proton extrusion and increased plasma membrane redox activity, which result in the protonation and activation of the extracellular CA.