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 exofaci al ferricyanide reduction at the plasma membrane of intact cells, inorganic carbon status of the cells, and extracellular carbonic anhydrase (CA) acti vity is proposed. In species with no extracellular CA activity under carbon -limited or carbon-replete conditions, barely detectable ferricyanide reduc tion was observed. Species in which extracellular CA was only detected unde r carbon-limited conditions showed high rates of exofacial ferricyanide red uction, as shown previously for Skeletonema costatum. Immunological analysi s has demonstrated that the CA protein was present in both carbon-limited a nd carbon-replete cells, even though the CA activity could only be detected when inorganic carbon was limiting. Incubation of the inactive extracellul ar CA protein from carbon-replete cells with DTT caused activation of the e nzyme. It is proposed that CA limitation in the light promotes proton extru sion and increased plasma membrane redox activity, which result in the prot onation and activation of the extracellular CA.