Relation between plasma membrane redox activity and extracellular carbonic anhydrase activity in marine phytoplankton

Exofacial ferricyanide reduction at the plasma membrane of intact cells, and the link between plasma membrane redox activity, inorganic carbon status of the cells and extracellular carbonic anhydrase (CAext) activity were assayed using 10 marine phytoplankton species. In species Chaetocceros compressus, Cocolithus pelagicus and Gephyrocapsa ocetanica with no extracellular CA activity under carbon-limited or carbon-replete conditions, barely detectable ferricyanide reduction was observed. Species Skeletonema costatum, Melosira sp., Thalassiosira rotula, Thalassiosira weisflogi and Pleurochrysis carterae in which extracellular CA activity was only detected under carbon-limited conditions showed high rates of exofacial ferricyanide reduction. Western blotting and immunolocalization showed the presence of enzyme protein under carbon-limited and replete conditions at the cell surface, even though the CA activity could only detected when inorganic carbon was limiting, which suggests that the development of extracellular CA in response to carbon limitation is an activation of a preexisting protein rather than de novo synthesis. The results suggest that inorganic carbon limitation in the light increases plasma membrane redox activity and promotes proton extrusion, which result in the protonation and activation of the extracellular CA.