Carbon dioxide-concentrating mechanism and the development of extracellular carbonic anhydrase in the marine picoeukaryote Micromonas pusilla.

A range of marine photosynthetic picoeukaryote phytoplankton species grown in culture were screened for the presence of extracellular carbonic anhydrase (CA ), a key enzyme in inorganic carbon acquisition under carbon- limiting conditions in some larger marine phytoplankton species. Of the species tested, extracellular carbonic anhydrase was detected only in Micromonas pusilla Butcher. The rapid, light-dependent development of CA when cells were transferred from carbon-replete to carbon-limiting conditions was regulated by the available free- CO concentration and not by total dissolved inorganic carbon. Kinetic studies provided support for a CO - concentrating mechanism in that the K [CO ] (i.e. the CO concentration required for the half-maximal rate of photosynthesis) was substantially lower than the K [CO ] of Rubisco from related taxa, whilst the intracellular carbon pool was at least seven fold greater than the extracellular DIC concentration, for extracellular DIC values ⩽1.0 mm. It is proposed that when the flux of CO into the cell is insufficient to support the photosynthetic rate at an optimum photon irradiance, the development of CA increases the availability of CO at the plasma membrane. This ensures rapid acclimation to environmental change and provides an explanation for the central role of M. pusilla as a carbon sink in oligotrophic environments.