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_ext), 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_ext when cells were transferred from carbon-replete to carbon-limiting conditions was regulated by the available free- CO₂ concentration and not by total dissolved inorganic carbon. Kinetic studies provided support for a CO₂- concentrating mechanism in that the K _0.5CO₂] (i.e. the CO₂ concentration required for the half-maximal rate of photosynthesis) was substantially lower than the K _mCO₂] 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 m m .
It is proposed that when the flux of CO₂ into the cell is insufficient to support the photosynthetic rate at an optimum photon irradiance, the development of CA_ext increases the availability of CO₂ 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.