Regulation of the expressions of HCO3- uptake and intracellular carbonic anhydrase in response to CO2 concentration in the marine diatom Phaeodactylum sp.

The marine diatom, Phaeodactylum tricornutum Bohlin, is probably one of the most extensively studied marine alga with respect to carbon acquisition and assimilation mechanisms. However, quantitative analyses of HCO3utilization and the detailed process of acclimation of cells from high CO2 to limited CO2 are yet to be done extensively. Suitable molecular markers for this acclimation process are not established, either. Recently, it became clear that the rate of CO2 formation in artificial seawater is about eight times slower than that in freshwater, and thatP. tricornutum cells utilize HCO3 quite efficiently. Despite their great capacity to take up HCO3, the signal controlling photosynthetic affinity for dissolved inorganic carbon has been shown to be CO2 in the medium. Furthermore, light seems to be required for this process. Internal carbonic anhydrase (CA) activity has been shown to be crucial for high-affinity photosynthesis in a number of algae, including marine diatoms. Internal β-type CA, which has been isolated in one strain of P. tricornutum, was clearly shown to be a low-CO2 inducible enzyme. This review paper additionally includes data showing that this CA occurs generally in P. tricornutum species.