Variation in the occurrence of external carbonic anhydrase among strains of the marine diatom Phaeodactylum tricornutum (Bacillariophyceae)

Eleven different strains of Phaeodactylum tricornutum Bohlin were obtained from three culture collections and were examined for the presence of external and internal carbonic anhydrase (CA). Cells of all strains, grown in standing culture at alkaline pH and low dissolved inorganic carbon had internal CA, but only eight were found to have external CA. External CA activity was reduced when cultures were bubbled with air and was completely repressed when they were grown on 5% CO2. Expression of external CA activity appears to be regulated by CO2 concentration in the growth medium, but within one species, there appears to be a variation in occurrence of external CA and consequently in the mode of inorganic carbon acquisition.