Regulation of dissolved inorganic carbon transport in green algae

Abstract

The regulation of the expression of the inorganic carbon concentrating mechanism (CCM) in aquatic photoautotrophs, particularly green algae, has been thought to require light and active photosynthesis. Recent studies, however, have indicated that there may be a light-independent pathway of signal transduction in green algae that may regulate the expression of CCMs in response to changes in ambient dissolved inorganic carbon (DIC) concentration. In the green alga, Chlorella ellipsoidea, changes in the rate of transport of both CO2 and HCO3- were shown to occur in response to the CO2 concentration in the bulk medium, independent of pH, whereas other inorganic carbon species, which might induce or repress DIC transport expression, were shown to change markedly with the pH of the medium. Furthermore, neither changes in the CO2 concentration around ribulose bisphosphate carboxylase-oxygenase (Rubisco) nor light were shown to be critical factors in regulating CCM expression in this alga. CO2-insensitive mutants of Chlorella ellipsoidea were recently isolated in which DIC transport operates constitutively at maximum activity. These results strongly suggest that a direct CO2 sensing mechanism may operate at the cell surface in Chlorella ellipsoidea and that this mechanism may trigger the repression of the expression of DIC transport in response to high CO2. Some supportive evidence for this hypothesis has also been obtained in other green algae, Chlamydomonas reinhardtii and Chlorella kessleri. The possibility of the occurrence of a direct sensing mechanism for CO2, its implications, and possible coexistence of other regulatory systems for CCM expression are discussed.Key words: green algae, Chlorella, signal transduction, CO2-insensitive mutants, CO2-sensing mechanism.