REGULATION OF DISSOLVED INORGANIC CARBON TRANSPORT IN GREEN-ALGAE

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 s tudies, however, have indicated that there may be a light-independent pathway of signal transduction in green algae that may regulate the ex pression 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, indepen dent of pH, whereas other inorganic carbon species, which might induce or repress DIC transport expression, were shown to change markedly wi th the pH of the medium. Furthermore, neither changes in the CO2 conce ntration around ribulose bisphosphate carboxylase-oxygenase (Rubisco) nor light were shown to be critical factors in regulating CCM expressi on in this al,aa. CO2-insensitive mutants of Chlorella ellipsoidea wer e recently isolated in which DIC transport operates constitutively at maximum activity. These results strongly suggest that a direct CO2 sen sing mechanism may operate at the cell surface in Chlorella ellipsoide a and that this mechanism may trigger the repression of the expression of DIC transport in response to high CO2. Some supportive evidence fo r this hypothesis has also been obtained in other green algae, Chlamyd omonas reinhardtii and Chlorella kessleri. The possibility of the occu rrence of a direct sensing mechanism for CO2, its implications, and po ssible coexistence of other regulatory systems for CCM expression are discussed.