Abstract
Marine diatoms are thought to contribute at least 25% of the global carbon fixation. Diatoms have been shown to possess the inorganic carbon-concentrating mechanism (CCM) that facilitates an ample flux of CO2 to photosynthesis under CO2-limited environment. The Diatom CCM is regulated primarily by [CO2] but not other Ci species in the bulk medium and elevated [CO2] has strong repressive effect on the high-affinity photosynthesis. However, detailed molecular mechanisms of CO2 sensing and CCM regulation in marine diatoms are not known. We show evidences that cAMP functions as a major signaling molecule for the changes in the ambient [CO2] and light in the marine diatom Phaeodactylum tricornutum. Increment of cAMP level in the cytosol appears to operate to repress chloroplastic carbonic anhydrase gene, ptca1, which is known to be a CO2-responsive gene, under elevated [CO2] condition via functions of a cAMP responsive element in the promoter region of ptca1.
