CO2 sensing at ocean surface mediated by cAMP in a marine diatom

Marine diatoms are known to be responsible for about a quarter of global primary production and their photosynthesis is sustained by inorganic carbon-concentrating mechanisms and/or C-4 metabolism. Activities of the inorganic carbon-concentrating mechanism are attenuated under enriched [CO2]; however, impacts of this factor on primary productivity and the molecular mechanisms of CO 2 responses in marine diatoms are unknown. In this study, transgenic cells were generated of the marine diatom Phaeodactylum tricornutum by the introduction of a beta-glucuronidase reporter gene under the control of an intrinsic CO2-responsive promoter, which is the sequence between 280 to 161 relative to the transcription start site of a chloroplastic-carbonic anhydrase gene, ptca1, obtained from P. tricornutum. The activity of the ptca1 promoter was effectively repressed in air-level CO2 by treating cells with a 1.0 mM cAMP analog, dibutyryl cAMP, or a cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. Deletion of the intrinsic cAMP-response element from the ptca1 promoter caused a lack of repression of the reporter gene uidA, even under elevated [CO2] and a null phenotype to the strong repressive effects of dibutyryl cAMP and 3-isobutyl-1-methylxanthine on the ptca1 promoter. Deletion of the cAMP-response element was also shown to cause derepression of the uidA reporter gene in the dark. These results indicate that the cytosolic cAMP level increases under elevated [CO2] and represses the ptca1 promoter. This strongly suggests the participation of cAMP metabolism, presumably at the cytosolic level, in controlling CO2-acquisition systems under elevated [CO2] at the ocean surface in a marine diatom.