Our previous study showed that three CO2/cAMP responsive elements (CCRE) 1, CCRE2, and CCRE3 in the promoter of the chloroplastic β-carbonic anhydrase 1 gene in the marine diatom Phaeodactylum tricornutum (Pptca1) were critical for the cAMP-mediated transcriptional response to ambient CO2 concentration. Pptca1 was activated under CO2 limitation, but the absence of light partially disabled this low-CO2-triggered transcriptional activation. This suppression effect disappeared when CCRE2 or two of three CCREs were replaced with a NotI restriction site, strongly suggesting that light signal cross-talks with CO2 on the cAMP-signal transduction pathway which targets CCREs. The paralogous chloroplastic carbonic anhydrase gene, ptca2 was also CO2/cAMP responsive. The upstream truncation assay of the ptca2 promoter (Pptca2) revealed a short sequence of -367 to -333 relative to the transcription-start site to be a critical regulatory region for the CO2 and light responses. This core-regulatory region comprises a CCRE1 and two CCRE2 sequences. Further detailed analysis of Pptca2 clearly indicates that two CCRE2s are the Cis-element governing the CO2/light response of Pptca2. The transcriptional activation of two Pptcas in CO2 limitation were evident under the illumination with photosynthetically active light wavelength, and an artificial electron acceptor from the reduction side of PSI efficiently inhibited Pptcas activation, while neither inhibition of the linear electron transport from PSII to PSI nor inhibition of ATP synthesis showed effect on the promoter activity, strongly suggesting a specific involvement of redox level of the stromal side of the PSI in the CO2/light cross-talk.