Marine diatoms, the major primary producer in ocean environment, are known to take up both CO(2) and HCO(3)(-) in seawater and efficiently concentrate them intracellularly, which enable diatom cells to perform high-affinity photosynthesis under limiting CO(2). However, mechanisms so far proposed for the inorganic carbon acquisition in marine diatoms are significantly diverse despite that physiological studies on this aspect have been done with only limited number of species. There are two major hypotheses about this; that is, they take up and concentrate both CO(2) and HCO(3)(-) as inorganic forms, and efficiently supply CO(2) to Rubisco by an aid of carbonic anhydrases (biophysical CO(2)-concentrating mechanism: CCM); and as the other hypothesis, biochemical conversion of HCO(3)(-) into C(4) compounds may play a major role to supply concentrated CO(2) to Rubisco. At moment however, physiological evidence for these hypotheses were not related well to molecular level evidence. In this study, recent progresses in molecular studies on diatom-carbon-metabolism genes were related to the physiological aspects of carbon acquisition. Furthermore, we discussed the mechanisms regulating CO(2) acquisition systems in response to changes in pCO(2). Recent findings about the participation of cAMP in the signaling pathway of CO(2) concentration strongly suggested the occurrences of mammalian-type-signaling pathways in diatoms to respond to changes in pCO(2). In fact, there were considerable numbers of putative adenylyl cyclases, which may take part in the processes of CO(2) signal capturing.