A recent report of a novel carbonic anhydrase (CDCA1) with Cd as its metal centre in the coastal diatom Thalassiosira weissflogii has led us to search for the occurrence of this Cd enzyme (CDCA) in other marine phytoplankton and in the environment. Using degenerate primers designed from the published sequences from T. weissflogii and a putative sequence in the genome of Thalassiosira pseudonana, we show that CDCA is widespread in diatom species and ubiquitous in the environment. All detected genes share more than 64% amino acid identity with the CDCA of T. pseudonana. Analysis of the amino acid sequence of CDCA shows that the putative Cd binding site resembles that of beta-class carbonic anhydrases (CAs). The prevalence of CAs in diatoms that presumably contain Cd at their active site probably reflects the very low concentration of Zn in the marine environment and the difficulty in acquiring inorganic carbon for photosynthesis. The cdca primers developed in this study should be useful for detecting cdca genes in the field, and studying the conditions under which they are expressed.