The enzyme carbonic anhydrase plays a key role in the acquisition of inorganic carbon for photosynthesis in phytoplankton. A recent study reported the isolation of the cadmium carbonic anhydrase (CDCA), a CA that can use either Zn or Cd as its metal center from the marine diatom, Thalassiosira weissflogii. Using degenerate primers designed from the sequences of T. weissflogii and a putative sequence in the genome of T. pseudonana, CDCA was shown to be widespread in diatom species and in the environment. Analysis of the amino acid sequence of CDCA showed that the putative Cd binding site resembles that of beta-class carbonic anhydrases. Because the use of Cd in CDCA is the only known biological function of Cd and is thought to explain the nutrient-like concentration profile of Cd in the oceans, the expression of CDCA was explored in cultures of Thalassiosira weissflogii and in samples from the Equatorial Pacific and coastal New Jersey. CDCA1 expression in T. weissflogii was highly modulated by pH changes, and induced by the addition of Cd. In samples from the Equatorial Pacific, peptide sequences matching CDCA1 were found. In samples from New Jersey coastal water, a high level of CDCA expression was found inversely correlated with pCO₂. The presence of cadmium carbonic anhydrases in diatoms and in environments probably reflects the difficulty in acquiring inorganic carbon for photosynthesis. Diatoms may have become successful as the primary producers due to the ability to use Cd in carbon acquisition under conditions of very low CO₂.