There is currently only one known biological role for cadmium, which is toxic in excess to animals, plants, and microbes. Carbonic anhydrases, which catalyze the interconversion of CO2 and bicarbonate, are critical enzymes in carbon acquisition of aquatic phototrophs, and they are, accordingly, important for productivity (Cannon et al., 2010). Zn ion is the usual catalyst in carbonic anhydrases. In a Zn-deficient environment, the usual enzyme in the diatom Thalassiosira weissflogii (TWCA1) is replaced by a Cd-containing isoform, CDCA1 (Lane and Morel, 2000). This enzyme is found in other diatoms and is stimulated by Cd availability.