The oceanic distribution of cadmium follows closely that of major algal nutrients such as phosphate. The reasons for this "nutrient- like" distribution are unclear, however, because cadmium is not generally believed to have a biological function. Herein. we pro- vide evidence of a biological role for Cd in the marine diatom Thalassiosira weissflogii under conditions of low zinc, typical of the marine environment. Addition of Cd to Zn-limited cultures en- hances the growth rate of T. weissflogii particularly at low pCO_2. This increase in growth rate is reflected in increased levels of cellular carbonic anhydrase (CA) activity, although the levels of TWCA1, the major intracellular Zn-requiring isoform of CA in T. weissflogii, remain low. 109~Cd label comigrates with a protein band that shows CA activity and is distinct from TWCA1 on native PAGE of radiolabeled T. weissflogii cell lysates. The levels of the Cd Protein are modulated by CO_2 in a manner that is consistent with a role for this enzyme in carbon acquisition. Purification of the CA-active fraction leads to the isolation of a Cd-containing protein 0f 43 kDa. It is now clear that T weissflogii expresses a Cd-specific CA, which, particularly under conditions of Zn limitation, can replace the Zn enzyme TWCA1 in its carbon-concentrating mechanism.
展开▼
