In vivo substitution of zinc by cobalt in carbonic anhydrase of a marine diatom

We examined the mechanism by which Co additions partially alleviate the growth limitation of marine diatoms in cultures with low Zn concentrations. Cultures of the coastal diatom Thalassiosira weissflogii were grown under low-Zn conditions and provided with low and high levels of Co. Enzyme assays and autoradiographs of electrophoresis gels of cellular proteins revealed that a large fraction of the Co is found in the main isoform of the enzyme carbonic anhydrase (CA), where most of the Zn is located. The affinity of CA for Co is lower than for Zn, and the Co-substituted enzyme is less active than the native Zn form. Comparisons of growth under different partial pressures of CO₂ show that Co can ameliorate the growth of carbon- limited cultures. We conclude that Co alleviates Zn limitation via direct substitution in CA, which is important for carbon acquisition.