Cobalt is a trace metal which behaves as a nutrient in some marine phytoplankton species. Although cobalt concentrations in the open ocean are typically at least an order of magnitude less than those of zinc, in some regions of the ocean where zinc is extremely depleted, cobalt concentrations approach those of zinc.

Laboratory cultures of marine phytoplankton demonstrate that cobalt additions to culture media can alleviate zinc limitation of growth. This substitution can restore growth to near maximum rates in zinc-limited oceanic species, but is less effective in coastal phytoplankton. The effectiveness of cobalt substitution is correlated to the concentrations of zinc at which species become zinc-limited: the lower the zinc concentration necessary for growth, the more effective the cobalt substitution. Analysis of cellular metal contents show that oceanic species have relatively high cobalt to zinc ratios, and thus a lesser preference for zinc over cobalt than do coastal species.

One mechanism by which the substitution of cobalt for zinc occurs in the coastal diatom THALASSIOSIRA WEISSFLOGII is by direct metal substitution in a soluble form of the zinc enzyme carbonic anhydrase, which is used in acquiring inorganic carbon. A similar soluble carbonic anhydrase was found in an oceanic diatom as well. The function of both zinc and cobalt in carbonic anhydrase of T. WEISSFLOGII is apparent from the alleviation of carbon limitation in cultures under low partial pressures of carbon dioxide (CO2) by addition of either zinc or cobalt to the culture medium.

Assays of soluble proteins in other phytoplankton indicate that another function in which cobalt may substitute for zinc is in the enzyme superoxide dismutase. In contrast, cobalt does not appear to substitute for zinc in RNA and DNA polymerases. Despite the lack of direct substitution of cobalt for zinc in the nucleic acid fraction of zinc-limited cells, cobalt additions cause redistribution of cellular zinc from the insoluble to the soluble and nucleic acid fractions of a cell. This effect, as well as the presence of a large part of total cellular zinc in the insoluble fraction of many phytoplankton, suggest that a major role of cobalt substitution for zinc occurs in proteins found in membranes. This insoluble protein may be a form of carbonic anhydrase, as a positive response to metal addition under low CO2 is found some phytoplankton species when zinc and cobalt are added, despite assays showing no carbonic anhydrase activity in the soluble fraction of these algae. Despite the fact that cobalt substitution for zinc may occur chiefly in membrane-bound proteins, nearly half the cellular cobalt is always found in an unknown low molecular weight compound. This compound may be a phytochelatin complex which acts as an internal cobalt buffer.

The effect of zinc and cobalt in alleviating carbon limitation may be reflected in the distributions of these metals in the surface oceans: the concentration of cobalt in surface waters follows nutrient-like vertical profiles where zinc is very depleted and surface temperature is high; the decreased solubility of CO2 warm waters may require phytoplankton in these regions to produce carbonic anhydrase to enhance inorganic carbon uptake and thus utilize cobalt in the absence of sufficient zinc.