Cadmium in marine phytoplankton

The distribution of cadmium in the ocean is very similar to that of major nutrients suggesting that it may be taken up by marine phytoplankton at the surface and remineralized at depth. This interpretation is supported by recent data on Cd isotope distribution showing an increase in the¹¹²Cd/ ¹¹⁰Cd ratio in Cd-depleted surface water. While at high concentrations, Cd is toxic to phytoplankton as it is to many organisms, at relatively low concentrations, Cd can enhance the growth of a number of phytoplankton species under zinc limitation. Kinetic studies suggest that Cd is taken up through either the Mn or the Zn transport system, depending on the ambient concentrations of these metals. In addition to inorganic Cd complexes (including the free Cd²⁺ ion), Cd complexes with relatively weak organic ligands may also be bioavailable. Cd is very effective to induce the production of phyto-chelatin and other thiols in phytoplankton, probably as a detoxification mechanism as well as a control of Cd homeostasis in cells. The only known biological function of Cd is to serve as a metal cofactor in Cd-carbonic anhydrase (CDCA) in diatoms. The expression of CDCA is regulated by Cd and Zn availabilities and by the pCO₂/pH of the ambient seawater in cultured diatoms and natural assemblages. The conformation of CDCA active site is similar to that of β-CA and both Zn and Cd can be used as its metal cofactor and exchanged for each other. Understanding of the biological role of Cd in marine phytoplankton provides insights into the biogeochemical cycling of this element in the ocean.