Morel, F.M.M., 1986. Trace metals-phytoplankton interactions : an overview. In: P. Lasserre and J.M. Martin (eds), Biogeochemical Processes at the Land-Sea Boundary. Elsevier, Amsterdam. As shown early in the century, the role of trace metals as nutrients or toxicants to phytoplankton is mediated by the aqueous speciation of the elements. More recently free metal ion activities have been demonstrated to be the parameters determining such physiological effects. It is argued that this result implies no particular role for the free hydrated metal ions but represents simply the thermodynamic tendancy of the elements to react chemically, at pseudo-equilibrium, with cellular functional groups and hence to be bioactive. At present the focus is on disequilibrium processes, particularly the reduction and photoreduction of the metals Fe and Mn at cell surfaces. Such a process may make these essential elements more available to algae. It may also enhance the specificity of the uptake system, other metals which act as coordinative analogs of the essential ones being toxic by interfering with their transport or utilization. These physiological processes correspond to a general ecological view in which aquatic micro-organisms are simultaneously limited in stable systems by several essential elements and their toxic chemical analogs.
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