Interactions between phytoplankton and trace metals in the ocean

This review assesses the degree to which phytoplankton in the contemporary oceans interact with the essential trace metals in their chemical environment as exemplified by the cycling of iron, manganese, cobalt, nickel and zinc. The toxic element cadmium is also considered because of the extent to which it is taken up. The stage is set by a brief consideration of the overall geochemical controls on the composition of sea water and their implications for the milieu within which life evolved. The utilization of the elements within the cells is addressed with the consequent implications for optimizing the uptake of essential elements and controlling the ingress of potentially toxic elements. The impact of the change from an anoxic to an oxygenated atmosphere some 2 billion years ago on the availability of the basic building blocks for living systems is considered. The essential elements have to be delivered to the centres of synthesis in the appropriate ratios. The optimal tailoring of the flows of material into and out of the cell to meet the requirements for maintenance, growth and reproduction requires a carefully regulated internal economy. This internal economy sets the guidelines for the interaction of phytoplankton with the chemistry of their aqueous environment. The uptake mechanisms are explored using data derived from culture experiments indicating a significant degree of biological influence on the chemistry of the essential elements close to the cell surface (the near-field chemistry). The perspective is then widened to consider the implications of these processes for the relative concentrations of the elements and their distribution throughout the world's oceans (the far-field chemistry). Case histories are followed for iron, manganese, copper, zinc, cobalt and nickel reviewing (a) their distribution in the oceans, (b) their biological availability, (c) their uptake and impact upon primary production. This external economy is intimately related to the feedback between the organisms and their environment. The extent of recycling within the ocean system by the mutually dependent processes of photosynthesis and respiration provides a clear measure of the regulatory power of the biological system itself. This is analysed in the context of the Gaia hypothesis. Although the biological processes, to a large degree, control the availability and distribution of the essential trace metals in the oceans, the system does not appear to be optimized. For example cadmium, generally considered to be a non-essential element, is recycled more vigorously than any other element. Zinc in contrast appears to be rendered less accessible as the result of biological activity, and phytoplankton cells in the open ocean are straining at the limit of diffusive transport to obtain sufficient supplies.