Trace Metal Interactions with Marine Phytoplankton

Abstract

Large spatial and temporal differences in both trace metal concentrations and chemical speciation in the sea have led to wide variations in biological availability of metals and their effects on phytoplankton. Trace metals are usually taken up by algae via the formation of coordination complexes with specialized transport ligands in their outer membranes, and metal uptake is determined by the interplay between redox, complexation, or oxide dissolution reactions of metals in seawater and ligand-exchange reactions at these sites. Some metals, such as copper and zinc, are heavily chelated by organic ligands in seawater, and their biological availability is determined by the concentrations of free metal ions or of kinetically labile inorganic species (free ions plus inorganic complexes). Once inside cells, trace metals influence metabolism primarily as a consequence of the role of many of these metals (Fe, Mn, Zn, Cu, Co, Mo, Ni) as essential cofactors in metalloenzymes. Trace metals may also inhibit metabolism when they bind to the wrong metabolic sites including those normally occupied by other essential nutrient metal ions. The interactions of metals at the biomolecular level determine their overall effect on cellular rate processes, such as growth, and on the productivity of the phytoplankton community as a whole. Evidence exists for limitation of algal productivity by soluble inorganic iron concentrations and manganese to cupric ion ratios in certain regions of the sea. However, wide differences exist among algal species in their requirements for nutrient metals or in their sensitivity to toxic metals, and, thus, the predominant effect of trace metals may well be on species composition of phytoplankton communities. The effects of metals on phytoplankton are reciprocal, and the phytoplankton community itself has a profound influence on the concentrations and chemical speciation of trace metals in seawater, providing important biological feedback.