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Recent work demonstrating the ecological importance of trace metals in aquatic plankton communities has stimulated interest in biological metal cycling and transfer in natural waters. Like major nutrients, sources of metals to phytoplankton can include both exogenous (new) and endogenous (recycled) supplies. Indirect evidence suggests that the subcellular location of metals in plankton cells - surface-adsorbed versus intracellular - is a primary factor controlling their biogeochemical fate after processing by zooplankton grazers. However, there are no methods presently available to directly measure the turnover of intra- and extracellular metal pools, or the uptake of new and regenerated metal sources by phytoplankton. We present here new techniques that use dual radioisotopes of the same trace element to make the first direct measurements of these biogeochemically important processes in marine and freshwater plankton communities. Since our methods depend on quantitative removal of surface-bound isotopes from plankton cells, we first compared the efficiency with Which a variety of published surface-wash techniques remove adsorbed trace metals from phytoplankton cells. A widely used Fe surface-wash method that employs a Ti(III)/citrate/EDTA reagent was the most effective technique examined for removing a variety of extracellular trace metals, including Co, Cd, Zn and Mn. We then demonstrated the use of the dual radiolabels 59Fe and 55Fe to compare new and regenerated Fe uptake by marine phytoplankton in laboratory and field experiments. Results indicated that new (dissolved) Fe was utilized more readily by phytoplankton than Fe regenerated from the intracellular pools of other plankton, but transfer of intracellular Fe was substantial. This dual isotope pair was also used to examine the fate of diatom intracellular 55Fe and surface-adsorbed 59Fe after grazing by copepods. Similar freshwater experiments examined the cycling of diatom intracellular 60Co and extracellular 57Co after grazing by cladocerans. Both grazing experiments showed that intracellular metals are most efficiently assimilated by zooplankton, while extracellular Fe and Co are preferentially regenerated to dissolved forms during grazing. Dual-labeling techniques promise to allow direct, unambiguous characterization of difficult-to-resolve portions of the biogeochemical cycles of many biologically important trace elements, including Fe, Co, Mn, Ni, Ag, Cd and Se

Dual-labeling techniques for trace metal biogeochemical investigations in aquatic plankton communities