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

Recent work demonstrating the ecological importance of trace metals in aqua tic plankton communities has stimulated interest in biological metal cyclin g and transfer in natural waters. Like major nutrients, sources of metals t o phytoplankton can include both exogenous (new) and endogenous (recycled) supplies. Indirect evidence suggests that the subcellular location of metal s in plankton cells-surf ace-adsorb ed versus intracellular-is a primary fa ctor controlling their biogeochemical fate after processing by zooplankton grazers. However, there are no methods presently available to directly meas ure the turnover of intra- and extracellular metal pools, or the uptake of new and regenerated metal sources by phytoplankton. We present here new tec hniques that use dual radioisotopes of the same trace element to make the f irst direct measurements of these biogeochemically important processes in m arine and freshwater plankton communities. Since our methods depend on quan titative removal of surface-bound isotopes from plankton cells, we first co mpared the efficiency with which a variety of published surface-wash techni ques remove adsorbed trace metals from phytoplankton cells. A widely used F e surface-wash method that employs a Ti(III)/citrate/EDTA reagent was the m ost effective technique examined for removing a variety of extracellular tr ace metals, including Co, Cd, Zn and Mn. We then demonstrated the use of th e dual radiolabels Fe-59 and Fe-55 to compare new and regenerated Fe uptake by marine phytoplankton in laboratory and field experiments. Results indic ated that new (dissolved) Fe was utilized more readily by phytoplankton tha n Fe regenerated from the intracellular pools of other plankton, but transf er of intracellular Fe was substantial. This dual isotope pair was also use d to examine the fate of diatom intracellular Fe-55 and surface-adsorbed Fe -59 after grazing by copepods. Similar freshwater experiments examined the cycling of diatom intracellular Co-60 and extracellular Co-57 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 o f difficult-to-resolve portions of the biogeochemical cycles of many biolog ically important trace elements, including Fe, Co, Mn, Ni, Ag, Cd and Se.