Regeneration of trace metals from picoplankton by nanoflagellate grazing

Rapid regeneration of trace metals from the particulate to the dissolved phase (<0.2 <mu>m) was observed in the laboratory with a simplified microbial food web composed of mixotrophic chrysophycean nanoflagellates (Ochromonas danica) grazing on picocyanobacteria (Synechococcus leopoliensis) that had been previously exposed to the radionuclides Gd-153(III), Zn-65(II), Cd-109(II), and Cs-137(I). These trace metals were chosen to represent a range of surface reactivities with particles (Gd>Zn, Cd>>Cs). Grazing experiments and the appropriate nongrazing controls were carried out in batch cultures over 43-49 h in defined, inorganic freshwater medium; metal partitioning among the consumer, prey, and dissolved phases was determined by sequential filtration (3 mum, 0.2 mum) at timed intervals. Most of the trace metals consumed as radioactive prey were regenerated into the dissolved phase. Regenerated Gd, Zn: and Cd present in the dissolved phase were less available for resorption by plankton than were the same radionuclides added in inorganic form to fresh growth medium. Our results suggest that where this grazing activity exists, it will serve to increase trace metal residence times in the water column.