The bioconcentration of metals in marine bacterioplankton has largely gone unexplored, even though bacterial cells represent the largest pool of living carbon in the ocean. Radioisotopes of several metals were used to investigate the magnitude of metal accumulation with different cultured bacteria, to determine the potential of metal transfer from bacteria to protistan grazers through trophic interactions, and to address the role of viral lysis on the release and bioavailability of bacterially-bound metals.

In order to quantify the accumulation of metals by marine bacteria, the radioisotopes ⁵⁴Mn, ⁵⁵Fe, ⁶⁵Zn, ¹⁰⁹Cd, ¹³⁷Cs, and ²⁴¹Am were used in laboratory experiments with 5 bacterial species. The bacteria were exposed to environmentally realistic metal concentrations in natural, unamended seawater. Interspecific differences in metal uptake tended to be small and were proportional to surface:volume ratios of the cells. The Q¹⁰ of Zn uptake in the two bacterial species examined was about 1, suggesting passive uptake of this metal. Given typical bacterial biomass in surface waters, I calculate that <1% of most metals, but ∼20% of Fe, should be associated with bacterial cells; these cells may serve as enriched sources of some metals for those organisms that consume them.

The radioisotopes ⁵⁵Fe, ⁶⁵Zn, ¹⁰⁹ Cd, and ²⁴¹Am were used to assess the release of bacterially-bound metals and their bioavailability from virally lysed and unlysed cells of another bacterial species and to diatoms in experiments conducted in natural seawater. The data indicate that viral lysis can enhance the release of at least some metals from bacterial cells to ambient seawater, but no consistent influence of viral lysis was observed on the relative bioavailability of released metals to bacterioplankton or phytoplankton.

Marine bacteria may serve as an enriched source of essential and non-essential metals for organisms that eat them. I evaluated this trophic transfer by measuring the accumulation of Fe, Zn, and Am by a planktonic ciliate isolate ( Uronema sp.) following the ingestion of radiolabeled bacteria ( Vibrio natriegens) in laboratory experiments. V. natriegens cells were allowed to accumulate ⁵⁵Fe, ⁶⁵ Zn, and ²⁴¹Am from seawater for 5 d and were subsequently fed to the ciliates. Uptake and depuration of the metals was monitored in the ciliates over time. The results suggest that bacteria present an alternate source to phytoplankton for some metals, especially Fe, to be introduced into metazoan food webs.

The accumulation of ⁵⁵Fe by three species of cultured heterotrophic bacteria (Roseobacter litoralis, Vibrio natriegens, and Halomonas aquamarina) was assessed in laboratory experiments using natural seawater from an oligotrophic ocean region by adding environmentally realistic levels of Fe in form of the radioisotope. An oxalate rinse was used to determine partitioning of extracellular and intracellular fractions of Fe. Fe accumulation proceeded rapidly over the first 10 h of the experiments, after which it approached a steady-state for all three bacterial species. Volume concentration factors varied from 1.3 x 10⁶ to 7.4 x 10⁷, with the highest values obtained for the bacterial species characterized by the smallest cell size and highest relative surface area. The maximum fraction of Fe removed using oxalate was 11%, indicating efficient Fe transport into the cells. Results are compared to results of Fe accumulation by the same bacterial species grown in more eutrophic natural seawater and to results on Fe uptake by naturally occurring picoplankton cells in the Equatorial Pacific Ocean. (Abstract shortened by UMI.)