Metal accumulation by heterotrophic marine bacterioplankton

Radioisotopes of six metals (⁵⁴Mn, ⁵⁵Fe, ⁶⁵Zn, ¹⁰⁹Cd, ¹³⁷Cs, and ²⁴¹Am) were used to assess the accumulation of these metals by five species of heterotrophic marine bacteria in laboratory cultures exposed to environmentally realistic metal concentrations in natural seawater. Typically, uptake proceeded rapidly for the first 24 h and slowed down over the following 72-96 h. At steady state, the bioconcentration of metals in cells was Fe > Am =* Mn = Cd > Zn > Cs. Concentration factors were 0.6-2.8 X 10* for Fe, 0.6-3.5 X 104 for Mn and Am, 0.24.8 X 104 for Cd, 0.5-1.6 X 103 for Zn, and 1.9 X 102 for Cs. Rinsing with oxalate did not result in significantly lower Fe concentrations, but rinsing the cells with ethylenediaminetetraacetic acid resulted in significantly lower concentration factors for Mn and Am in one species. Interspecific differences in metal uptake were small under the conditions tested and were proportional to surface : volume ratios of the cells. The Qm of Zn uptake in two bacterial species examined was about 1, suggesting passive uptake. The Q₁₀ of Mn uptake was in the range of 1.83.6, indicating active uptake and transport into the cells. Given typical bacterial biomass in surface waters, we calculate that < 1 % of most metals, but ~ 20% of Fe, should be associated with bacterial cells; they may serve as enriched sources of some metals for those organisms that consume them.