Characterization of Heavy Metal Resistance of Metal-Reducing Shewanella Isolates From Marine Sediments

This study focuses on heavy metal resistance of marine, benthic Mn(IV)- and Fe(III)-reducing bacteria and their potential to mobilize heavy metals from sedimentary phases, as hydrous ferric oxides (HFO) and Mn(IV)-oxides (δ -MnO ₂ ). One isolate was obtained from enrichments of metal-polluted sediment with δ -MnO ₂ (strain MB4, 99% similarity to S. marisflavi), and two strains were isolated from enrichments on HFO (strain FB18 and FS8, 98 and 97% 16S rRNA gene similarity to Shewanella collwelliana). The 16S rRNA sequences similar to isolates MB4 and FS8 were detected previously in DGGE profiles and clone libraries of the original sediment samples. Toxicity tests under aerobic conditions showed that the latter two ceased growth at 150 μ M Cu, but strain MB4 and reference strain S. oneidensis MR1 were more tolerant to copper; growth with 150 μ M Cu reached 56–58 ± 0.1% of maximal optical density, OD_max, in control cultures. Similar experiments conducted under anaerobic conditions with fumarate indicated no significant change in copper tolerance in strain MB4 (66 ± 3% OD_max at 150 μ M). Biphasic experiments with δ -MnO ₂ -reduction followed by use of fumarate, furthermore indicated that the presence of manganese oxides decreased bio-availability of copper through sorption processes, thereby alleviating the toxicity of copper to strain MB4 to some extent. Scanning electron microscopic images showed the initial amorphous Mn(IV)-oxides and newly formed, highly crystalline, lemon-shaped, particles making up the precipitate that remained after microbial reduction. Concomitant electron dispersive x-ray spectrometry confirmed presence of copper in the initial sample, yet detected no copper in the precipitate after microbial reduction, indicating that the Mn(IV)-reducing Shewanella strain MB4 mobilized copper adsorbed to δ -MnO ₂ .