Bacterial reduction of copper-contaminated ferric oxide: copper toxicity and the interaction between fermentative and...

Fe(III) oxide is an important heavy-metal sink, and bacteria are responsible for much of the Fe(III) reduced in nonsulfidogenic aquatic environments, yet factors governing the bacterial reduction of heavy metal-contaminated iron oxide are largely unknown. In this study with a stabilized bacterial consortium enriched from metal-contaminated sediments, we demonstrate that Cu toxicity impedes anaerobic carbon oxidation and bacterial reduction of hydrous ferric oxide (HFO). In the enrichment culture, a Clostridium sp. fermented lactate to propionate and acetate and Fe(III) reducers coupled acetate oxidation to HFO reduction. Increasing the amount of Cu in the culture medium significantly extended the time before Fe(III) reduction occurred and decreased the reduction rate, but did not affect the extent of HFO reduction. The Clostridium had a higher Cu-complexation capacity than the Fe(III) reducer Shewanella alga. Iron reduction was inhibited until almost all of the dissolved Cu was removed from solution and occurred two to seven times faster if the sediment enrichment culture was fed lactate instead of acetate. Our findings suggest that fermentative bacteria play a role in ameliorating heavy metal toxicity to iron-reducing bacteria. Fermenters may therefore enhance metal release in sediments by facilitating the bacterial reductive dissolution of heavy metal-contaminated HFO.