The Use of Biologically Produced Ferrihydrite for the Isolation of Novel Iron-Reducing Bacteria

Ferric iron was produced anaerobically from ferrous iron through the metabolic activity of recently described ferrous iron-oxidizing, nitrate-reducing bacteria. It was identified as poorly crystallized 2-line ferrihydrite with a particle size of 1–2 nm. This biologically produced ferrihydrite was shown to be a suitable electron acceptor for dissimilatory ferric iron-reducing bacteria in freshwater enrichment cultures, and was completely reduced to the ferrous state; no magnetite formation occurred. Geobacter metallireducens was also able to completely reduce the biologically produced ferrihydrite. These results indicate the possibility of an anaerobic, microbial cycling of iron. Using the biologically produced ferric iron, two isolates of obligately anaerobic, dissimilatory ferric iron-reducing bacteria, strains Dfrl and Dfr2, were obtained from freshwater enrichment cultures. Analysis of 16S rRNA gene sequences revealed an affiliation with the Geobacter cluster within the family Geobacteraceae. The sequence similarity between strains Dfrl and Dfr2 is 92.5%. The closest relative of strain Dfrl is Geobacter sulfurreducens with 92.9%, and of strain Dfr2 Geobacter chapelleii with 93.7% sequence similarity. In addition, strains Dfrl and Dfr2 are both able to grow by dissimilatory reduction of Mn(IV), S°, and fumarate. Furthermore, strain Dfr2 is able to reduce akaganeite (β-FeOOH), a more crystallized type of ferric iron oxide.