Biogenic magnetite production under low pH conditions.

The biogenic production of magnetite is an interesting phenomenon, which has unresolved mechanisms of production in the geological record. Magnetite is of interest for various industrial applications and also an agent relevant for in situ contaminant destruction in contaminated groundwater aquifers. Certain ferric iron-reducing microorganisms form magnetite from goethite or ferrihydrite under circumneutral or alkaline pH conditions, but magnetite formation is not favorable at low pH. Little is known about microbial magnetite formation at acidic pH. We obtained a microbial enrichment culture from tropical forest soil that reduces ferrihydrite at pH 5 with acetate as electron donor. Magnetic susceptibility measurements of the precipitate by this enrichment culture suggested that the precipitate was entirely magnetite. X-ray diffraction measurements determined that the precipitate consisted of two mineral phases, magnetite and vivianite. Preliminary data suggest that magnetite was the dominant precipitate, and a Rietveld refinement is being performed to determine the percentage of each mineral phase. Vivianite formed in abiotic control incubations, but ferrihydrite reduction and magnetite formation required live cells, suggesting that the only mineral precipitate made by the microbial community is magnetite. 16S rRNA gene clone libraries did not detect common ferric iron-reducing bacteria (e.g., Geobacter, Shewanella, Anaeromyxobacter), and detailed sequencing efforts are underway to profile the microbial community producing magnetite at pH 5.