Virus and Bacterial Community Responses to Anaerobic Fe(III) Bioreduction Vary with Down-Gradient Distance from the Point of Electron-Donor Addition.

In situ anaerobic bioremediation (AB) has been widely used to degrade or transform subsurface organic and inorganic contaminants. During anaerobic bioremediation, microorganisms are stimulated to degrade/transform contaminants under anaerobic subsurface conditions by introducing a variety of organic substrates that generally stimulate Fe(III)-oxide reducing bacteria (e.g. Geobacter species). In the course of evaluating the secondary impacts of AB on chemical, physical, and biological properties of porous media, microbial communities and viral assemblages in iron-rich soil aggregates were investigated after 60 d of Fe(III)-bioreduction. Laboratory-scale columns packed with water stable soil aggregates, inoculated with Geobacter sulfureducens were injected with a continuous flow of artificial groundwater media with or without acetate to simulate anaerobic Fe(III)-bioreduction. After 60 d the columns were sectioned and bacterial communities characterized via sequencing of 16S rRNA gene libraries. Overall Proteobacteria, Firmicutes, Bacteroidetes, Acidobactria and Actinobacteria were most abundant but the community composition varied with distance along the flow path. The microbial communities within all fractions of the columns treated with acetate were similarly dominated by genus Geobacter nearest the influent end of the column. More Fe(III)-bioreduction, aggregate breakdown, and colloid dispersion were observed in fractions where there were more abundant Geobacter suggesting these were the most active zones of iron reduction. Similarly, viral abundance was greatest near the inlet end of the treated columns and viral composition varied correspondingly along the flow path. This research demonstrates that biological iron reduction can alter the physical structure of porous media and that viral assemblages respond in concert with the microbial community in response to environmental stimuli.