Iron oxides influence bacterial community structure and the spatial distribution of the aerobic methanotrophs and sulphate reducers in granitic aquifers

The effect of bacteriogenic iron oxides (BIOS) on microbial community structure in subterranean igneous rock aquifers was evaluated. Of interest was to understand how bacteria are partitioned between groundwater and BIOS and to determined whether such spatial separation within the same environment confers niche-specific selectivity. The distribution of the methane-oxidizing and sulphate-reducing bacteria (SRB) was examined closely because the former are important for redox maintenance in this predominantly anaerobic environment. The SRB are ecologically important in organic carbon mineralization representing the most abundant and diverse metabolic group in the aquifers. The corrosive hydrogen sulphide they produce poses a threat to the integrity of the copper canisters that will store spent nuclear waste deposited in these aquifers. Widespread deposits of BIOS occur at microaerophilic interfaces where anaerobic ground water interacts with oxygen. Applying molecular techniques, BIOS deposited along microaerophilic zones in the aquifers was found to select for bacterial phylotypes normally not predominant in groundwater. More than 70% of the SRB occurred in association with the BIOS-rich zones. The native SRB widespread in the aquifers poorly colonized environments associated with BIOS. As much as 50% of the total bacterial counts present in the BIOS communities could be identified as type I methanotrophs predominated by filamentous forms. In addition to biogeochemical factors such as salinity and depth, this study suggests that BIOS environments may constitute a distinct niche with unique properties that enrich for microorganisms different from those commonly reported in the non-BIOS aquifers