Organotrophic diversity of bacteria in deep continental bedrock fracture fluid assessed with acetate-amended microcosms

This study sheds light on acetate metabolism of microbial communities from indigenous fracture fluid attained from a Finnish crystalline bedrock groundwater system, targeting on the microbial community residing in the fracture zone at 967 m depth of the Outokumpu Deep Drill Hole. Fracture fluid was anaerobically collected and amended with acetate, acetate+sulfate, only with sulfate or left unamended as control. We studied changes in the microbial communities with community fingerprinting technique as well as high-throughput 16S rRNA gene amplicon sequencing. The amended microcosms hosted more diverse bacterial communities compared to the treatment without amendments and the intrinsic fracture zone community. The majority of the bacterial populations enriched with acetate belonged to a-proteobacteria and clostridial hydrogenotrophic thiosulfate reducers affiliating with groups earlier found from subsurface and groundwater environments. No clear acetate-utilizing bacterial groups were detected during the incubation, but added H2 in the microcosm headspace probably played a role in modifying the bacterial community.