Abstract
Crystalline bedrock has been chosen for deep geologic long-term storage of used nuclear fuel in Finland. The risks generated by the deep subsurface microbial communities in these disposal sites need to be well characterised in advance to ensure safety. Deep subsurface microbial communities in a steady state are unlikely to contribute to known risk factors, such as corrosion or gas production. However, the construction of the geological final-disposal facility, bedrock disturbances, and hydraulic gradients cause changes that affect the microbial steady-state. To study the induced metabolism of deep microbial communities in changing environmental conditions, the activating effect of different electron donors and acceptors were measured with redox sensing fluorescent dyes (5-Cyano-2,3-ditolyl tetrazolium chloride, CTC and RedoxSensor™ Green, RSG). Fluids originating from two different fracture zones of the Finnish disposal site in Olkiluoto were studied. These fracture fluids were very dissimilar both chemically and in terms of bacterial and archaeal diversity. However, the microbial communities of both fracture fluids were activated, especially with acetate, which indicates the important role of acetate as a preferred electron donor for Olkiluoto deep subsurface communities.
Highlights
Microbial communities inhabiting deep terrestrial groundwater systems utilise all available energy sources [1]
The deep subsurface life in undisturbed condition exists in a balance that is maintained in a steady state by one or a few limiting energetic or nutrient factors
Deep subsurface microbial communities in a steady state are unlikely to contribute to known risk factors, such as corrosion or gas formation
Summary
Microbial communities inhabiting deep terrestrial groundwater systems utilise all available energy sources [1]. These energy sources and the biogeochemical cycles of elements, such as carbon, nitrogen, and sulphur, are connected to each other. Diverse communities may form in such conditions, the core microbiome often consist of only a few species [3,4], or even of a single-species ecosystem [5]. Such microbial communities may achieve relatively stable community structures over the course of time. The deep subsurface life in undisturbed condition exists in a balance that is maintained in a steady state by one or a few limiting energetic or nutrient factors
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