Acetate Activates Deep Subsurface Fracture Fluid Microbial Communities in Olkiluoto, Finland

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.

Microbial Community Composition in Deep‐Subsurface Reservoir Fluids Reveals Natural Interwell Connectivity

“…Similarly, Borehole OT and Borehole GC (which were ~50 m apart); Borehole B and Borehole D (which were ~10 m apart); Fractures A, B, and C (which were distributed ~10–20 m apart from each other); and Port 17Ledge and Borehole 8 (which were ~50 m apart) also had distinct microbial community profiles despite almost identical geochemistry between some sample pairs (Table ). This microbial heterogeneity is consistent with our expectation as well as with prior studies in deep‐subsurface fracture‐dominated flow systems with 10‐m to 2‐km scale (Miettinen et al, ; Osburn et al, ; Purkamo et al, ; Rajala & Bomberg, ). Such heterogeneity within a single geographic location is likely because of the largely varying environmental factors (lithology, water chemistry, in situ temperature and pressure, etc.)…”

“…The large spatial variability in reservoir conditions including lithology, porosity, and state of stress leads to heterogeneity in temperature, pressure, water saturation, water chemistry, oxidation states, and so on, all of which are key factors shaping each location's microbial community composition, that is, the presence/absence of each microbial taxon and its relative abundance with respect to the entire community. Such heterogeneity in subsurface environmental conditions is therefore likely to support distinct microbial communities in each environment, which form a relatively stable community structure according to local environmental conditions over the course of time (Miettinen et al, 2018). The composition of the communities, probed by modern highthroughput DNA sequencing technologies on the indigenous fluids, may therefore reflect the combination of subsurface parameters they once resided in, which allows fluids produced from different sources/compartments to be distinguished unambiguously.…”

Epilithic Microbial Community Functionality in Deep Oligotrophic Continental Bedrock

“…Two of these matched with Desulfosporosinus , which is a known sulfate reducing genus and known to produce acetate either from H 2 and CO 2 or via an electrosynthetic pathway ( Agostino et al, 2020 ). Acetate is an important intermediate compound in deep subsurface providing substrates for methanogenesis ( Lever, 2012 ; Purkamo et al, 2017 ; Miettinen et al, 2018 ).…”

Advances in Defining Ecosystem Functions of the Terrestrial Subsurface Biosphere

“…A variety of other locations provide fracture fluids within metamorphic units in the Fennoscandian Shield. Shallow boreholes provide access to Precambrian gneisses (e.g., at Olkiluoto, Romuvarra), where emphasis on cultivation and identification of sulfate reducing bacteria, nitrate reducing bacteria, acetogens, and methanogens has been the focus of much previous research ( Nyyssönen et al, 2012 ; Pedersen, 2013 ; Bomberg et al, 2015 , 2016 ; Kutvonen et al, 2015 ; Miettinen et al, 2018 ). For example, in shallow fluids (up to 600 m), sulfate reduction and methanotrophy were induced when hydrogen and methane were introduced in flow cells using fluid from a borehole in the ONKALO tunnel (Olkiluoto, Finland; Pedersen, 2013 ), and active nitrate and ammonium based catabolism were also indicated ( Kutvonen et al, 2015 ; Miettinen et al, 2018 ).…”