Abstract
Hydrogen storage in salt caverns is important for supporting the energy transition. However, there is limited knowledge about microbial communities within these caverns and associated risks of hydrogen loss. In this study we characterised a salt-saturated brine from a salt cavern and found a high sulphate content (4.2 g/L) and low carbon content (84.9 mg/L inorganic, 7.61 mg/L organic). The brine contained both Bacteria and Archaea, and 16S rRNA gene analysis revealed a halophilic community with members of Acetohalobium, Thiohalorhabdus, Salinibacter and up to 40% of unknown sequences. Within the Archaea, Euryarchaeota and the symbiotic Nanohaloarcheaota were dominant. Growth experiments showed that some microbes are resistant to autoclaving and pass through 0.22 μm filters. Heyndrickxia-related colonies grew on aerobic plates up to 10% salt, indicating the presence of inactive spores. The highest anaerobic activity was observed at 30°C, including glucose- and yeast extract fermentation, hydrogen-oxidation, lactate-utilisation, methane- and acetate-formation and sulphate-reduction, which was observed up to 80°C. However, microbial activity was slow, with incubations taking up to 1 year to measure microbial products. This study indicates that artificial salt caverns are an extreme environment containing potential hydrogen-consuming microbes.