Exploring microbiological dynamics in a salt cavern with potential use for hydrogen storage

Hydrogen storage in salt caverns is key to support the energy transition. However, little is known about microbial communities living within these caverns and associated risks of hydrogen loss. We carefully characterized 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 contains both Bacteria and Archaea and 16S DNA 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 dominate. Growth experiments revealed that some microbes are resistant to autoclaving and pass through 0.2µm filters. Heyndrickxia-related colonies grew on aerobic plates up to 10% salt, indicating the presence of inactive spores. We observed highest anaerobic activity at 30°C including glucose- and yeast extract fermentation, hydrogen-oxidation, lactate-utilization, methane- and acetate-formation and active sulphate-reduction up to 80°C. However, activity was very slow, and incubations took up to one year to measure any significant microbial products, which hints to a strong nutrient limitation in the cavern. Our study shows that artificial salt caverns are a fascinating extreme environment with the potential discovery of yet unknown polyextremophilic microbes.