Microbial activity in salt cavern brine
H2 storage in salt caverns changes microbial nutrient availability and eventually leads to enhanced metabolic rates. Due to the geochemical composition of salt rock, sulfate is the primary electron acceptor in the system and its reduction can lead to considerable amounts of corrosive sulfide. Growth of endogenous microorganisms is thought to be limited by high salinity in the salt cavern brine. Therefore, an investigation of microbial activity in salt cavern brine is necessary to understand the dynamics of microbial growth and metabolism in a future H2 atmosphere. To do so, we sampled brine of natural gas storage caverns intended for H2 UGS and investigated their potential to allow for microbial growth. We performed anaerobic cultivation experiments with brine and H2. An extremely halophilic, sulfate reducing enrichment culture served as positive control. No microbial activity was detected in the natural brine samples by means of H2 oxidation and sulfate reduction, although cell number increased in all setups. Contrary, H2 and sulfate were actively consumed in the brine samples that were inoculated with the positive control. Subsequent metagenome sequencing confirmed the genetic potential for dissimilatory sulfate reduction and autotrophic CO2 fixation via the Wood-Ljungdahl pathway. Due to the limited availability of salt cavern brine and the inherent need to identify microbial growth within a reasonable time frame, we used a dual stable isotope probing experiment (dual-SIP) to identify microbial activity on the single-cell level based on incorporation of deuterium from heavy water and 13C from labelled bicarbonate. This method finally allowed the identification of active microorganisms in the cavern brine that initially escaped detection by means of H2 oxidation und sulfate reduction rates and is a promising tool to sensitively detect changes in microbial activity during H2 UGS.