Microbiology of salt caverns: description of extremely halotolerant microbiomes and a monitoring guideline for operators. Microbiology of salt caverns

Green hydrogen is key in a climate-neutral economy, as it allows the concept of storing renewable electricity in form of gas. Salt caverns are a safe and well-proven reservoir for large-scale natural gas storage and hence, a potential hydrogen storage. Contrary to natural gas, hydrogen is a favorable energy source for microorganisms at various conditions. Therefore, future hydrogen underground gas storage requires careful examination of the cavern microbiome to evaluate possible unwanted microbial reactions during storage. With this work we provide insight into the hydrochemical composition and microbial diversity of five natural gas storage caverns located in the Zechstein formation of Central Germany. NaCl and sulfate concentrations in the brine were 4.7 M and 40.8 mM. Further, methanol (11.9-37.5 mM), ethanol (0.4-6.9 mM), isopropanol (≤0.9 mM), butyrate (≤0.7 mM) and ammonium (0.2-0.7 mM) were found in all cavern brines, whereas formate (≤0.08 mM), acetate (≤0.5 mM) and lactate (≤0.06 mM) were detected in some brines (. The brines of all five caverns were colonized by extremely halophilic microorganisms , with an average cell number of 3.9E+6 cells ml -1 and characterized by 16S rRNA gene amplicon sequencing Their relative abundance varied between the different caverns, but the microbial core community was found to consist of different members of the Halobacteria, Halanaerobiales and Balneolales. Further, a phylotype belonging to the extremely halophilic, lithoautotrophic and sulfate-reducing genus Desulfovermiculus was found. Differences in the microbial community structure go along with differences in the hydrochemistry of each cavern, thus indicating microbial activity in the salt cavities. A guideline is given for microbial characterization of salt cavern brines including devices for sampling and subsequent chemical and molecular biological analysis.