Anaerobic Hydrogen Consumption of nutrient-limited Aquifer Sediment microbial Communities examined by stable Isotope Analysis
Underground storage of hydrogen (H2) in porous aquifers is an option to bridge seasonal and daily fluctuations of renewably energy. However, the biogeochemical consequences of H2 underground storage are poorly understood, for example the determining factors of the possible consumption of H2 by autochthonous microorganisms. With this study, the effects of nutrient limitations on H2-oxidation of an aquifer microbial community in sediment microcosms were investigated in order to evaluate possible responses to otherwise naturally unusual high H2 partial pressures. We analyzed the hydrogen and carbon stable isotopes of H2, carbon dioxide (CO2) and methane (CH4), as well as the diversity of the microbial community to verify microbial H2 oxidation and to differentiate H2-related metabolic pathways. Hydrogen isotope analyses of H2 yielded isotopic depletion in all biotic setups indicating microbial H2 consumption. Carbon isotope analyses of CO2 showed isotopic enrichment in all H2-supplemented biotic setups indicating H2-dependent consumption of CO2 by methanogens or homoacetogens. Consumption of CO2 and H2 varied along the differently nutrient-amended setups, as did the onset of CH4 production. Compound-specific isotope analysis of carbon and hydrogen of CH4 showed distinct differences in early- and late-phase CH4 production along H2 consumption. Firstly, hydrogenotrophically produced CH4 was detected, followed by acetoclastic production of CH4. The microbial communities of the different nutrient-amended microcosms, determined by sequence analyses of the genes for 16S rRNA and methyl coenzyme A (mcrA), showed only slight variations. Most abundant phylotypes belonged to typical ferric iron reducers, indicating that besides CO2, Fe(III) was an important electron acceptor. Methanobacterium sp. was the dominate methanogen. In summary, our study has provided evidence for the adaptability of subsurface microbial communities under different nutrient-deficient conditions to elevated H2 partial pressures. Compound-specific stable isotope analysis (CSIA) was shown to be an effective method to monitor microbial processes upon H2 oxidation.