Hydrogen competition between a gas reservoir community indicates available CO2 as main limiting factor

Hydrogen (H2) underground storage in salt caverns or depleted gas reservoirs has been proposed as an economically viable option for storing large volumes of H2 to overcome seasonal fluctuations in energy production/demand. However, subsurface reservoirs often contain microbial communities that can consume H2 as an electron donor over time. To study the possible activity and H2 consumption within a reservoir, we sampled brine from a depleted gas field in Austria. Initial DNA community sequencing showed that the brine contained a diverse microbial community including sulfur-cycling microbes, methanogens, acetogens and general fermenters. In batch cultivation tests, we exposed the brine to different ratios of H2/CO2 (10, 40, and 95% H2 with 3-7% CO2). H2 was consumed in all cases, and we observed initial H2S formation, followed by what appeared to be simultaneous methanogenesis and acetogenesis. Activity ceased when CO2/dissolved carbonate was depleted. H2 consumption restarted as soon as CO2 was resupplied. Addition of original rock material, which is rich in carbonate minerals (26-50 Ma.-% Calcite), boosted activity. The results show that for a mixed H2-consuming community, the limiting factor is CO2 when H2 is in excess. This will have direct effects on pH and the competing activity of methanogens and acetogens.