Aquifer heat storage: abundance and diversity of the microbial community with acetate at increased temperatures

The temperature affects the availability of organic carbon and terminal electron acceptors (TEA) as well as the microbial community composition of the subsurface. To investigate the impact of thermal energy storage on the indigenous microbial communities and the fluid geochemistry, lignite aquifer sediments were flowed through with acetate-enriched water at temperatures of 10, 25, 40, and 70 °C in sediment column experiments. Genetic fingerprinting revealed significant differences in the microbial community compositions with respect to the different temperatures. The highest bacterial diversity was found at 70 °C. Carbon and TEA mass balances showed that the aerobic degradation of organic matter and sulfate reduction were the primary processes that occurred in all the columns, whereas methanogenesis only played a major role at 25 °C. The methanogenic activity corresponded to the highest abundance of an acetoclastic Methanosaeta concilii-like archaeon and the most efficient degradation of acetate. This study suggests a significant impact of geothermal energy storage on the natural microbial community and various metabolic activities because of increased temperatures in sediments with a temperature-related sediment organic matter release.