Sulfidic acetate mineralization at 45°C by aquifer microbes

Aquifer Thermal Energy Storage (ATES) is a promising substitute for the sustainable provision of thermal energy to reduce dependence on fossil fuels for heating and cooling purposes, especially in urban areas. Most ATES reported are low temperature operations (LT-ATES), where regulatory thresholds are generally about 25°C whereas thresholds of for High-Temperature ATES (HT-ATES) are much higher. Temperatures = 45°C, which are relevant in the core and close-by impact zones of HT-ATES systems, may dramatically influence the structure and function of indigenous aquifer microbial communities. Here, we investigated the temperature behavior of an acetate-mineralizing, sulfate-reducing aquifer microbial community adapted to 45°C. Acetate mineralization was strongly inhibited at temperatures = 25°C and 60°C. Prolonged incubation at 12°C and 25°C resulted in acetate mineralization recovery after 40-80 days whereas acetate was not mineralized at 60°C within 100 days. Cultures adapted to 45°C and inhibited for 28 days by incubation at 12°C, 25°C or 60°C recovered after changing the temperature back to 45°C. The results of these heat exchange experiments indicate that the community was made up of distinct acetate mineralizing organisms adapted to temperatures between 12°C and 45°C and resilient to temporary heat stress. Phylotypes affiliated to the order Spirochaetales and to endospore-forming sulfate reducers of the order Clostridiales were abundant in microcosms being active at 45°C highlighting their key role at 45°C. The results suggest that prolonged incubation at 45°C will result in active microbial communities mainly consisting of organisms adapted to temperatures between the typical temperature range of mesophiles and thermophiles and being resilient to temporary heat changes.