Heterotrophic nitrate reduction potential of an aquifer microbial community from psychrophilic to thermophilic conditions

High temperature-aquifer thermal energy storage (HT-ATES) aims at the seasonal storage and extraction of large quantities of heat in the subsurface. However, the impacts of temperature fluctuations caused by HT-ATES toward biodiversity and ecosystem services in the subsurface environment with respect to the nitrogen cycle remain unclear. Hence, understanding possible temperature adaptation mechanisms of aquifer microbial communities is crucial to assess potential environmental risks associated with HT-ATES. In this study, we investigated the effects of temperatures between 12 °C and 80 °C on a pristine aquifer microbial community and its capacity to reduce nitrate, a common global groundwater contaminant. ¹³C-labeled acetate was used as easily consumable carbon and energy source for nitrate reduction, allowing precise activity measurement by analysis of released ¹³CO₂. We observed nitrate reduction coupled to acetate mineralization at 12 °C, 25 °C, 38 °C, 45 °C and 60 °C but not at 80 °C. The rates of acetate mineralization at 12 °C to 38 °C were significantly higher than rates at 45 °C and 60 °C. Temperature significantly affected the composition of the acetate-mineralizing, nitrate-reducing microbial communities. Members of the families Pseudomonadaceae and Comamonadaceae mainly developed in enrichments incubated at 12 °C and 25 °C, whereas phylotypes affiliated to Rhizobiaceae dominated at 38 °C. At 45 °C and 60 °C, phylotypes belonging to Symbiobacteriaceae, Paenibacillaceae and Planococcaceae mainly developed. These findings indicate that the indigenous aquifer microbiome can maintain the ability to reduce nitrate over a wide temperature range, providing support that HT-ATES may allow thermal energy storage while simultaneously attenuating nitrate pollution.