Temperature management potentially affects carbon mineralization capacity and microbial community composition of a shallow aquifer.

High-temperature aquifer thermal energy storage (HT-ATES) is a promising technique to reduce the CO2 footprint of heat supply in the frame of transitioning to renewable energies. However, HT-ATES causes temperature fluctuations in groundwater ecosystems potentially affecting important microbial-mediated ecosystem services. Hence, assessing the impact of increasing temperatures on the structure and functioning of aquifer microbiomes is crucial to evaluate potential environmental risks associated with HT-ATES. In this study, we investigated the effects of temperature variations (12-80°C) on microbial communities and their capacity to mineralize acetate in aerobically incubated sediments sampled from a pristine aquifer. Compared to natural conditions (12°C), increased acetate mineralization rates were observed at 25°C, 37°C and 45°C, whereas mineralization was decelerated at 60°C and absent at 80°C. Sequencing of 16S rRNA genes revealed that the bacterial diversity in acetate-amended and non-acetate-amended sediments decreased with rising temperatures. Distinct communities dominated by bacterial groups affiliated with meso- and thermophilic bacteria established at 45°C and 60°C, respectively, while the number of archaeal phylotypes decreased. The changes in microbial diversity observed at 45°C and 60°C indicate an increasing loss of ecosystem functioning, functional redundancy, and resilience, at higher temperatures, whereas 80°C results in a collapse of the ecosystem integrity.