Impact of thermal disturbance on heterotrophic denitrification by aquifer microbiota in simulated Aquifer Thermal Energy Storage (ATES) systems
High-temperature aquifer thermal energy storage (HT-ATES) is a promising and sustainable method for seasonal heat storage in the subsurface. While its technical feasibility is established, ecological impacts—particularly on subsurface biodiversity and nitrogen cycling—remain poorly understood. This dissertation investigates the effects of HT-ATES operations on groundwater microbial communities and their nitrate (NO3⁻) reduction capacity using laboratory batch experiments. Chapter 2 examines how temperature gradients representative of HT-ATES influence native aquifer microbiomes. Results show that acetate mineralization coupled with nitrate reduction occurs across 12–60 °C but is inhibited at 80 °C, suggesting adaptation to moderate thermal stress but uncertain recovery under extreme conditions. Chapter 3 explores the combined effects of periodic heating and water exchange, highlighting microbial resilience to thermal and hydraulic disturbances. Cable bacteria, which couple electrogenic sulfur oxidation with nitrate reduction, are identified as potentially important in subsurface nitrogen cycling. Due to technical limitations, their direct study was not possible, but collaboration with Prof. Lars Peter Nielsen at Aarhus University led to a co-authored review on cable bacteria-mediated sulfur oxidation (Chapter 4). Integrating these studies, this dissertation evaluates HT-ATES impacts on nitrate reduction and provides insights into microbial mechanisms mediating nitrogen transformations under thermal stress, contributing to more sustainable HT-ATES strategies.
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