Sulfidic toluene mineralization by aquifer microbial communities at different temperatures

High-temperature aquifer thermal energy storage (HT-ATES) is a carbon-neutral technology in the heating and cooling sector particularly suitable for urban areas, where aquifers are often contaminated with hydrocarbons. How HT-ATES could influence the natural degradation of contaminants such as hydrocarbons has hardly been investigated. Here, we determined the effects of temperature and temperature shifts on the capability of aquifer microbial communities to mineralize the model hydrocarbon toluene at sulfate-reducing conditions. Distinct toluene-mineralizing, sulfate-reducing consortia were enriched from material of two hydrocarbon-contaminated field sites at 12°C, 20°C, 25°C, 38°C, and 45°. Lowest toluene mineralization rates were observed at 38°C, and highest rates were observed at 45°C. Consortia adapted to 12°C or 25°C were generally negatively impacted by temporary or permanent temperature shifts to temperatures ≥ 38°C. Desulfosporosinus phylotypes dominated enrichments at 12°C, indicating a major role for toluene mineralization at in situ temperatures. At 20°C-25°C, typical sulfate-reducing genera such as Desulfoprunum, Desulfallas or Pelotomaculum were abundant, indicating synergistic relationships of various toluene degraders belonging to different taxa. The communities grown at 45°C were dominated by putative thermophilic phylotypes affiliated to the phyla Bacillota or Caldiserica. Overall, our data indicate that 45°C is the upper limit for anaerobic toluene mineralization of the investigated communities.