Assessing the effects of temperature of sulfidic toluene mineralisation on aquifer microbial communities using various sediments in a microcosm study

High temperature aquifer thermal energy storage (HT-ATES) is a promising carbon-neutral technology in the heating and cooling sector. There is a particular need for these systems in urban areas, where aquifers are often contaminated with hydrocarbons which are usually slowly biodegraded at anoxic in situ conditions. The effects of temperature increases on the degradation behavior of microbial communities have hardly been investigated; principally, combining HT-ATES and in situ bioremediation in a synergistic approach is a promising approach. In this laboratory microcosm study, we investigated systematically the effects of temperature and temperature shifts on the capacity of aquifer microbial communities to mineralize the model hydrocarbon toluene at sulfate-reducing conditions. Toluene-mineralizing, sulfate-reducing cultures were successfully enriched from sediment of two hydrocarbon-contaminated field sites (Zeitz (Zt) and Weißandt-Gölzau (Wg)) at 12°C, 20°C, 25°C, 38°C and 45°C. Each temperature resulted in the development of a distinct microbial community based on Bray-Curtis dissimilarities. Toluene mineralisation rates ranged from 2.7 ± 0.4 µM d-1 to 16.2 ± 0.6 µM d-1 with generally corresponding sulfide production rates. Lowest rates were observed at 38°C, showing that toluene mineralization rates did not follow simple Arrhenius relationships. No toluene mineralization was observed at 60°C within 15 to 21 months incubation. Several cultures adapted to 12°C or 25°C were severely impacted by transient or permanent temperature shifts to temperatures =38°C. Desulfosporosinus phylotypes dominated enrichments at 12°C, indicating a major role for degradation of toluene (and possibly other hydrocarbons) at in situ conditions at both investigated sites. At 25°C, putative sulfate reducing genera such as Desulfoprunum, Desulfotomaculum or Pelotomaculum were abundant, indicating synergistic relationships upon toluene mineralization or the activity of various toluene degraders belonging to different taxa. The community grown at 45°C was dominated by putative thermophilic phylotypes belonging to the Thermoanaerobacteraceae and Caldisericaceae. The permanent or temporary exposure of temperature-adapted cultures to different temperatures either diversified the communities or preserved the state in case of loss of activity. Overall, this research indicates that 38°C -45°C is the upper limit for anaerobic toluene mineralization of the investigated communities.