Effects of temperature on TCE reductive dechlorination, methanogenesis and microbial community composition

Chlorinated ethenes, perchloroethene (PCE), trichloroethene (TCE), dichloroethene (DCE) and vinyl chloride (VC) are common groundwater contaminants, with VC being particularly toxic and carcinogenic. Chlorinated ethene contamination is linked with anthropogenic activity leading to occurrences in the subsurface. There, the PCE and TCE can undergo biotransformation to the harmless product, ethene but are likely inhibited by factors including temperature and the underlying native microbial composition. In regions with temperate climates, the demand for energy in densely populated areas often integrates heating and cooling systems nearby. Recently, to divest from fossil fuel industries, municipalities have begun exploring and implementing aquifer thermal energy storage (ATES) systems in their areas. The aim was to couple energy storage and enhancing contaminant bioremediation, including contaminated ethenes. Of late, there has been interest to upgrade the storage limit capacity from 25°C in the low temperature (LT)-ATES to up to 80°C as a high temperature (HT)-ATES application. However, it is still unclear how the native microbial consortium would respond to temperature changes especially in their composition and reductive dechlorination ability. Our study aims to evaluate the impact of temperature on dechlorination of chlorinated ethenes using fine- and coarse-grained sediment samples from a contaminated field site in Ferrara (northern Italy). Laboratory scale microcosms were designed, supplemented with TCE (electron acceptor) and lactate (electron donor) and incubated at six different temperatures (10-60°C). Activity was monitored for up to 105 d and afterwards the microbial communities were analyzed. Results showed complete reductive dechlorination at 10-20°C and were putatively linked to the abundant Dehalogenimonas. Substantial cis-DCE and VC accumulated at 30°C and 40°C, respectively, while dechlorination activity was absent at 50 and 60°C suggesting temperature-related inhibition. Dehalogenimonas was notably absent, and phylotypes likely affiliated with Peptococcaceae abundance may be involved only in the initial transformation steps. Methanogenesis was observed between 20-40°C, and much later after 105 d at 10°C, while activity was absent >40°C. Methanosarcina, present at 10-40°C, are likely involved in methanogenesis. Meanwhile, Methanothrix was less TCE tolerant and more adapted to 30°C and 40°C, Methanobacterium was abundant at 40°C. Phylotypes such as Methanoculleus were likely more TCE tolerant but temperature-dependent. These findings aim to complement the current understanding on temperature-related transformation ability of the inoculum sediment, prediction of the transformation products and as a guide for future optimization HT-ATES targeted studies.