Dichloromethane biodegradation in multi-contaminated groundwater: Insights from biomolecular and compound-specific isotope analyses.

Dichloromethane (DCM) is a widespread and toxic industrial solvent which often co-occurs with chlorinated ethenes at polluted sites. Biodegradation of DCM occurs under both oxic and anoxic conditions in soils and aquifers. Here we investigated in situ and ex situ biodegradation of DCM in groundwater sampled from the industrial site of Themeroil (France), where DCM occurs as a major co-contaminant of chloroethenes. Carbon isotopic fractionation (ε) for DCM ranging from -46 to -22‰ were obtained under oxic or denitrifying conditions, in mineral medium or contaminated groundwater, and for laboratory cultures of Hyphomicrobium sp. strain GJ21 and two new DCM-degrading strains isolated from the contaminated groundwater. The extent of DCM biodegradation (B%) in the aquifer, as evaluated by compound-specific isotope analysis (δC), ranged from 1% to 85% applying DCM-specific ε derived from reference strains and those determined in this study. Laboratory groundwater microcosms under oxic conditions showed DCM biodegradation rates of up to 0.1 mM·day, with concomitant chloride release. Dehalogenase genes dcmA and dhlA involved in DCM biodegradation ranged from below 4 × 10 (boundary) to 1 × 10 (source zone) copies L across the contamination plume. High-throughput sequencing on the 16S rrnA gene in groundwater samples showed that both contaminant level and terminal electron acceptor processes (TEAPs) influenced the distribution of genus-level taxa associated with DCM biodegradation. Taken together, our results demonstrate the potential of DCM biodegradation in multi-contaminated groundwater. This integrative approach may be applied to contaminated aquifers in the future, in order to identify microbial taxa and pathways associated with DCM biodegradation in relation to redox conditions and co-contamination levels.