Coexistence of anaerobic and aerobic dechlorinating bacterial communities in 1,2-dichloroethane contaminated groundwater

Chlorinated Aliphatic Hydrocarbons (CAHs) are widespread, persistent and toxic environmental pollutants. Bioremediation of CAHs-contaminated aquifers is generally achieved through anaerobic processes mediated by specialized microorganisms known as OrganoHalide Respiring Bacteria (OHRB), but aerobic and co-metabolic processes can also co-occur. The aim of this work is to investigate the intrinsic biodegradation potential of the autochthonous microbial communities from an aquifer mainly contaminated by 1,2-dichloroethane (1,2-DCA), in response to anaerobic and aerobic biostimulation treatments in microcosm. Microcosms under aerobic and anaerobic conditions were set up from contaminated groundwater using mineral salt media amended with 1,2-DCA and appropriate substrates, if required (lactate or no additive amendment in anaerobic microcosms; volatile hydrocarbon mixture or no additive amendment in aerobic microcosms). The bacterial communities from the aquifer and microcosms were characterized by 16S rRNA gene Metagenomic Sequencing and 16S rRNA OHRB specific probes. Although low relative abundances of known dechlorinating anaerobic and aerobic bacteria were detected in the aquifer, the biostimulation treatments allowed the enrichment of both anaerobic and aerobic 1,2-DCA dechlorinating communities as revealed by the removal of 1,2-DCA monitored over time by GC-MS. Known dechlorinating genera detected in the anaerobic (e.g. Dehalococcoides and Desulfuromonas) and aerobic (Starkeya and Ancylobacter) enriched cultures may mediate the biodegradation of 1,2-DCA under anaerobic and aerobic conditions respectively. The presence of catabolic genes rdhA and dhlA, involved in reductive and hydrolytic dechlorination respectively, was confirmed by PCR assays with degenerated and specific primers and sequencing. In conclusion, although the site shows a poor intrinsic catabolic potential, the detection of aerobic and anaerobic 1,2-DCA degraders and dechlorination genes in enriched cultures suggests the coexistence of aerobic and anaerobic metabolisms that could be biostimulated and exploited for the bioremediation of the site.