Microcosm biostimulation of anaerobic and aerobic dechlorinating bacteria in 1,2-dichloroethane contaminated water

Chlorinated Aliphatic Hydrocarbons (CAHs) are persistent and toxic environmental pollutants. In contaminated groundwater, CAHs can be biodegraded by anaerobic Organohalide Respiring Bacteria (OHRB)[1] in the dehalorespiration process mediated by reductive dehalogenase enzymes. Nevertheless, aerobic oxidative processes can also co-occur, mediated by haloalkane dehalogenase enzymes, or cometabolically[2]. Specialized dechlorinating microorganisms can be stimulated by addition of proper amendments and exploited in bioremediation strategies. The aim of this work is to investigate the intrinsic biodegradation potential of a CAHs contaminated aquifer (mainly 1,2-dichloroethane, 1,2-DCA) by evaluating the response of the autochthonous microbial communities to biostimulation treatments in microcosm, under anaerobic and aerobic conditions. Microcosms were set up from contaminated groundwater with appropriate mineral media under anaerobic conditions with (or without) lactate as electron donor to stimulate dehalorespiration, and under aerobic conditions with (or without) volatile hydrocarbon mixture to stimulate cometabolic or direct degradation. Parallel abiotic control microcosms were also set up. Chemical monitoring by Gas Chromatography-Mass Spectrometry over time revealed 1,2-DCA removal in all tested conditions. Illumina 16S rRNA gene sequencing revealed the enrichment of dechlorinating taxa in anaerobic microcosms, including known OHRB Dehalococcoides and Desulfuromonas and 1,2-DCA degraders such as Ancylobacter and Starkeya[3] in aerobic microcosms. The catabolic genes rdhA and dhlA, involved in reductive and hydrolytic dechlorination respectively, were detected in metagenomic DNA from microcosms by probing with specific primers. The biostimulation of anaerobic and aerobic 1,2-DCA degrading bacteria from the same groundwater suggests that both processes can be exploited for bioremediation purposes in site.