Microbial Communities Associated with Sustained Anaerobic Reductive Dechlorination of α‑, β‑, γ‑, and δ‑Hexachlorocyclohexane Isomers to Monochlorobenzene and Benzene

Intensive historical and worldwide use of pesticide formulations
containing hexachlorocyclohexane (HCH) has led to widespread contamination.
We derived four anaerobic enrichment cultures from HCH-contaminated
soil capable of sustainably dechlorinating each of α-, β-,
γ-, and δ-HCH isomers stoichiometrically to benzene and
monochlorobenzene (MCB). For each isomer, the dechlorination rates,
inferred from production rates of the dechlorinated products, MCB
and benzene, increased progressively from <3 to ∼12 μM/day
over 2 years. The molar ratio of benzene to MCB produced was a function
of the substrate isomer and ranged from β (0.77 ± 0.15),
α (0.55 ± 0.09), γ (0.13 ± 0.02), to δ
(0.06 ± 0.02) in accordance with pathway predictions based on
prevalence of antiperiplanar geometry. Data from 16S rRNA gene amplicon
sequencing and quantitative PCR revealed significant increases in
the absolute abundances of Pelobacter and Dehalobacter, most notably in
the α-HCH and δ-HCH cultures. Cultivation with a different
HCH isomer resulted in distinct bacterial communities, but similar
archaeal communities. This study provides the first direct comparison
of shifts in anaerobic microbial communities induced by the dechlorination
of distinct HCH isomers. It also uncovers candidate microorganisms
responsible for the dechlorination of α-, β-, γ-,
and δ-HCH, a key step toward better understanding and monitoring
of natural attenuation processes and improving bioremediation technologies
for HCH-contaminated sites.