We examined the potential for CH4 oxidation to be coupled with oxygen derived from the dissimilatory reduction of perchlorate, chlorate, or via chlorite (ClO(-) 2) dismutation. Although dissimilatory reduction of ClO(-) 4 and ClO(-) 3 could be inferred from the accumulation of chloride ions either in spent media or in soil slurries prepared from exposed freshwater lake sediment, neither of these oxyanions evoked methane oxidation when added to either anaerobic mixed cultures or soil enriched in methanotrophs. In contrast, ClO(-) 2 amendment elicited such activity. Methane (0.2 kPa) was completely removed within several days from the headspace of cell suspensions of Dechloromonas agitata CKB incubated with either Methylococcus capsulatus Bath or Methylomicrobium album BG8 in the presence of 5 mM ClO(-) 2. We also observed complete removal of 0.2 kPa CH4 in bottles containing soil enriched in methanotrophs when co-incubated with D. agitata CKB and 10 mM ClO(-) 2. However, to be effective these experiments required physical separation of soil from D. agitata CKB to allow for the partitioning of O2 liberated from chlorite dismutation into the shared headspace. Although a link between ClO(-) 2 and CH4 consumption was established in soils and cultures, no upstream connection with either ClO(-) 4 or ClO(-) 3 was discerned. This result suggests that the release of O2 during enzymatic perchlorate reduction was negligible, and that the oxygen produced was unavailable to the aerobic methanotrophs.

Acetylene (C₂H₂) can be generated in contaminated groundwater sites as a consequence of chemical degradation of trichloroethene (TCE) by in situ minerals, and C₂H₂ is known to inhibit bacterial dechlorination. In this study, we show that while high C₂H₂ (1.3 mM) concentrations reversibly inhibit reductive dechlorination of TCE by Dehalococcoides mccartyi isolates as well as enrichment cultures containing D. mccartyi sp., low C₂H₂ (0.4 mM) concentrations do not inhibit growth or metabolism of D. mccartyi. Cocultures of Pelobacter SFB93, a C₂H₂-fermenting bacterium, with D. mccartyi strain 195 or with D. mccartyi strain BAV1 were actively sustained by providing acetylene as the electron donor and carbon source while TCE or cis-DCE served as the electron acceptor. Inhibition by acetylene of reductive dechlorination and methanogenesis in the enrichment culture ANAS was observed, and the inhibition was removed by adding Pelobacter SFB93 into the consortium. Transcriptomic analysis of D. mccartyi strain 195 showed genes encoding for reductive dehalogenases (e.g., tceA) were not affected during the C₂H₂-inhibition, while genes encoding for ATP synthase, biosynthesis, and Hym hydrogenase were down-regulated during C₂H₂ inhibition, consistent with the physiological observation of lower cell yields and reduced dechlorination rates in strain 195. These results will help facilitate the optimization of TCE-bioremediation at contaminated sites containing both TCE and C₂H₂.

In aquifers, acetylene (C₂H₂) is a product of abiotic degradation of trichloroethene (TCE) catalyzed by in situ minerals. C₂H₂ can, in turn, inhibit multiple microbial processes including TCE dechlorination and metabolisms that commonly support dechlorination, in addition to supporting the growth of acetylenotrophic microorganisms. Previously, C₂H₂ was shown to support TCE reductive dechlorination in synthetic, laboratory-constructed cocultures containing the acetylenotroph Pelobacter sp. strain SFB93 and Dehalococcoides mccartyi strain 195 or strain BAV1. In this study, we demonstrate TCE and perchloroethene (PCE) reductive dechlorination by a microbial community enriched from contaminated groundwater and amended with C₂H₂ as the sole electron donor and organic carbon source. The metagenome of the stable, enriched community was analyzed to elucidate putative community functions. A novel anaerobic acetylenotroph in the phylum Actinobacteria was identified using metagenomic analysis. These results demonstrate that the coupling of acetylenotrophy and reductive dechlorination can occur in the environment with native bacteria and broaden our understanding of biotransformation at contaminated sites containing both TCE and C₂H₂IMPORTANCE Understanding the complex metabolisms of microbial communities in contaminated groundwaters is a challenge. PCE and TCE are among the most common groundwater contaminants in the United States that, when exposed to certain minerals, exhibit a unique abiotic degradation pathway in which C₂H₂ is a product. C₂H₂ can act as both an inhibitor of TCE dechlorination and of supporting metabolisms and an energy source for acetylenotrophic bacteria. Here, we combine laboratory microcosm studies with computational approaches to enrich and characterize an environmental microbial community that couples two uncommon metabolisms, demonstrating unique metabolic interactions only yet reported in synthetic, laboratory-constructed settings. Using this comprehensive approach, we have identified the first reported anaerobic acetylenotroph in the phylum Actinobacteria, demonstrating the yet-undescribed diversity of this metabolism that is widely considered to be uncommon.