Methanosarcina spp. Drive Vinyl Chloride Dechlorination via Interspecies Hydrogen Transfer

Abstract

Two highly enriched cultures containing Dehalococcoides spp. were used to study the effect of aceticlastic methanogens on reductive vinyl chloride (VC) dechlorination. In terms of aceticlastic methanogens, one culture was dominated by Methanosaeta, while the other culture was dominated by Methanosarcina, as determined by fluorescence in situ hybridization. Cultures amended with 2-bromoethanesulfonate (BES), an efficient inhibitor of methanogens, exhibited slow VC dechlorination when grown on acetate and VC. Methanogenic cultures dominated by Methanosaeta had no impact on dechlorination rates, compared to BES-amended controls. In contrast, methanogenic cultures dominated by Methanosarcina displayed up to sevenfold-higher rates of VC dechlorination than their BES-amended counterparts. Methanosarcina-dominated cultures converted a higher percentage of [2-14 C]acetate to 14 CO 2 when concomitant VC dechlorination took place, compared to nondechlorinating controls. Respiratory indices increased from 0.12 in nondechlorinating cultures to 0.51 in actively dechlorinating cultures. During VC dechlorination, aqueous hydrogen (H 2 ) concentrations dropped to 0.3 to 0.5 nM. However, upon complete VC consumption, H 2 levels increased by a factor of 10 to 100, indicating active hydrogen production from acetate oxidation. This process was thermodynamically favorable by means of the extremely low H 2 levels during dechlorination. VC degradation in nonmethanogenic cultures was not inhibited by BES but was limited by the availability of H 2 as electron donor, in cultures both with and without BES. These findings all indicate that Methanosarcina (but not Methanosaeta), while cleaving acetate to methane, simultaneously oxidizes acetate to CO 2 plus H 2 , driving hydrogenotrophic dehalorespiration of VC to ethene by Dehalococcoides.Contamination with chlorinated ethenes, an almost ubiquitous class of pollutants, presently poses a serious threat to groundwater quality in industrialized countries (1). Microbial reductive dechlorination is the major pathway of degradation and detoxification of chloroethenes under anaerobic conditions (26). Dehalorespiring bacteria (DRB; synonym, chlororespiring bacteria) are able to use the energy available from reductive dechlorination in a respiratory process (27, 28); however, sequential dechlorination may result in transient buildup of the highly toxic metabolite vinyl chloride (VC) (15,44). DRB growing with vinyl chloride as an electron acceptor are to date restricted to the Dehalococcoides group of bacteria (11,12,16,23). All known Dehalococcoides isolates require hydrogen (H 2 ) as a direct electron donor (3,23,34). However, in anaerobic substrate degradation, a considerable fraction of organic carbon is converted to methane via acetate. The latter may also be oxidized by sulfate-reducing, Fe(III)-reducing, or denitrifying bacteria (49), while known VC-dechlorinating DRB are unable to use acetate as a direct electron donor. H 2 production from acetate is an obligately syntrophic process that may...

Methanosarcinaceae and Acetate-Oxidizing Pathways Dominate in High-Rate Thermophilic Anaerobic Digestion of Waste-Activated Sludge

“…The thermodynamic shifts in methanogenic pathways with respect to increase in temperature have also been reported (16,42), suggesting that high temperature and short HRT favor the syntrophic association between acetate oxidizers and hydrogenotrophic methanogens, which may include Methanosarcinaceae and Methanobacteriaceae. Methanobacteriaceae have previously been found only to be hydrogen and formate utilizing, while Methanosarcinaceae can also cleave acetate, with the metabolic possibility also for acetate oxidation to hydrogen (43,44). The high level of formate activity may be due to activity by Methanobacteriaceae, since Methanosarcinaceae have not been previously observed to grow on formate (though the genes for this do exist [45]).…”

Sustainable syntrophic growth of Dehalococcoides ethenogenes strain 195 with Desulfovibrio vulgaris Hildenborough and Methanobacterium congolense: global transcriptomic and proteomic analyses

“…In addition, methanogens were found to compete for hydrogen with dehalogenators (Smatlak et al, 1996;Fennell et al, 1997;Yang and McCarty, 1998). Although there have been a number of studies evaluating the link between methanogens and Dehalococcoides (Fathepure and Boyd, 1988;Freedman and Gossett 1989;Smatlak et al, 1996;Fennell et al, 1997;Lö ffler et al, 1997;Yang and McCarty, 1998;Booker and Pavlostathis, 2000;Heimann et al, 2006), the role of methanogens in dechlorinating communities remains unclear.…”

Biodiversity of Dehalorespiring Bacteria with Special Emphasis on Polychlorinated Biphenyl/Dioxin Dechlorinators

“…Therefore, the co-existence of synergistic hydrogen producers and the concentration of hydrogen produced are the key factors for reductive dehalogenation 29) . For example, interspecies transfer of H2 produced via acetate oxidation by methanogens triggers hydrogenotrophic dechlorination of VC to ethene by "Dehalococcoides" 59) . A synergetic dehalogenating process with "Dehalococcoides" and a hydrogen-producing fermenting bacterium exists in the above-noted culture TUT2264 (unpublished data).…”