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 [2-14C]acetate to 14CO2 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 (H2) concentrations dropped to 0.3 to 0.5 nM. However, upon complete VC consumption, H2 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 H2 levels during dechlorination. VC degradation in nonmethanogenic cultures was not inhibited by BES but was limited by the availability of H2 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 CO2 plus H2, driving hydrogenotrophic dehalorespiration of VC to ethene by Dehalococcoides.

Coaggregation Facilitates Interspecies Hydrogen Transfer between Pelotomaculum thermopropionicum and Methanothermobacter thermautotrophicus

ABSTRACT A thermophilic syntrophic bacterium, Pelotomaculum thermopropionicum strain SI, was grown in a monoculture or coculture with a hydrogenotrophic methanogen, Methanothermobacter thermautotrophicus strain ΔH. Microscopic observation revealed that cells of each organism were dispersed in a monoculture independent of the growth substrate. In a coculture, however, these organisms coaggregated to different degrees depending on the substrate; namely, a large fraction of the cells coaggregated when they were grown on propionate, but relatively few cells coaggregated when they were grown on ethanol or 1-propanol. Field emission-scanning electron microscopy revealed that flagellum-like filaments of SI cells played a role in making contact with ΔH cells. Microscopic observation of aggregates also showed that extracellular polymeric substance-like structures were present in intercellular spaces. In order to evaluate the importance of coaggregation for syntrophic propionate oxidation, allowable average distances between SI and ΔH cells for accomplishing efficient interspecies hydrogen transfer were calculated by using Fick's diffusion law. The allowable distance for syntrophic propionate oxidation was estimated to be approximately 2 μm, while the allowable distances for ethanol and propanol oxidation were 16 μm and 32 μm, respectively. Considering that the mean cell-to-cell distance in the randomly dispersed culture was approximately 30 μm (at a concentration in the mid-exponential growth phase of the coculture of 5 × 107 cells ml−1), it is obvious that close physical contact of these organisms by coaggregation is indispensable for efficient syntrophic propionate oxidation.

Tetrachloroethene Dehalorespiration and Growth of Desulfitobacterium frappieri TCE1 in Strict Dependence on the Activity of Desulfovibrio fructosivorans

ABSTRACT Tetrachloroethene (PCE) dehalorespiration was investigated in a continuous coculture of the sulfate-reducing bacterium Desulfovibrio fructosivorans and the dehalorespiring Desulfitobacterium frappieri TCE1 at different sulfate concentrations and in the absence of sulfate. Fructose (2.5 mM) was the single electron donor, which could be used only by the sulfate reducer. With 2.5 mM sulfate, the dehalogenating strain was outnumbered by the sulfate-reducing bacterium, sulfate reduction was the dominating process, and only trace amounts of PCE were dehalogenated by strain TCE1. With 1 mM sulfate in the medium, complete sulfate reduction and complete PCE dehalogenation to cis-dichloroethene (cis-DCE) occurred. In the absence of sulfate, PCE was also completely dehalogenated to cis-DCE, and the population size of strain TCE1 increased significantly. The results presented here describe for the first time dehalogenation of PCE by a dehalorespiring anaerobe in strict dependence on the activity of a sulfate-reducing bacterium with a substrate that is exclusively used by the sulfate reducer. This interaction was studied under strictly controlled and quantifiable conditions in continuous culture and shown to depend on interspecies hydrogen transfer under sulfate-depleted conditions. Interspecies hydrogen transfer was demonstrated by direct H2 measurements of the gas phase and by the production of methane after the addition of a third organism, Methanobacterium formicicum.