Microbial formate production and consumption during syntrophic conversion of ethanol or lactate to methane was examined in purified flocs and digestor contents obtained from a whey-processing digestor. Formate production by digestor contents or purified digestor flocs was dependent on CO/sub 2/ and either ethanol or lactate but not H/sub 2/ gas as an electron donor. Floc preparations accumulated fourfold-higher levels of formate (40 ..mu..M) than digestor contents, and the free flora was the primary site for formate cleavage to CO/sub 2/ and H/sub 2/ (90 ..mu..M formate per h). Inhibition of methanogenesis by CHCl/sub 3/ resulted in formate accumulation and suppression of syntrophic ethanol oxidation. H/sub 2/ gas was an insignificant intermediary metabolite of syntrophic ethanol conversion by flocs, and it exogenous addition neither stimulated methanogenes nor inhibited the initial rate of ethanol oxidation. These results demonstrated that >90% of the syntrophic ethanol conversion to methane by mixed cultures containing primarily Desulfovibrio vulgaris and Methanobacterium formicicum was mediated via interspecies formate transfer and the <10% was mediated via interspecies H/sub 2/ transfer. The results are discussed in relation to biochemical thermodynamics. A model is presented which describes the dynamics of a bicarbonate-formate electron shuttle mechanism for control of carbon andmore » electron flow during syntrophic methanogenesis and provides a novel mechanism for energy conservation by syntrophic acetogens.« less

Control of interspecies electron flow during anaerobic digestion: significance of formate transfer versus hydrogen transfer during syntrophic methanogenesis in flocs. [Methanobacterium formicicum; Desulfovibrio vulgaris]

Interspecies electron transfer is common in methanogenic consortia. Syntrophic partners cooperate by transferring electrons from one species to the other using H2, formate or other pathways, and maintaining H2 at low levels, so that the overall reactions are thermodynamically more favorable. Here we reviewed the mechanisms of interspecies electron transfer involved in methanogenic consortia, i.e., interspecies H2 transfer, interspecies formate transfer, and direct interspecies electron transfer (DIET) via pili, and/or outer membrane cytochromes. Although the relative importance of different mechanisms is uncertain, one mechanism may predominate under specific circumstances. Interspecies electron transfer is a probable mechanism in anaerobic granules formed in upflow anaerobic sludge bed (UASB) reactors, though direct evidence is limited. Studies of interspecies electron transfer without extracellular shuttles are mainly confined to Shewanella and Geobacter species. These mechanisms could be more energy-conserving than interspecies H2 or formate transfer, but the relationship between them is yet unelucidated. These studies are meaningful in that they could enable us a deeper understanding of how microorganisms degrade waste and produce biogas, and how current waste treatment and methane production strategies could be improved.

Interspecies electron transfer in syntrophic methanogenic consortia: From cultures to bioreactors

Role of biochar in the granulation of anaerobic sludge and improvement of electron transfer characteristics.

Long chain fatty acids degradation in anaerobic digester: Thermodynamic equilibrium consideration

A mixed culture from an anaerobic biowaste digester was enriched on propionate and used to investigate interspecies hydrogen transfer in dependence of spatial distances between propionate degraders and methanogens. From 20.3 mM propionate, 20.8 mM acetate and 15.5 mM methane were formed. Maximum specific propionate oxidation and methane formation rates were 49 and 23 mmol mg−1 day−1, respectively. Propionate oxidation was inhibited by only 20 mM acetate by about 50 %. Intermediate formate formation during inhibited methanogensis was observed. The spatial distribution and the biovolume fraction of propionate degraders and of methanogens in relation to the total population during aggregate formation were determined. Measurements of interbacterial distances were conducted with fluorescence in situ hybridization by application of group-specific 16S rRNA-targeted probes and 3D image analyses. With increasing incubation time, floc formation and growth up to 54 μm were observed. Propionate degraders and methanogens were distributed randomly in the flocs. The methanogenic biovolume fraction was high at the beginning and remained constant over 42 days, whereas the fraction of propionate degraders increased with time during propionate feeding. Interbacterial distances between propionate degraders and methanogens decreased with time from 5.30 to 0.29 μm, causing an increase of the maximum possible hydrogen flux from 1.1 to 10.3 nmol ml−1 min−1. The maximum possible hydrogen flux was always higher than the hydrogen formation and consumption rate, indicating that reducing the interspecies distance by aggregation is advantageous in complex ecosystems.

Lun Shi-yi

Papers

The contribution of interspecies hydrogen transfer to the substrate removal in methanogenesis