Fatty acids and alcohols are key intermediates in the methanogenic degradation of organic matter, e.g., in anaerobic sewage sludge digestors or freshwater lake sediments. They are produced by classical fermenting bacteria for disposal of electrons derived in simultaneous substrate oxidations. Methanogenic bacteria can degrade primarily only one-carbon compounds. Therefore, acetate, propionate, ethanol, and their higher homologs have to be fermented further to one-carbon compounds. These fermentations are called secondary or syntrophic fermentations. They are endergonic processes under standard conditions and depend on intimate coupling with methanogenesis. The energetic situation of the prokaryotes cooperating in these processes is problematic: the free energy available in the reactions for total conversion of substrate to methane attributes to each partner amounts of energy in the range of the minimum biochemically convertible energy, i.e., 20 to 25 kJ per mol per reaction. This amount corresponds to one-third of an ATP unit and is equivalent to the energy required for a monovalent ion to cross the charged cytoplasmic membrane. Recent studies have revealed that syntrophically fermenting bacteria synthesize ATP by substrate-level phosphorylation and reinvest part of the ATP-bound energy into reversed electron transport processes, to release the electrons at a redox level accessible by the partner bacteria and to balance their energy budget. These findings allow us to understand the energy economy of these bacteria on the basis of concepts derived from the bioenergetics of other microorganisms.

/pdf/energetics-of-syntrophic-cooperation-in-methanogenic-42tfc2jdem.pdf

Energetics of syntrophic cooperation in methanogenic degradation.

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_57-58/010025232.pdf

Production, oxidation, emission and consumption of methane by soils: A review

Soil microorganisms as controllers of atmospheric trace gases (H2, CO, CH4, OCS, N2O, and NO).

Although of limited metabolic diversity, methanogenic archaea or methanogens possess great phylogenetic and ecological diversity. Only three types of methanogenic pathways are known: CO(2)-reduction, methyl-group reduction, and the aceticlastic reaction. Cultured methanogens are grouped into five orders based upon their phylogeny and phenotypic properties. In addition, uncultured methanogens that may represent new orders are present in many environments. The ecology of methanogens highlights their complex interactions with other anaerobes and the physical and chemical factors controlling their function.

Metabolic, phylogenetic, and ecological diversity of the methanogenic archaea.

A culture-independent molecular phylogenetic approach was used to survey constituents of microbial communities associated with an aquifer contaminated with hydrocarbons (mainly jet fuel) and chlorinated solvents undergoing intrinsic bioremediation. Samples were obtained from three redox zones: methanogenic, methanogenic-sulfate reducing, and iron or sulfate reducing. Small-subunit rRNA genes were amplified directly from aquifer material DNA by PCR with universally conserved or Bacteria- or Archaea-specific primers and were cloned. A total of 812 clones were screened by restriction fragment length polymorphisms (RFLP), approximately 50% of which were unique. All RFLP types that occurred more than once in the libraries, as well as many of the unique types, were sequenced. A total of 104 (94 bacterial and 10 archaeal) sequence types were determined. Of the 94 bacterial sequence types, 10 have no phylogenetic association with known taxonomic divisions and are phylogenetically grouped in six novel division level groups (candidate divisions WS1 to WS6); 21 belong to four recently described candidate divisions with no cultivated representatives (OP5, OP8, OP10, and OP11); and 63 are phylogenetically associated with 10 well-recognized divisions. The physiology of two particularly abundant sequence types obtained from the methanogenic zone could be inferred from their phylogenetic association with groups of microorganisms with a consistent phenotype. One of these sequence types is associated with the genus Syntrophus; Syntrophus spp. produce energy from the anaerobic oxidation of organic acids, with the production of acetate and hydrogen. The organism represented by the other sequence type is closely related to Methanosaeta spp., which are known to be capable of energy generation only through aceticlastic methanogenesis. We hypothesize, therefore, that the terminal step of hydrocarbon degradation in the methanogenic zone of the aquifer is aceticlastic methanogenesis and that the microorganisms represented by these two sequence types occur in syntrophic association.

https://espace.library.uq.edu.au/view/UQ:262655/UQ262655_OA.pdf

Microbial diversity in a hydrocarbon- and chlorinated-solvent-contaminated aquifer undergoing intrinsic bioremediation

The effect of temperature on CH4 production, turnover of dissolved H2, and enrichment of H2-utilizing anaerobic bacteria was studied in anoxic paddy soil and sediment of Lake Constance. When anoxic paddy soil was incubated under an atmosphere of H2/CO2, rates of CH4 production increased 25°C, but decreased at temperatures lower than 20°C. Chloroform completely inhibited methano-genesis in anoxic paddy soil and lake sediment, but did not or only partially inhibit the turnover of dissolved H2, especially at low incubation temperatures. Cultures with H2 as energy source resulted in the enrichment of chemolithotrophic homoacetogenic bacteria whenever incubation temperatures were lower than 20°C. Hydrogenotrophic methanogens could only be enriched at 30°C from anoxic paddy soil. A homoacetogen

Hydrogen turnover by psychrotrophic homoacetogenic and mesophilic methanogenic bacteria in anoxic paddy soil and lake sediment

Interspecies H2 transfer within methanogenic bacterial associations (MBA) accounted for 95–97% of the conversion of 14CO2 to 14CH4 in anoxic paddy soil. Only 3–5% of the 14CH4 were produced from the turnover of dissolved H2. The H2-syntrophic MBA developed within 5 days after the paddy soil had been submerged and placed under anoxic atmosphere. Afterwards, both the contribution of MBA to H2-dependent methanogenesis and the turnover of dissolved H2 did not change significantly for up to 7 months of incubation. However, while the rates of H2-dependent methanogenesis stayed relatively constant, the rates of total methanogenesis decreased. The contribution of MBA to H2-dependent methanogenesis was further enhanced to 99% when the temperature was shifted from 30°C to 17°C, or when the soil had been planted with rice. This enhancement was partially due to an increased utilization of dissolved H2 by chloroform-insensitive non-methanogenic bacteria, most probably homoacetogens, so that CH4 production was almost completely restricted to H2-syntrophic MBA. The activity of MBA, as measured by the conversion of 14CO2 to 14CH4, was stimulated by glucose, lactate, and ethanol to a similar or greater extent than by exogenous H2. Propionate and acetate had no effect.

Temporal change of gas metabolism by hydrogen-syntrophic methanogenic bacterial associations in anoxic paddy soil

Acetate turnover was measured in slurries of anoxic methanogenic paddy soil after addition of carrier-free [2-14C]-acetate. Acetate concentrations stayed fairly constant for about 1–2 days indicating steady state between production and consumption reactions. Depending on the experiment, acetate concentrations were between 100 and 3000 μM. Turnover rates were determined from the logarithmic decrease of [2-14C]-acetate or from the accumulation of acetate in the presence of chloroform resulting in similar values, i.e. 12–13 nmol h−1g−1d.w. soil at 17°C and 36–88 nmol h−1g−1d.w. at 30°C. Acetate consumption was completely inhibited by chloroform. The respiratory index (RI) was < 0.27. Hence, acetate was apparently consumed by methanogenic bacteria. About 80–90% of the CH4 produced originated from the methyl group of acetate. The role of homoacetogenesis for acetate production was studied by measuring the incorporation of radioactive bicarbonate into acetate. In different experiments, CO2 incorporation accounted for fractions of 1–60% of the acetate produced, about 10% being the most likely value for steady-state conditions. The fraction increased at high H2 concentrations and decreased at high acetate concentrations. The rate of H2 production that was required for chemolithotrophic acetate production from CO2 was two orders of magnitude higher than the actually measured rate. Hence, most of the CO2 incorporation into acetate was caused by electron donors other than H2 (e.g., carbohydrates) and/or by exchange reactions.

Hans-Peter Mayer

Papers

Hydrogen turnover by psychrotrophic homoacetogenic and mesophilic methanogenic bacteria in anoxic paddy soil and lake sediment

Temporal change of gas metabolism by hydrogen-syntrophic methanogenic bacterial associations in anoxic paddy soil

Bicarbonate-dependent production and methanogenic consumption of acetate in anoxic paddy soil