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.

Potential rates of both methane production and methane consumption vary over three orders of magnitude and their distribution is skew. These rates are weakly correlated with ecosystem type, incubation temperature, in situ aeration, latitude, depth and distance to oxic/anoxic interface. Anaerobic carbon mineralisation is a major control of methane pro- duction. The large range in anaerobic CH4:CO2 production rates indicate that a large part of the anaerobically mineralised carbon is used for reduction of electron acceptors, and, hence, is not available for methanogenesis. Consequently, cycling of electron acceptors needs to be studied to understand methane production. Methane and oxygen half saturation constants for methane oxidation vary about one order of magnitude. Potential methane oxidation seems to be correlated with methanotrophic biomass. Therefore, variation in potential methane oxidation could be related to site characteristics with a model of methanotrophic biomass.

/pdf/methane-production-and-methane-consumption-a-review-of-2zsauk07pa.pdf

Methane production and methane consumption: a review of processes underlying wetland methane fluxes.

Oxygen and methane metabolism were measured using intact sediment cores taken from the profundal (147 m depth) of Lake Constance. Vertical O2 profiles were determined with O2 microelectrodes. Oxygen penetrated into the sediment to a depth of about 1.5–2.5 mm. The potential O2 consumption rates did not differ significantly between various sampling dates and sampling sites on the deep lake floor. Dissolved CH4 increased linearly between 2 and 20 cm depth resulting linearly between 2 and 10 cm depth resulting in a diffusive flux of about 369 μmol CH4 m−2 d−1 into the oxic sediment surface layer as calculated form Fick's law. Activities of methanogenesis were measured in slurried sediment subcores. Integration of these activities over 2–10 cm depth indicated a total production of 1400 μmol CH4 m−2 d−1. Incubation of intact sediment cores overlaid with O2-containing hypolimnetic water resulted in a flux of about 35 μmol CH4 m−2 d−1 out of the sediment into the water. However, as soon as dissolved O2 had decreased to less than about 18 μM O2, the CH4 flux abruptly increased to about 480 μmol CH4 m−2 d−1. This anaerobic CH4 flux was similar to the CH4 production estimated from the vertical distribution of dissolved CH4, but was much higher than the CH4 flux measured under aerobic conditions. Therefore, about 93% of the produced CH4 must have been oxidized within the oxic sediment surface layer by aerobic methanotrophic bacteria which consumed about > 9% of the O2 flux into the sediment.

Oxidation of methane in the oxic surface layer of a deep lake sediment (Lake Constance)

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

Maximum rates of CH4 production in the littoral sediment were observed in 2–5 cm depth The CH4 production rates increased during the year from about 5 mmol m−2d−1 in December to a maximum of about 95 mmol m−2d−1 in September CH4 production rates showed a temperature optimum at 30°C and an apparent activation energy of 76 kJ mol−1 A large part of the seasonality of CH4 production could be ascribed to the change of the sediment temperature Most of the produced CH4 was lost by ebullition Gas bubbles contained about 60–70% CH4 with an average δ13C of −562% and δD of −354%, and 2% CO2 with an average δ13C of −141% indicating that CH4 was produced from methyl carbon, ie mainly using acetate as methanogenic substrate This result was confirmed by inhibition of methanogenesis with chloroform which resulted in an accumulation rate of acetate equivalent to 81% of the rate of CH4 production Most probable numbers of methanogenic bacteria were in the order of 104 bacteria g−1dw sediment for acetate-, methanol- or formate-utilizing, and of 105 for H2-utilizing methanogens The turnover times of acetate were in the order of 23–48 h which, with in situ acetate concentrations of about 25–50 μM, resulted in rates of acetate turnover which were comparable to the rates of CH4 production The respiratory index (RI) showed that [2−14C]acetate was mainly used by methanogenesis rather than by respiratory processes, although the zone of CH4 production in the sediment overlapped with the zone of sulfate reduction

Methane production in littoral sediment of Lake Constance

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.

Bernward Thebrath

Papers

Oxidation of methane in the oxic surface layer of a deep lake sediment (Lake Constance)

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

Methane production in littoral sediment of Lake Constance

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