Showing papers on "Methanogenesis published in 1994"

Iron and manganese in anaerobic respiration: environmental significance, physiology, and regulation

Abstract: Dissimilatory iron and/or manganese reduction is known to occur in several organisms, including anaerobic sulfur-reducing organisms such as Geobacter metallireducens or Desulfuromonas acetoxidans, and facultative aerobes such as Shewanella putrefaciens. These bacteria couple both carbon oxidation and growth to the reduction of these metals, and inhibitor and competition experiments suggest that Mn(IV) and Fe(III) are efficient electron acceptors similar to nitrate in redox abilities and capable of out-competing electron acceptors of lower potential, such as sulfate (sulfate reduction) or CO2 (methanogenesis). Field studies of iron and/or manganese reduction suggest that organisms with such metabolic abilities play important roles in coupling the oxidation of organic carbon to metal reduction under anaerobic conditions. Because both iron and manganese oxides are solids or colloids, they tend to settle downward in aquatic environments, providing a physical mechanism for the movement of oxidizing potential into anoxic zones. The resulting biogeochemical metal cycles have a strong impact on many other elements including carbon, sulfur, phosphorous, and trace metals.

Metabolic interactions between anaerobic bacteria in methanogenic environments.

Abstract: In methanogenic environments organic matter is degraded by associations of fermenting, acetogenic and methanogenic bacteria. Hydrogen and formate consumption, and to some extent also acetate consumption, by methanogens affects the metabolism of the other bacteria. Product formation of fermenting bacteria is shifted to more oxidized products, while acetogenic bacteria are only able to metabolize compounds when methanogens consume hydrogen and formate efficiently. These types of metabolic interaction between anaerobic bacteria is due to the fact that the oxidation of NADH and FADH2 coupled to proton or bicarbonate reduction in thermodynamically only feasible at low hydrogen and formate concentrations. Syntrophic relationships which depend on interspecies hydrogen or formate transfer were described for the degradation of e.g. fatty acids, amino acids and aromatic compounds.

Methanogenesis : Ecology, Physiology, Biochemistry and Genetics

Abstract: Preface-- J G Ferry Contributors An historical overview of methanogenesis-- R S Wolfe Part I: Microbiology: Diversity and taxonomy of methanogens -- D R Boone, W B Whitman, and P Rouviere Microscopy-- G D Sprott and T H Beveridge Physiological ecology of methanogens-- S H Zinder Part II: Biochemistry: Reactions and enzymes involved in methanogenesis from CO2 and H2-- R K Thauer, R Hedderich, and R Fischer Conversion of methanol and methylamines to methane and carbon dioxide-- J T Keltjens and G D Vogels Fermentation of acetate-- J G Ferry Redox enzymes of methanogens: physiochemical properties of selected, purified oxidoreductases-- D A Grahame and T C Stadtman Bioenergetics of methanogens-- V Muller, M Blaut and G Gottschalk Part III: Biosynthesis: Biosynthesis of the coenzymes in methanogens -- R H White and D Zhou Anabolic pathways in methanogens -- P G Simpson and W B Whitman Nitrogen and phosphorus metabolism of methanogens-- E DeMoll Part IV: Genetics: Structure and organization of genes-- J N Reeve Index

Sulfate reduction in methanogenic bioreactors

Abstract: In the anaerobic treatment of sulfate-containing wastewater, sulfate reduction interferes with methanogenesis. Both mutualistic and competitive interactions between sulfate-reducing bacteria and methanogenic bacteria have been observed. Sulfate reducers will compete with methanogens for the common substrates hydrogen, formate and acetate. In general, sulfate reducers have better growth kinetic properties than methanogens, but additional factors which may be of importance in the competition are adherence properties, mixed substrate utilization, affinity for sulfate of sulfate reducers, relative numbers of bacteria, and reactor conditions such as pH, temperature and sulfide concentration. Sulfate reducers also compete with syntrophic methanogenic consortia involved in the degradation of substrates like propionate and butyrate. In the absence of sulfate these methanogenic consortia are very important, but in the presence of sulfate they are thought to be easily outcompeted by sulfate reducers. However, at relatively low sulfate concentrations, syntrophic degradation of propionate and butyrate coupled to HZ removal via sulfate reduction rather than via methanogenesis may become important. A remarkable feature of some sulfate reducers is their ability to grow fermentatively or to grow in syntrophic association with methanogens in the absence of sulfate.

Anaerobic degradation of toluene and o-xylene by a methanogenic consortium.

Abstract: Toluene and o-xylene were completely mineralized to stoichiometric amounts of carbon dioxide, methane, and biomass by aquifer-derived microorganisms under strictly anaerobic conditions. The source of the inoculum was creosote-contaminated sediment from Pensacola, Fla. The adaptation periods before the onset of degradation were long (100 to 120 days for toluene degradation and 200 to 255 days for o-xylene). Successive transfers of the toluene- and o-xylene-degrading cultures remained active. Cell density in the cultures progressively increased over 2 to 3 years to stabilize at approximately 10(9) cells per ml. Degradation of toluene and o-xylene in stable mixed methanogenic cultures followed Monod kinetics, with inhibition noted at substrate concentrations above about 700 microM for o-xylene and 1,800 microM for toluene. The cultures degraded toluene or o-xylene but did not degrade m-xylene, p-xylene, benzene, ethylbenzene, or naphthalene. The degradative activity was retained after pasteurization or after starvation for 1 year. Degradation of toluene and o-xylene was inhibited by the alternate electron acceptors oxygen, nitrate, and sulfate. Degradation was also inhibited by the addition of preferred substrates such as acetate, H2, propionate, methanol, acetone, glucose, amino acids, fatty acids, peptone, and yeast extract. These data suggest that the presence of natural organic substrates or contaminants may inhibit anaerobic degradation of pollutants such as toluene and o-xylene at contaminated sites.

Ecosystem and physiological controls over methane production in northern wetlands

Abstract: Peat chemistry appears to exert primary control over methane production rates in the Canadian Northern Wetlands Study (NOWES) area. We determined laboratory methane production rate potentials in anaerobic slurries of samples collected from a transect of sites through the NOWES study area. We related methane production rates to indicators of resistance to microbial decay (peat C: N and lignin: N ratios) and experimentally manipulated substrate availability for methanogenesis using ethanol (EtOH) and plant litter. We also determined responses of methane production to pH and temperature. Methane production potentials declined along the gradient of sites from high rates in the coastal fens to low rates in the interior bogs and were generally highest in surface layers. Strong relationships between CH4 production potentials and peat chemistry suggested that methanogenesis was limited by fermentation rates. Methane production at ambient pH responded strongly to substrate additions in the circumneutral fens with narrow lignin: N and C: N ratios (delta CH4/delta EtOH = 0.9-2.3 mg/g) and weakly in the acidic bogs with wide C: N and lignin: N ratios (delta CH4/delta EtOH = -0.04-0.02 mg/g). Observed Q(sub 10) values ranged from 1.7 to 4.7 and generally increased with increasing substrate availability, suggesting that fermentation rates were limiting. Titration experiments generally demonstrated inhibition of methanogenesis by low pH. Our results suggest that the low rates of methane emission observed in interior bogs during NOWES likely resulted from pH and substrate quality limitation of the fermentation step in methane production and thus reflect intrinsically low methane production potentials. Low methane emission rates observed during NOWES will likely be observed in other northern wetland regions with similar vegetation chemistry.

Temporal and spatial changes of terminal electron‐accepting processes in a petroleum hydrocarbon‐contaminated aquifer and the significance for contaminant biodegradation

Abstract: Measurements of dissolved hydrogen and other biologically active solutes in groundwater from a shallow petroleum hydrocarbon-contaminated aquifer indicate that the distribution of microbial terminal electron-accepting processes (TEAPs), such as methanogenesis, sulfate reduction, and ferric iron (Fe (III)) reduction, is highly dynamic in both time and space. Delivery of sulfate to methanogenic zones by infiltrating recharge or lateral transport can result in a TEAP shift from methanogenesis to sulfate reduction. Conversely, lack of recharge and consumption of available sulfate can result in a shift from sulfate reduction to methanogenesis. Temporal shifts between sulfate and Fe (III) reduction were also observed. Time lags associated with TEAP shifts ranged from less than 10 days to about months. The relation between TEAP and biodegradation rates of a variety of organic compounds indicate that biodegradation rates of petroleum hydrocarbons probably vary temporally and spatially in a contaminated aquifer.

Methane production during zooplankton grazing on marine phytoplankton

Abstract: Zooplankton produced CH, while grazing on marine phytoplankton during laboratory experiments. We consider anaerobic microniches within zooplankton digestive tracts to be the most likely site for such methanogenesis. Methane production appeared to be zooplankton species-specific rather than dependent on the phytoplankton species being grazed. The amount of CH, produced (4-20 nmol copepod-’ d-l) was sufficient under experimental conditions to make a significant contribution to the formation and maintenance of oceanic subsurface CH, maxima. The methanogenic substrates monomethylamine and trimethylamine were constituents of the diatom, dinoflagellate, and flagellate phytoplankton species used in the CH, production feeding experiments. Dimethylamine was present in two of the three algal species. Trimethylamine was the most abundant methylamine in all cases and was present in sufficient quantity to account for the CH, production observed during copepod feeding experiments. Methanogens in these experiments would need to convert only 412% of the phytoplankton methylamines available to copepods during grazing to produce CH, concentrations observed in the experimental flasks.

Performance of a high‐solids anaerobic digestion process under various ammonia concentrations

Abstract: Ammonia is produced during anaerobic digestion of protein-containing materials. While ammonia can be utilized by some members of the anaerobic population, excess ammonia can inhibit methanogenesis. High-solids anaerobic digestion may be especially sensitive to the effects of ammonia overproduction. This study explored the performance of a pilot scale high-solids anaerobic digestion process under various total ammonia nitrogen (TAN) concentrations. In general the high-solids digester could operate up to 1000 mg dm -3 without any inhibitory effect. The digester performed best when it was operated at TAN concentrations in the narrow range of 600 to 800 mg dm -3

The anaerobic treatment of low strength soluble wastewaters.

Abstract: Low strength soluble wastewaters with chemical oxygen demand (COD) of less than 2000 mg/I are mostly from food processing industries. They commonly contain simple substrates such as short- chain fatty acids, alcohols and carbohydrates. The application of anaerobic technology has been mostly directed towards the treatment of medium and high strength wastewaters rather than those of low strength. Problems limiting the treatment of dilute wastewaters are related to the wastewater and the reactor design. This dissertation investigates the application of the conventional upflow anaerobic sludge blanket (UASB) and its modification, the expanded granular sludge bed (EGSB), for the treatment of low strength soluble wastewaters. The main topics studied concern the wastewater related problems. Ile effect of dissolved oxygen on the methanogenic activity of granular sludges and the effect of low substrate levels inside reactors on the treatment performance were evaluated. Moreover, some aspects of reactor design related problems such as the retention of biomass and wastewaterbiomass contact were considered.Methanogens located in granular sludge have a high tolerance to oxygen. The concentration of oxygen found to cause 50% inhibition to methanogenic activity was between 7% and 41 % oxygen in the head space of flasks, which corresponded to 0.05 mg/ l and 6 mg/ l of dissolved oxygen prevailing in the media, respectively. The most important mechanism for the tolerance was the consumption of oxygen by facultative bacteria while metabolizing substrates. The most highly tolerant sludges had the highest respiration rates. The hypothesis considered is that anaerobic microenvironments are created inside granules protecting the methanogens. The absence of facultative substrate for respiring oxygen decreases the tolerance of methanognens to O 2 . The coexistence of methanogenic and facultative bacteria competing for substrate in one single bioreactor was explored under highly aerobic conditions, in order to verify the possible application of anaerobic-aerobic cocultures for the removal of recalcitrant pollutants. Simultaneous methane production and oxygen uptake occurred in an oxygen tolerant sludge while at least 2 mg/ l of dissolved oxygen was present in the media. The healthy co-culture was evident even after longer periods of oxygen exposure, when methane oxidizing bacteria eventually also developed.The feasibility of UASB and EGSB reactors at 30°C was demonstrated. In UASB reactors, COD removal efficiencies exceeded 95% at organic loading rates (OLR) up to 6.8 g COD/ l .d and influent COD concentrations (COD in ) ranging from 422 to 943 mg/ l , during the treatment of ethanol substrate. The efficiencies exceeded 86% at OLR up to 3.9 mg COD/ l .d when whey was used as a substrate. Below 630 mg COD/ l , acidification of whey instead of methanogenesis was the rate limiting step. The retention of biomass is not a problem in the UASB, but the mixing intensity is not high enough to decrease the biofilm diffusion limitation of substrate transport into granular biofilms. The EGSB was shown to have superior potentials compared with the UASB. COD removal efficiencies were above 80% at OLRs up to 12 g COD/ l .d with COD in as low as 100 to 200 mg/ l . The effect of low substrate levels was not significant in the EGSB due to the intense turbulent mixing regime obtained by applying high hydraulic and organic loads. The very low apparent K S value of 9.8 mg COD/ l found for the biofilms in the reactor, was comparable to the intrinsic K S values determined for the most predominant acetoclastic methanogen found in anaerobic bioreactors, Methanothrix soehngenii. This indicates that all transport limitations of substrate movement into the biofilms were overcome. Optimized operation without sludge washout is achieved when liquid upflow velocities (V up ) below 5.5 m/h are applied. The problem of sludge retention is also restricted when sludge flotation occurs due to the buoyancy forces of gas attached to biofilms. The required equilibrium between mixing intensity and sludge retention limits the operation of the EGSB to OLRs up to 7 g COD/ l .d and V up values ranging from 2.5 and 5.5 m/h. Both reactor studies confirmed that in practice dissolved oxygen does not constitute any detrimental effect on the treatment performance. Improved mixing intensity in the UASB and improved sludge retention in the EGSB will enable higher OLRs and lower COD in which can be tolerated, compared with those of this study.

Depth distribution of microbial production and oxidation of methane in northern boreal peatlands

Abstract: The depth distributions of anaerobic microbial methane production and potential aerobic microbial methane oxidation were assessed at several sites in both Sphagnum- and sedge-dominated boreal peatlands in Sweden, and compared with net methane emissions from the same sites. Production and oxidation of methane were measured in peat slurries, and emissions were measured with the closed-chamber technique. Over all eleven sites sampled, production was, on average, highest 12 cm below the depth of the average water table. On the other hand, highest potential oxidation of methane coincided with the depth of the average water table. The integrated production rate in the 0-60 cm interval ranged between 0.05 and 1.7 g CH4 m (-2) day(-) and was negatively correlated with the depth of the average water table (linear regression: r (2) = 0.50, P = 0.015). The depth-integrated potential CH4-oxidation rate ranged between 3.0 and 22.1 g CH4 m(-2) day(-1) and was unrelated to the depth of the average water table. A larger fraction of the methane was oxidized at sites with low average water tables; hence, our results show that low net emission rates in these environments are caused not only by lower methane production rates, but also by conditions more favorable for the development of CH4-oxidizing bacteria in these environments.

Carbon isotope effects associated with aceticlastic methanogenesis.

Abstract: The carbon isotope effects associated with synthesis of methane from acetate have been determined for Methanosarcina barkeri 227 and for methanogenic archaea in sediments of Wintergreen Lake, Michigan. At 37 degrees C, the 13C isotope effect for the reaction acetate (methyl carbon) --> methane, as measured in replicate experiments with M. barkeri, was - 21.3% +/- 0.3%. The isotope effect at the carboxyl portion of acetate was essentially equal, indicating participation of both positions in the rate-determining step, as expected for reactions catalyzed by carbon monoxide dehydrogenase. A similar isotope effect, - 19.2% +/- 0.3% was found for this reaction in the natural community (temperature = 20 degrees C). Given these observations, it has been possible to model the flow of carbon to methane within lake sediment communities and to account for carbon isotope compositions of evolving methane. Extension of the model allows interpretation of seasonal fluctuations in 13C contents of methane in other systems.

Establishment of hydrogen-utilizing bacteria in the rumen of the newborn lamb

Abstract: The development of hydrogenotrophic bacteria in the rumen of lambs was investigated by culture and labeling experiments. 14CO2 and 13CO2 incorporation by the rumen microflora of a 24-h-old lamb showed that while there was no labeled methane, double-labeled acetate was formed indicating the presence of hydrogen-dependent acetogenesis. In vitro counts from rumen fluid of 20-h-old lambs confirmed an extensive colonization of acetogenic bacteria while methanogens were absent. Methanogens appeared in the rumen of 30-h-old lambs, and as they developed there was a proportional decrease in the numbers of acetogens, indicating a competition for hydrogen between these two groups. Hydrogen-utilizing sulfate-reducing bacteria, which were established by the 3rd day after birth, did not seem to be affected by this competition.

Mesophilic syntrophic acetate oxidation during methane formation by a triculture at high ammonium concentration

Abstract: In a mesophilic (37°C) triculture at a high ammonium concentration and pH8, methanogenesis from acetate occurred via syntrophic acetate oxidation. Studies with 14C-labelled substrates showed that the amount of labelled methane formed from 1-14C-labelled acetate was equal to that formed from 2-14C-labelled acetate. Labelled methane was also formed from H14CO3-. These results clearly showed that both the methyl and carboxyl groups of acetate were oxidized to CO2 and that CO2 was reduced to CH4 through hydrogenotrophic methanogenesis. During growth of the triculture, a significant isotopic exchange between the carboxyl group of acetate and bicarbonate occurred. As a result, there was a decrease in the specific activity of 1-14C-acetate, and the production of 14CO2 was slightly higher from 1-14C- than from 2-14C-acetate. For each mole acetate degraded, 0.94 mol methane was formed; 9.2 mmol acetate was metabolized during the 294 days of incubation.

Metabolism of methanogens

Abstract: Methanogenic archaea convert a few simple compounds such as H2 + CO2, formate, methanol, methylamines, and acetate to methane. Methanogenesis from all these substrates requires a number of unique coenzymes, some of which are exclusively found in methanogens. H2-dependent CO2 reduction proceeds via carrier-bound C1 intermediates which become stepwise reduced to methane. Methane formation from methanol and methylamines involves the disproportionation of the methyl groups. Part of the methyl groups are oxidized to CO2, and the reducing equivalents thereby gained are subsequently used to reduce other methyl groups to methane. This process involves the same C1 intermediates that are formed during methanogenesis from CO2. Conversion of acetate to methane and carbon dioxide is preceded by its activation to acetyl-CoA. Cleavage of the latter compound yields a coenzyme-bound methyl moiety and an enzyme-bound carbonyl group. The reducing equivalents gained by oxidation of the carbonyl group to carbon dioxide are subsequently used to reduce the methyl moiety to methane. All these processes lead to the generation of transmembrane ion gradients which fuel ATP synthesis via one or two types of ATP synthases. The synthesis of cellular building blocks starts with the central anabolic intermediate acetyl-CoA which, in autotrophic methanogens, is synthesized from two molecules of CO2 in a linear pathway.

Importance of cobalt for individual trophic groups in an anaerobic methanol-degrading consortium.

Abstract: Methanol is an important anaerobic substrate in industrial wastewater treatment and the natural environment. Previous studies indicate that cobalt greatly stimulates methane formation during anaerobic treatment of methanolic wastewaters. To evaluate the effect of cobalt in a mixed culture, a sludge with low background levels of cobalt was cultivated in an upflow anaerobic sludge blanket reactor. Specific inhibitors in batch assays were then utilized to study the effect of cobalt on the growth rate and activity of different microorganisms involved in the anaerobic degradation of methanol. Only methylotrophic methanogens and acetogens were stimulated by cobalt additions, while the other trophic groups utilizing downstream intermediates, H2-CO2 or acetate, were largely unaffected. The optimal concentration of cobalt for the growth and activity of methanol-utilizing methanogens and acetogens was 0.05 mg liter-1. The higher requirement of cobalt is presumably due to the previously reported production of unique corrinoid-containing enzymes (or coenzymes) by direct utilizers of methanol. This distinctly high requirement of cobalt by methylotrophs should be considered during methanolic wastewater treatment. Methylotroph methanogens presented a 60-fold-higher affinity for methanol than acetogens. This result in combination with the fact that acetogens grow slightly faster than methanogens under optimal cobalt conditions indicates that acetogens can outcompete methanogens only when reactor methanol and cobalt concentrations are high, provided enough inorganic carbon is available.

Methanogens outcompete sulphate reducing bacteria for H2 in the human colon.

Abstract: Methanogens and sulphate reducing bacteria compete for H2 in the human colon, and, as a result, faeces usually contain high concentrations of just one of these two organisms. There is controversy over which of these organisms wins the competition for H2, although theoretical data suggest that sulphate reducing bacteria should predominate. To elucidate this question experiments were undertaken in which sulphate enriched homogenates of human sulphate reducing faeces and methane producing faeces were incubated separately or mixed together. Co-incubation of sulphate reducing faeces with methanogenic faeces resulted in a sixfold reduction in the activity of the sulphate reducing bacteria (measured as sulphide production), whereas methane production was not inhibited by co-incubation with sulphate reducing bacteria. Methanogenic faeces also consumed H2 more rapidly and reduced the H2 tension of the homogenate to a lower value than did sulphate reducing faecal samples. In these experiments, methanogens seem to outcompete sulphate reducing bacteria for H2.

Impact of gypsum application on the methane emission from a wetland rice field.

Abstract: Methane emission from Philippine rice paddies was monitored with a closed chamber technique during the 1991 and 1992 wet season. The methane emission from plots amended with 6.66 tons.ha−1 gypsum was reduced by 55–70% compared to non-amended plots. Although CH4 emission from fields with a high input of fresh organic matter was strongly enhanced, the experiments showed that the relative reduction in CH4 emission upon gypsum application was independent of organic matter addition. The reduced CH4 emission upon gypsum application was most likely due to inhibition of methanogenesis by sulfate-reducing bacteria. Observed SO42− concentrations in the soil solution of gypsum-amended plots were well above minimum concentrations reported in the literature for successful competition of sulfate-reducing bacteria with methanogens. The data provide a base for reducing the estimates of CH4 emissions from rice grown on high-sulfate containing soils or gypsum-amended soils.

Methane Metabolism in a Temperate Swamp

Abstract: Comparisons between in situ CH4 concentration and potential factors controlling its net production were made in a temperate swamp. Seasonal measurements of water table level and depth profiles of pH, dissolved CH4, CO2, O2, SO42-, NO3-, formate, acetate, propionate, and butyrate were made at two adjacent sites 1.5 to 2 m apart. Dissolved CH4 was inversely correlated to O2 and, in general, to NO3- and SO42-, potential inhibitors of methanogenesis. At low water table levels (August 1992), maximal CH4 (2 to 4 μM) occurred below 30 cm, whereas at high water table levels (October 1992) or under flooded conditions (May 1993), CH4 maxima (4 to 55 μM) occurred in the top 10 to 20 cm. Higher CH4 concentrations were likely supported by inputs of fresh organic matter from decaying leaf litter, as suggested by high acetate and propionate concentrations (25 to 100 μM) in one of the sites in fall and spring. Measurements of potential CH4 production (and consumption) showed that the highest rates generally occurred in the top 10 cm of soil. Soil slurry incubations confirmed the importance of organic matter to CH4 production but also showed that competition for substrates by nonmethanogenic microorganisms could greatly attenuate its effect.

Production and emission of methane in a brackish and a freshwater wetland

Abstract: There is no clear consensus on how environmental and biotic factors control microbiallymediated methane production in wetlands, as well as emission of this important ‘greenhouse gas’ from wetlands into the atmosphere. To provide insight, I studied rates of methane production and emission into the atmosphere, as well as factors controlling those rates, along a toposequence from non-flooded to seasonally flooded in a coastal meadow and in a fen in Denmark. Methane production was estimated from anaerobic soil slurries while emission was estimated from static flux chambers. Methane emission into the atmosphere averaged 0.04 μg C-CH4 dm−2 h-−1 in the coastal meadow and 1.9 μg C-CH4 dm−2 h−1 in the fen. A comparison of potential CH4 production and CH4 emission into the atmosphere showed that in the coastal meadow, but not in the fen, emission increased when production increased during summer. Relationships between potential CN4 production and soil water content as well as soil temperature are discussed. Arrhenius plots indicated strikingly similar temperature responses of CH4 production in the two wetlands. Also, both wetlands showed different temperature responses in saturated soils (Q10 = 3.1 and 3.6; Eh=79 and 84 kJ mol−1) compared to unsaturated soils (Q10 = 8.1 and 8.7; eh = 138 and 142 kJ mol−1). My results suggest that different types of methanogens inhabit saturated and unsaturated soils in both a coastal meadow and a fen. Overall, the study indicates that CH4 production in wetlands and CH4 emission into the atmosphere from wetlands are controlled by a complex set of environmental and biotic factors which differ between wetlands.

Spatial variability in biodegradation rates as evidenced by methane production from an aquifer.

Abstract: Accurate predictions of carbon and energy cycling rates in the environment depend on sampling frequencies and on the spatial variability associated with biological activities. We examined the variability associated with anaerobic biodegradation rates at two sites in an alluvial sand aquifer polluted by municipal landfill leachate. In situ rates of methane production were measured for almost a year, using anaerobic wells installed at two sites. Methane production ranged from 0 to 560 μmol · m-2 · day-1 at one site (A), while a range of 0 to 120,000 μmol · m-2 · day-1 was measured at site B. The mean and standard deviations associated with methane production at site A were 17 and 57 μmol · m-2 · day-1, respectively. The comparable summary statistics for site B were 2,000 and 9,900 μmol · m-2 · day-1. The coefficients of variation at sites A and B were 340 and 490%, respectively. Despite these differences, the two sites had similar seasonal trends, with the maximal rate of methane production occurring in summer. However, the relative variability associated with the seasonal rates changed very little. Our results suggest that (i) two spatially distinct sites exist in the aquifer, (ii) methanogenesis is a highly variable process, (iii) the coefficient of variation varied little with the rate of methane production, and (iv) in situ anaerobic biodegradation rates are lognormally distributed.

Competition between reductive acetogenesis and methanogenesis in the pig large-intestinal flora.

Abstract: Washed bacterial suspensions obtained from the pig hindgut were incubated under 13CO2 in a buffer containing NaH13CO3 and carbohydrates. Incorporation of 13C into short chain fatty acids was assayed by quantitative nuclear magnetic resonance. The effects of different levels of H2 added to the gas phase (0, 20 and 80% v/v) and of the specific methanogenesis inhibitor 2-bromoethane-sulphonic acid (BES) were determined. In control incubations increasing the concentration of H2 markedly increased methane production. Single- and double-labelled acetate and butyrate were formed in all incubations. In the absence of BES, increasing H2 significantly increased the incorporation of 13CO2 into butyrate and the proportion of double-labelled acetate in total labelled acetate. The addition of BES proved to be very successful as a methane inhibitor and greatly enhanced the amount of mono- and double-labelled acetate, especially at the highest H2 partial pressure. The results suggest that methanogenesis inhibited both routes of reductive acetogenesis, i.e. the homoacetate fermentation of hexose (represented for the most part by single labelling) and the synthesis of acetate from external CO2 and H2 (represented mostly by double labelling). A highly significant interaction between BES and H2 concentration was observed. At the highest pH2 BES increased the proportion of labelled acetate in total acetate from 17.1% for the control to 50.9%. It was concluded that although acetogenesis and methanogenesis can occur simultaneously in the pig hindgut, reductive acetogenesis may become a significant pathway of acetate formation in the absence of methanogenesis.

Syntrophic Acetate Oxidation and “Reversible Acetogenesis”

Abstract: Acetate is an important CH4 precursor in nature, accounting for two-thirds of the CH4 produced in many natural habitats and in anaerobic bioreactors. Although microbial methanogenesis from acetate was first described in the early 1900s, the mechanism of methanogenesis from acetate was controversial until 1978, when it was demonstrated that a pure culture of Methanosarcina barkeri could grow on acetate (Mah et al., 1978; Smith and Mah, 1978; Weimer and Zeikus, 1978) and convert acetate to CH4 by a decarboxylation mechanism sometimes called the aceticlastic reaction. With the description of a similar mechanism for Methanothrix soehngenii in 1980 (Zehnder et al., 1980), it appeared that acetate decarboxylation was “the” mechanism for methanogenesis from acetate.

Microbial colonization of different support materials used to enhance the methanogenic process

Abstract: Macrobial colonization of the different support materials used to enhance methane production in anaerobic digestors is rapid and occurs in the first 24 h of sludge incubation. Scanning electron microscopy studies reveal a predominant presence of filamentous methanogenic forms, closely resemblingMethanosaeta (Methanothrix), which are located on the outer layer and in the bacterial framework of the biofilm. These findings are consistent with the results obtained from microbial counts using both the most probable number and epifluorescence microscopic techniques, which show an increase in the numbers of aceticlastic methanogens compared to other microbial groups involved, such as sulphate-reducing bacteria, the numbers of which are similar to those obtained under the initial conditions. Moreover, a sharp increase in the bacterial counts is observed by using the epifluorescence microscopic technique applied to homogenized samples, probably due to the count of bacteria released from the support materials.