Microbial methanogenesis and acetate metabolism in a meromictic lake.

Methanogenesis and the anaerobic metabolism of acetate were examined in the sediment and water column of Knaack Lake, a small biogenic meromictic lake located in central Wisconsin. The lake was sharply stratified during the summer and was anaerobic below a depth of 3 m. Large concentrations (4,000 mumol/liter) of dissolved methane were detected in the bottom waters. A methane concentration maximum occurred at 4 m above the sediment. The production of (14)CH(4) from (14)C-labeled HCOOH, HCO(3) (-), and CH(3)OH and [2-(14)C]acetate demonstrated microbial methanogenesis in the water column of the lake. The maximum rate of methanogenesis calculated from reduction of H(14)CO(3) (-) by endogenous electron donors in the surface sediment (depth, 22 m) was 7.6 nmol/h per 10 ml and in the water column (depth, 21 m) was 0.6 nmol/h per 10 ml. The methyl group of acetate was simultaneously metabolized to CH(4) and CO(2) in the anaerobic portions of the lake. Acetate oxidation was greatest in surface waters and decreased with water depth. Acetate was metabolized primarily to methane in the sediments and water immediately above the sediment. Sulfide inhibition studies and temperature activity profiles demonstrated that acetate metabolism was performed by several microbial populations. Sulfide additions (less than 5 mug/ml) to water from 21.5 m stimulated methanogenesis from acetate, but inhibited CO(2) production. Sulfate addition (1 mM) had no significant effect on acetate metabolism in water from 21.5 m, whereas nitrate additions (10 to 14,000 mug/liter) completely inhibited methanogenesis and stimulated CO(2) formation.     

Effect of Fall Turnover on Terminal Carbon Metabolism in Lake Mendota Sediments

The carbon and electron flow pathways and the bacterial populations responsible for the transformation of H₂-CO₂, formate, methanol, methylamine, acetate, ethanol, and lactate were examined in eutrophic sediments collected during summer stratification and fall turnover. The rate of methane formation averaged 1,130 μmol of CH₄ per liter of sediment per day during late-summer stratification versus 433 μmol of CH₄ per liter of sediment per day during the early portion of fall turnover, whereas the rate of sulfate reduction was 280 μmol of sulfate per liter of sediment per day versus 1,840 μmol of sulfate per liter of sediment per day during the same time periods, respectively. The sulfate-reducing population remained constant while the methanogenic population decreased by one to two orders of magnitude during turnover. The acetate concentration increased from 32 to 81 μmol per liter of sediment while the acetate transformation rate constant decreased from 3.22 to 0.70 per h, respectively, during stratification versus turnover. Acetate accounted for nearly 100% of total sedimentary methanogenesis during turnover versus 70% during stratification. The fraction of ¹⁴CO₂ produced from all ¹⁴C-labeled substrates examined was 10 to 40% higher during fall turnover than during stratification. The addition of sulfate, thiosulfate, or sulfur to stratified sediments mimicked fall turnover in that more CO₂ and CH₄ were produced. The addition of Desulfovibrio vulgaris to sulfate-amended sediments greatly enhanced the amount of CO₂ produced from either [¹⁴C]methanol or [2-¹⁴C]acetate, suggesting that H₂ consumption by sulfate reducers can alter methanol or acetate transformation by sedimentary methanogens. These data imply that turnover dynamically altered carbon transformation in eutrophic sediments such that sulfate reduction dominated over methanogenesis principally as a consequence of altering hydrogen metabolism.     

Partitioning Effects during Terminal Carbon and Electron Flow in Sediments of a Low-Salinity Meltwater Pond near Bratina Island, McMurdo Ice Shelf, Antarctica

A study of anaerobic sediments below cyanobacterial mats of a low-salinity meltwater pond called Orange Pond on the McMurdo Ice Shelf at temperatures simulating those in the summer season (<5°C) revealed that both sulfate reduction and methane production were important terminal anaerobic processes. Addition of [2-¹⁴C]acetate to sediment samples resulted in the passage of label mainly to CO₂. Acetate addition (0 to 27 mM) had little effect on methanogenesis (a 1.1-fold increase), and while the rate of acetate dissimilation was greater than the rate of methane production (6.4 nmol cm⁻³ h⁻¹ compared to 2.5 to 6 nmol cm⁻³ h⁻¹), the portion of methane production attributed to acetate cleavage was <2%. Substantial increases in the methane production rate were observed with H₂ (2.4-fold), and H₂ uptake was totally accounted for by methane production under physiological conditions. Formate also stimulated methane production (twofold), presumably through H₂ release mediated through hydrogen lyase. Addition of sulfate up to 50-fold the natural levels in the sediment (interstitial concentration, ~0.3 mM) did not substantially inhibit methanogenesis, but the process was inhibited by 50-fold chloride (36 mM). No net rate of methane oxidation was observed when sediments were incubated anaerobically, and denitrification rates were substantially lower than rates for sulfate reduction and methanogenesis. The results indicate that carbon flow from acetate is coupled mainly to sulfate reduction and that methane is largely generated from H₂ and CO₂ where chloride, but not sulfate, has a modulating role. Rates of methanogenesis at in situ temperatures were four- to fivefold less than maximal rates found at 20°C.     

Terminal Processes in the Anaerobic Degradation of an Algal-Bacterial Mat in a High-Sulfate Hot Spring

The algal-bacterial mat of a high-sulfate hot spring (Bath Lake) provided an environment in which to compare terminal processes involved in anaerobic decomposition. Sulfate reduction was found to dominate methane production, as indicated by comparison of initial electron flow through the two processes, rapid conversion of [2-¹⁴C]acetate to ¹⁴CO₂ and not to ¹⁴CH₄, and the lack of rapid reduction of NaH¹⁴CO₃ to ¹⁴CH₄. Sulfate reduction was the dominant process at all depth intervals, but a marked decrease of sulfate reduction and sulfate-reducing bacteria was observed with depth. Concurrent methanogenesis was indicated by the presence of viable methanogenic bacteria and very low but detectable rates of methane production. A marked increased in methane production was observed after sulfate depletion despite high concentrations of sulfide (>1.25 mM), indicating that methanogenesis was not inhibited by sulfide in the natural environment. Although a sulfate minimum and sulfide maximum occurred in the region of maximal sulfate reduction, the absence of sulfate depletion in interstitial water suggests that methanogenesis is always severely limited in Bath Lake sediments. Low initial methanogenesis was not due to anaerobic methane oxidation.     

Substrates for Sulfate Reduction and Methane Production in Intertidal Sediments

The activity of and potential substrates for methane-producing bacteria and sulfate-reducing bacteria were examined in marsh, estuary, and beach intertidal sediments. Slow rates of methane production were detected in all sediments, although rates of sulfate reduction were 100- to 1,000-fold higher. After sulfate was depleted in sediments, the rates of methane production sharply increased. The addition of methylamine stimulated methanogenesis in the presence of sulfate, and [¹⁴C]methylamine was rapidly converted to ¹⁴CH₄ and ¹⁴CO₂ in freshly collected marsh sediment. Acetate, hydrogen, or methionine additions did not stimulate methanogenesis. [methyl-¹⁴C]methionine and [2-¹⁴C]acetate were converted to ¹⁴CO₂ and not to ¹⁴CH₄ in fresh sediment. No reduction of ¹⁴CO₂ to ¹⁴CH₄ occurred in fresh sediment. Molybdate, an inhibitor of sulfate reduction, inhibited [2-¹⁴C]acetate metabolism by 98.5%. Fluoracetate, an inhibitor of acetate metabolism, inhibited sulfate reduction by 61%. These results suggest that acetate is a major electron donor for sulfate reduction in marine sediments. In the presence of high concentrations of sulfate, methane may be derived from novel substrates such as methylamine.     

Fate of Immediate Methane Precursors in Low-Sulfate, Hot-Spring Algal-Bacterial Mats

The fates of acetate and carbon dioxide were examined in several experiments designed to indicate their relative contributions to methane production at various temperatures in two low-sulfate, hot-spring algal-bacterial mats. [2-¹⁴C]acetate was predominantly incorporated into cell material, although some ¹⁴CH₄ and ¹⁴CO₂ was produced. Acetate incorporation was reduced by dark incubation in short-term experiments and severely depressed by a 2-day preincubation in darkness. Autoradiograms showed that acetate was incorporated by long filaments resembling phototrophic microorganisms of the mat communities. [³H]acetate was not converted to C³H₄ in samples from Octopus Spring collected at the optimum temperature for methanogenesis. NaH¹⁴CO₃ was readily converted to ¹⁴CH₄ at temperatures at which methanogenesis was active in both mats. Comparisons of the specific activities of methane and carbon dioxide suggested that of the methane produced, 80 ± 6% in Octopus Spring and 71 ± 21% in Wiegert Channel were derived from carbon dioxide. Addition of acetate to 1 mM did not reduce the relative importance of carbon dioxide as a methane precursor in samples from Octopus Spring. Experiments with pure cultures of Methanobacterium thermoautotrophicum suggested that the measured ratio of specific activities might underestimate the true contribution of carbon dioxide in methanogenesis.     

Methane production in meromictic Ace Lake,Antarctica

Methane occurred in the monimolimnion, at depths greater than 11 m, of an antarctic meromictic lake, Ace Lake (depth 24.7 m). Although the water of the lake was of approximate marine salinity, bottom waters were depleted in sulfate (less than 1 mmol 1^–1). The temperature of the bottom waters of the lake were constantly between 1 °C and 2 °C. Rates of methanogenesis from ¹⁴C-labelled precursors (bicarbonate, formate and acetate) were determined in time course experiments with the detection of ¹⁴CH₄ produced by a gas chromatography-gas proportional counting system. Rates of ¹⁴CH₄ production were difficult to determine as the reactions were always near our limit of detection.Reliable determinations of rates of methanogenesis at some depths using some precursors were obtained, the fastest rate being 2.5 µmol kg^–1 day^–1 at depth 20 m. Assuming constant rates of methanogenesis with time, this would equate to a turnover of methane in the lake every two years.The slow rate of methanogenesis suggests that the methanogens in Ace Lake may be working at well below their optimum temperature although definitive statements regarding the presence of psychrophilic methanogens in this antarctic lake must await isolation attempts or longer field studies using alternative methodologies.     

Selective Inhibition by 2-Bromoethanesulfonate of Methanogenesis from Acetate in a Thermophilic Anaerobic Digestor

The effects of 2-bromoethanesulfonate, an inhibitor of methanogenesis, on metabolism in sludge from a thermophilic (58°C) anaerobic digestor were studied. It was found from short-term experiments that 1 μmol of 2-bromoethanesulfonate per ml completely inhibited methanogenesis from ¹⁴CH₃COO⁻, whereas 50 μmol/ml was required for complete inhibition of ¹⁴CO₂ reduction. When 1 μmol of 2-bromoethanesulfonate per ml was added to actively metabolizing sludge which was then incubated for 24 h. it caused a 60% reduction in methanogenesis and a corresponding increase in acetate accumulation; at 50 μmol/ml it caused complete inhibition of methanogenesis and accumulation of acetate. H₂, and ethanol.     

Carbon and Electron Flow in Mud and Sandflat Intertidal Sediments at Delaware Inlet, Nelson, New Zealand

An investigation of carbon and electron flow in mud and sandflat intertidal sediments showed that the terminal electron acceptor was principally sulfate and that the carbon flow was mainly to CO₂. Studies with thin layers of sediment exposed to H₂ showed that methane production accounted for virtually none of the H₂ utilized, whereas sulfate reduction accounted for a major proportion of the gas uptake. At all sampling sites except one (site B7), rates of methanogenesis were low but sulfate concentrations in the interstitial water were high (>18 mM). At site B7, the sulfate concentrations declined with depth from 32 mM at 2 cm to <1 mM at 10 cm or below, and active methanogenesis occurred in the low-sulfate zone. Sulfate-reducing activity at this site initially decreased and then increased with depth so that elevated rates occurred in both the active and nonactive methanogenic zones. The respiratory index (RI) [RI = ¹⁴CO₂/(¹⁴CO₂ + ¹⁴CH₄)] for [2-¹⁴C]acetate catabolism at site B7 ranged from 0.98 to 0.2 in the depth range of 2 to 14 cm. Addition of sulfate to sediment from the low-sulfate zone resulted in an increase in RI and a decrease in methanogenesis. At all other sites examined, RI ranged from 0.97 to 0.99 and was constant with depth. The results suggested that although methanogenesis was inhibited by sulfate (presumably through the activity of sulfate-reducing bacteria), it was not always limited by sulfate reduction.     

Intermediary Metabolism of Organic Matter in the Sediments of a Eutrophic Lake

The rates, products, and controls of the metabolism of fermentation intermediates in the sediments of a eutrophic lake were examined. ¹⁴C-fatty acids were directly injected into sediment subcores for turnover rate measurements. The highest rates of acetate turnover were in surface sediments (0- to 2-cm depth). Methane was the dominant product of acetate metabolism at all depths. Simultaneous measurements of acetate, propionate, and lactate turnover in surface sediments gave turnover rates of 159, 20, and 3 μM/h, respectively. [2-¹⁴C]propionate and [U-¹⁴C]lactate were metabolized to [¹⁴C]acetate, ¹⁴CO₂, and ¹⁴CH₄. [¹⁴C]formate was completely converted to ¹⁴CO₂ in less than 1 min. Inhibition of methanogenesis with chloroform resulted in an immediate accumulation of volatile fatty acids and hydrogen. Hydrogen inhibited the metabolism of C₃-C₅ volatile fatty acids. The rates of fatty acid production were estimated from the rates of fatty acid accumulation in the presence of chloroform or hydrogen. The mean molar rates of production were acetate, 82%; propionate, 13%; butyrates, 2%; and valerates, 3%. A working model for carbon and electron flow is presented which illustrates that fermentation and methanogenesis are the predominate steps in carbon flow and that there is a close interaction between fermentative bacteria, acetogenic hydrogen-producing bacteria, and methanogens.     

Substrate limitation of sediment methane flux,methane oxidation and use of stable isotopes for assessing methanogenesis pathways in a small arctic lake

Among predicted impacts of climate change in the Arctic are greater thaw depth and shifts in vegetation patterns and hydrology that are likely to increase organic carbon and nutrient loading to lakes. We measured substrate limitation of sediment methane (CH₄) flux, examined pathways of methanogenesis, and potential CH₄ oxidation using stable isotope labeled acetate in intact sediment cores from arctic lake GTH 112 (68°40′20″N, 149°14′57″W). We hypothesized that the acetoclastic pathway would dominate methanogenesis, reflecting dissolved organic carbon supply from the surrounding landscape, and that sediment CH₄ flux would be stimulated by addition of acetate. Experiments demonstrated acetate limitation of sediment CH₄ flux with short-term CH₄ flux response to availability of acetate, high rates of CH₄ oxidation, and strong dominance of the acetoclastic over the hydrogenotrophic methanogenic pathway. The experiments also indicated that isotopic fractionation effects during isotope enrichment experiments are large during methanogenesis and can alter the methanogenic pathways being investigated. Under oxic conditions, CH₄ oxidation at the sediment–water interface or in the water column is likely to account for much of diffusive CH₄ flux, but under anoxic hypolimnetic conditions and increased substrate availability, conditions that are likely to occur with climate change, sediment CH₄ flux will likely increase, with oxidation utilizing a smaller portion of sediment CH₄ production.     

Methanogenic Pathway and Archaeal Community Structure in the Sediment of Eutrophic Lake Dagow: Effect of Temperature

Methanogenic degradation of organic matter is an important microbial process in lake sediments. Temperature may affect not only the rate but also the pathway of CH₄ production by changing the activity and the abundance of individual microorganisms. Therefore, we studied the function and structure of a methanogenic community in anoxic sediment of Lake Dagow, a eutrophic lake in north-eastern Germany. Incubation of sediment samples (in situ 7.5°C) at increasing temperatures (4, 10, 15, 25, 30°C) resulted in increasing production rates of CH₄ and CO₂ and in increasing steady-state concentrations of H₂. Thermodynamic conditions for H₂/CO₂ -dependent methanogenesis were only exergonic at 25 and 30°C. Inhibition of methanogenesis with chloroform resulted in the accumulation of methanogenic precursors, i.e., acetate, propionate, and isobutyrate. Mass balance calculations indicated that less CH₄ was formed via H₂ at 4°C than at 30°C. Conversion of ¹⁴CO₂ to ¹⁴CH₄ also showed that H₂/CO₂ -dependent methanogenesis contributed less to total CH₄ production at 4°C than at 30°C. [2–¹⁴ C]Acetate turnover rates at 4°C accounted for a higher percentage of total CH₄ production than at 30°C. Collectively, these results showed a higher contribution of H₂-dependent methanogenesis and a lower contribution of acetate-dependent methanogenesis at high versus low temperature. The archaeal community was characterized by cloning, sequencing, and phylogenetic analysis of the 16S rRNA genes retrieved from the sediment. Sequences were affiliated with Methanosaetaceae, Methanomicrobiaceae, and three deeply branching euryarchaeotal clusters, i.e., group III, Rice cluster V, and a novel euryarchaeotal cluster, the LDS cluster. Terminal restriction fragment length polymorphism (T-RFLP) analysis showed that 16S rRNA genes affiliated to Methanosaetaceae (20–30%), Methanomicrobiaceae (35–55%), and group III (10–25%) contributed most to the archaeal community. Incubation of the sediment at different temperatures (4–30°C) did not result in a systematic change of the archaeal community composition, indicating that change of temperature primarily affected the activity rather than the structure of the methanogenic community.     

Measurement of Monosaccharides and Conversion of Glucose to Acetate in Anoxic Rice Field Soil

Degradation of glucose has been implicated in acetate production in rice field soil, but the abundance of glucose, the temporal change of glucose turnover, and the relationship between glucose and acetate catabolism are not well understood. We therefore measured the pool sizes of glucose and acetate in rice field soil and investigated the turnover of [U-¹⁴C]glucose and [2-¹⁴C]acetate. Acetate accumulated up to about 2 mM during days 5 to 10 after flooding of the soil. Subsequently, methanogenesis started and the acetate concentration decreased to about 100 to 200 μM. Glucose always made up >50% of the total monosaccharides detected. Glucose concentrations decreased during the first 10 days from 90 μM initially to about 3 μM after 40 days of incubation. With the exception at day 0 when glucose consumption was slow, the glucose turnover time was in the range of minutes, while the acetate turnover time was in the range of hours. Anaerobic degradation of [U-¹⁴C]glucose released [¹⁴C]acetate and ¹⁴CO₂ as the main products, with [¹⁴C]acetate being released faster than ¹⁴CO₂. The products of [2-¹⁴C]acetate metabolism, on the other hand, were ¹⁴CO₂ during the reduction phase of soil incubation (days 0 to 15) and ¹⁴CH₄ during the methanogenic phase (after day 15). Except during the accumulation period of acetate (days 5 to 10), approximately 50 to 80% of the acetate consumed was produced from glucose catabolism. However, during the accumulation period of acetate, the rate of acetate production from glucose greatly exceeded that of acetate consumption. Under steady-state conditions, up to 67% of the CH₄ was produced from acetate, of which up to 56% was produced from glucose degradation.     

Die-back of Phragmites australis: influence on the distribution and rate of sediment methanogenesis

Methanogenesis was measured during the summer of 1994, in sediment coresand bulk samples from a Phragmites australis wetland in northern Jutland,Denmark. We compared sediment from healthy reed and dying-back reed, andan open lagoon resulting from die-back. Cores revealed variability withdepth and between sites, with the highest rates coinciding with layers oforganic gyttja, and negligible methane production from the underlying sandbase. Methanogenesis rates in the lagoon and die back sites were higher(up to 100–150 nmol h^-1 g^-1dry wt. sediment) than in the healthy reed (50–80 nmolh^-1 g^-1), with the highest rates being recordedfrom May to July. At these times, methanogenesis was markedly temperature-limited; samples incubated at 30 °C anon-limiting temperature, gave rates as high as 200–400nmol h^-1 g^-1 for the lagoon and die-backareas and 150 nmol h^-1 g^-1 for the healthyarea. Addition of 8 mM acetate and H₂/CO₂headspace suggested that both acetate-fermenting andCO₂-reducing bacteria were present. Acetate additions suggested some co-limitation by substrate availability, with acetate limitation occurring in the healthy site during July and in the die-back site during August. Lower rates during August, especially in the healthy area, were associated with low water levels which resulted in more oxidized sediments. The data reveal highly variable methanogenesis in the sediment which, when considered with sediment depths, indicates that sites of Phragmites die-back have significantly greater rates of anaerobic mineralization than surrounding healthy wetland, and may be intense sources of methane.     

Seasonal Rates of Methane Oxidation in Anoxic Marine Sediments

Methane concentrations and rates of methane oxidation were measured in intact sediment cores from an inshore marine sediment at Jutland, Denmark. The rates of methane oxidation, determined by the appearance of ¹⁴CO₂ from injected ¹⁴CH₄, varied with sediment depth and season. Most methane oxidation was anoxic, but oxygen may have contributed to methane oxidation at the sediment surface. Cumulative rates (0- to 12-cm depth) for methane oxidation at Kysing Fjord were 3.34, 3.48, 8.60, and 17.04 μmol m⁻² day⁻¹ for April (4°C), May (13°C), July (17°C), and August (21°C), respectively. If all of the methane was oxidized by sulfate, it would account for only 0.01 to 0.06% of the sulfate reduction. The data indicate that methane was produced, in addition to being oxidized, in the 0- to 18-cm sediment stratum.     

Effect of Temperature on Anaerobic Ethanol Oxidation and Methanogenesis in Acidic Peat from a Northern Wetland

The effects of temperature on rates and pathways of CH₄ production and on the abundance and structure of the archaeal community were investigated in acidic peat from a mire in northern Scandinavia (68°N). We monitored the production of CH₄ and CO₂ over time and measured the turnover of Fe(II), ethanol, and organic acids. All experiments were performed with and without specific inhibitors (2-bromoethanesulfonate [BES] for methanogenesis and CH₃F for acetoclastic methanogenesis). The optimum temperature for methanogenesis was 25°C (2.3 μmol CH₄ · g [dry weight]⁻¹ · day⁻¹), but the activity was relatively high even at 4°C (0.25 μmol CH₄ · g [dry weight]⁻¹ · day⁻¹). The theoretical lower limit for methanogenesis was calculated to be at −5°C. The optimum temperature for growth as revealed by real-time PCR was 25°C for both archaea and bacteria. The population structure of archaea was studied by terminal restriction fragment length polymorphism analysis and remained constant over a wide temperature range. Hydrogenotrophic methanogenesis accounted for about 80% of the total methanogenesis. Most 16S rRNA gene sequences that were affiliated with methanogens and all McrA sequences clustered with the exclusively hydrogenotrophic order Methanobacteriales, correlating with the prevalence of hydrogenotrophic methanogenesis. Fe reduction occurred parallel to methanogenesis and was inhibited by BES, suggesting that methanogens were involved in Fe reduction. Based upon the observed balance of substrates and thermodynamic calculations, we concluded that the ethanol pool was oxidized to acetate by the following two processes: syntrophic oxidation with methanogenesis (i) as an H₂ sink and (ii) as a reductant for Fe(III). Acetate accumulated, but a considerable fraction was converted to butyrate, making volatile fatty acids important end products of anaerobic metabolism.     

One-carbon metabolism in methanogens: evidence for synthesis of a two-carbon cellular intermediate and unification of catabolism and anabolism in Methanosarcina barkeri

One-carbon metabolic transformations associated with cell carbon synthesis and methanogenesis were analyzed by long- and short-term ¹⁴CH₃OH or ¹⁴CO₂ incorporation studies during growth and by cell suspensions. ¹⁴CH₃OH and ¹⁴CO₂ were equivalently incorporated into the major cellular components (i.e., lipids, proteins, and nucleic acids) during growth on H₂-CO₂-methanol. ¹⁴CH₃OH was selectively incorporated into the C-3 of alanine with decreased amounts fixed in the C-1 and C-2 positions, whereas ¹⁴CO₂ was selectively incorporated into the C₁ moiety with decreasing amounts assimilated into the C-2 and C-3 atoms. Notably, ¹⁴CH₄ and [3-¹⁴C]alanine synthesized from ¹⁴CH₃OH during growth shared a common specific activity distinct from that of CO₂ or methanol. Cell suspensions synthesized acetate and alanine from ¹⁴CO₂. The addition of iodopropane inhibited acetate synthesis but did not decrease the amount of ¹⁴CH₃OH or ¹⁴CO₂ fixed into one-carbon carriers (i.e., methyl coenzyme M or carboxydihydromethanopterin). Carboxydihydromethanopterin was only labeled from ¹⁴CH₃OH in the absence of hydrogen. Cell extracts catalyzed the synthesis of acetate from ¹⁴CO (~1 nmol/min per mg of protein) and an isotopic exchange between CO₂ or CO and the C-1 of pyruvate. Acetate synthesis from ¹⁴CO was stimulated by methyl B₁₂ but not by methyl tetrahydrofolate or methyl coenzyme M. Methyl coenzyme M and coenzyme M were inhibitory to acetate synthesis. Cell extracts contained high levels of phosphotransacetylase (>6 μmol/min per mg of protein) and acetate kinase (>0.14 μmol/min per mg of protein). It was not possible to distinguish between acetate and acetyl coenzyme A as the immediate product of two-carbon synthesis with the methods employed.     

Influence of pH on Terminal Carbon Metabolism in Anoxic Sediments from a Mildly Acidic Lake

The carbon and electron flow pathways and the bacterial populations responsible for transformation of H₂-CO₂, formate, methanol, methylamine, acetate, glycine, ethanol, and lactate were examined in sediments collected from Knaack Lake, Wis. The sediments were 60% organic matter (pH 6.2) and did not display detectable sulfate-reducing activity, but they contained the following average concentration (in micromoles per liter of sediment) of metabolites and end products: sulfide, 10; methane, 1,540; CO₂, 3,950; formate, 25; acetate, 157; ethanol, 174; and lactate, 138. Methane was produced predominately from acetate, and only 4% of the total CH₄ was derived from CO₂. Methanogenesis was limited by low environmental temperature and sulfide levels and more importantly by low pH. Increasing in vitro pH to neutral values enhanced total methane production rates and the percentage of CO₂ transformed to methane but did not alter the amount of ¹⁴CO₂ produced from [2-¹⁴C]acetate (~24%). Analysis of both carbon transformation parameters with ¹⁴C-labeled tracers and bacterial trophic group enumerations indicated that methanogenesis from acetate and both heterolactic- and acetic acid-producing fermentations were important to the anaerobic digestion process.     

Methane Metabolism in a Temperate Swamp

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, SO₄^2-, 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 SO₄^2-, 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.     

Kinetic Parameters of the Conversion of Methane Precursors to Methane in a Hypereutrophic Lake Sediment

The kinetic parameters K_m, V_max, T_t (turnover time), and v (natural velocity) were determined for H₂ and acetate conversion to methane by Wintergreen Lake sediment, using short-term (a few hours) methods and incubation temperatures of 10 to 14°C. Estimates of the Michaelis-Menten constant, K_m, for both the consumption of hydrogen and the conversion of hydrogen to methane by sediment microflora averaged about 0.024 μmol g⁻¹ of dry sediment. The maximal velocity, V_max, averaged 4.8 μmol of H₂ g⁻¹ h⁻¹ for hydrogen consumption and 0.64 μmol of CH₄ g⁻¹ h⁻¹ for the conversion of hydrogen to methane during the winter. Estimated natural rates of hydrogen consumption and hydrogen conversion to methane could be calculated from the Michaelis-Menten equation and estimates of K_m, V_max, and the in situ dissolved-hydrogen concentration. These results indicate that methane may not be the only fate of hydrogen in the sediment. Among several potential hydrogen donors tested, only formate stimulated the rate of sediment methanogenesis. Formate conversion to methane was so rapid that an accurate estimate of kinetic parameters was not possible. Kinetic experiments using [2-¹⁴C]acetate and sediments collected in the summer indicated that acetate was being converted to methane at or near the maximal rate. A minimum natural rate of acetate conversion to methane was estimated to be about 110 nmol of CH₄ g⁻¹ h⁻¹, which was 66% of the V_max (163 nmol of CH₄ g⁻¹ h⁻¹). A 15-min preincubation of sediment with 5.0 × 10⁻³ atm of hydrogen had a pronounced effect on the kinetic parameters for the conversion of acetate to methane. The acetate pool size, expressed as the term K_m + S_n (S_n is in situ substrate concentration), decreased by 37% and T_t decreased by 43%. The V_max remained relatively constant. A preincubation with hydrogen also caused a 37% decrease in the amount of labeled carbon dioxide produced from the metabolism of [U-¹⁴C]valine by sediment heterotrophs.