Oxygen effects on methane production and oxidation in humid tropical forest soils

We investigated the effects of oxygen (O₂) concentration on methane (CH₄) production and oxidation in two humid tropical forests that differ in long‐term, time‐averaged soil O₂ concentrations. We identified sources and sinks of CH₄ through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH₄ oxidized in situ. Complementary laboratory experiments were conducted to determine the effects of O₂ concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH₄ production occurred in soils that contained between 9±1.1% and 19±0.2% O₂. For example, we observed CH₄ concentrations in excess of 3% in soils with 9±1.1% O₂. CH₄ emissions from the lower O₂ sites were high (22–101 nmol CH₄ m^?2 s^?1), and were equal in magnitude to CH₄ emissions from natural wetlands. During peak periods of CH₄ efflux, carbon dioxide (CO₂) emissions became enriched in ¹³C because of high methanogenic activity. Gross CH₄ production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48–78% of the CH₄ produced was oxidized prior to atmospheric egress. O₂ availability influenced CH₄ oxidation more strongly than methanogenesis. Gross CH₄ production was relatively insensitive to O₂ concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH₄ production with increasing O₂ concentration, shifting the δ¹³C composition of CH₄ to values that were more positive. Isotopic measurements suggested that CO₂ was an important source of carbon for methanogenesis in humid forests. The δ¹³C value of methanogenesis was between ?84‰ and ?98‰, which is well within the range of CH₄ produced from CO₂ reduction, and considerably more depleted in ¹³C than CH₄ formed from acetate.     

Shifting microbial communities sustain multiyear iron reduction and methanogenesis in ferruginous sediment incubations

Reactive Fe(III) minerals can influence methane (CH₄) emissions by inhibiting microbial methanogenesis or by stimulating anaerobic CH₄ oxidation. The balance between Fe(III) reduction, methanogenesis, and CH₄ oxidation in ferruginous Archean and Paleoproterozoic oceans would have controlled CH₄ fluxes to the atmosphere, thereby regulating the capacity for CH₄ to warm the early Earth under the Faint Young Sun. We studied CH₄ and Fe cycling in anoxic incubations of ferruginous sediment from the ancient ocean analogue Lake Matano, Indonesia, over three successive transfers (500 days in total). Iron reduction, methanogenesis, CH₄ oxidation, and microbial taxonomy were monitored in treatments amended with ferrihydrite or goethite. After three dilutions, Fe(III) reduction persisted only in bottles with ferrihydrite. Enhanced CH₄ production was observed in the presence of goethite, highlighting the potential for reactive Fe(III) oxides to inhibit methanogenesis. Supplementing the media with hydrogen, nickel and selenium did not stimulate methanogenesis. There was limited evidence for Fe(III)‐dependent CH₄ oxidation, although some incubations displayed CH₄‐stimulated Fe(III) reduction. 16S rRNA profiles continuously changed over the course of enrichment, with ultimate dominance of unclassified members of the order Desulfuromonadales in all treatments. Microbial diversity decreased markedly over the course of incubation, with subtle differences between ferrihydrite and goethite amendments. These results suggest that Fe(III) oxide mineralogy and availability of electron donors could have led to spatial separation of Fe(III)‐reducing and methanogenic microbial communities in ferruginous marine sediments, potentially explaining the persistence of CH₄ as a greenhouse gas throughout the first half of Earth history.     

The methane cycle in ferruginous Lake Matano

In Lake Matano, Indonesia, the world’s largest known ferruginous basin, more than 50% of authigenic organic matter is degraded through methanogenesis, despite high abundances of Fe (hydr)oxides in the lake sediments. Biogenic CH₄ accumulates to high concentrations (up to 1.4 mmol L^?1) in the anoxic bottom waters, which contain a total of 7.4 × 10⁵ tons of CH₄. Profiles of dissolved inorganic carbon (ΣCO₂) and carbon isotopes (δ¹³C) show that CH₄ is oxidized in the vicinity of the persistent pycnocline and that some of this CH₄ is likely oxidized anaerobically. The dearth of NO₃^? and SO₄^2? in Lake Matano waters suggests that anaerobic methane oxidation may be coupled to the reduction of Fe (and/or Mn) (hydr)oxides. Thermodynamic considerations reveal that CH₄ oxidation coupled to Fe(III) or Mn(III/IV) reduction would yield sufficient free energy to support microbial growth at the substrate levels present in Lake Matano. Flux calculations imply that Fe and Mn must be recycled several times directly within the water column to balance the upward flux of CH₄. 16S gene cloning identified methanogens in the anoxic water column, and these methanogens belong to groups capable of both acetoclastic and hydrogenotrophic methanogenesis. We find that methane is important in C cycling, even in this very Fe‐rich environment. Such Fe‐rich environments are rare on Earth today, but they are analogous to conditions in the ferruginous oceans thought to prevail during much of the Archean Eon. By analogy, methanogens and methanotrophs could have formed an important part of the Archean Ocean ecosystem.     

Methane production in littoral sediment of Lake Constance

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 −56.2% and δD of −354%, and 2% CO₂ with an average δ¹³C of −14.1% indicating that CH₄ was produced from methyl carbon, i.e. 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⁻¹d.w. sediment for acetate-, methanol- or formate-utilizing, and of 10⁵ for H₂-utilizing methanogens. The turnover times of acetate were in the order of 2.3–4.8 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.     

Magnetite accelerates syntrophic acetate oxidation in methanogenic systems with high ammonia concentrations

Ammonia accumulation is a major inhibitory substance causing anaerobic digestion upset and failure in CH₄ production. At high ammonia levels, CH₄ production through syntrophic acetate oxidization (SAO) pathways is more tolerant to ammonia toxicity than the acetoclastic methanogenesis pathway, but the low CH₄ production rate through SAO constitutes the main reason for the low efficiency of energy recovery in anaerobic digesters treating ammonia‐rich substrates. In this study, we showed that acetate fermentation to CH₄ and CO₂ occurred through SAO pathway in the anaerobic reactors containing a high ammonia concentration (5.0 g l^?1 NH₄⁺–N), and the magnetite nanoparticles supplementation increased the CH₄ production rates from acetate by 36–58%, compared with the anaerobic reactors without magnetite under the same ammonia level. The mechanism of facilitated methanogenesis was proposed to be the establishment of direct interspecies electron transfer (DIET) for SAO, in which magnetite facilitated DIET between syntrophic acetate oxidizing bacteria and methanogens. High‐throughput 16S rRNA gene sequencing analysis revealed that the bacterial Geobacteraceae and the archaeal Methanosarcinaceae and Methanobacteriaceae might be involved in magnetite‐mediated DIET for SAO and CH₄ production. This study demonstrated that magnetite supplementation might provide an effective approach to accelerate CH₄ production rates in the anaerobic reactors treating wastewater containing high ammonia.     

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.     

Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

Wetlands are important sources of methane (CH₄) and sinks of carbon dioxide (CO₂). However, little is known about CH₄ and CO₂ fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH₄ and CO₂ in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO₂ but strong sources of CH₄, particularly during inundation when reducing conditions in soils increase CH₄ production and limit CO₂ release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH₄‐C m^?2 d^?1 and absorbed 1.6 ± 0.2 g CO₂‐C m^?2 d^?1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH₄ emissions decreased significantly (0.002 ± 0.001 g CH₄‐C m^?2 d^?1) but remained a net source, while the net CO₂ flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH₄ fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO₂ and CH₄ were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m^?2 (as CH₄‐C + CO₂‐C) in anaerobic phases and emitting 76 g C m^?2in aerobic phases), high CH₄ effluxes during the anaerobic flooded phase and modest CH₄ effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.     

The carbon and hydrogen stable isotope composition of bacteriogenic methane: A laboratory study using a landfill inoculum

Anaerobic bacterial degradation of landfill waste produces a globally significant source of the greenhouse gas methane. Stable isotopic measurements of methane [δ^I3C(CH₄) and δD(CH₄)] can often fingerprint different sources of methane (natural vs. anthro‐pogenic) and help identify the bacterial processes involved in methane production. Landfill microbial communities are complex and diverse, and hence so too is the biogeochem‐istry of methane formation. To investigate the influence of (l) the methane formation pathway (acetoclastic methanogenesis and CO₂ reduction), and (2) SD of water on the stable isotopic composition of landfill methane, two model butyrate‐degrading landfill systems were established. The systems were inoculated with domestic refuse from a landfill and incubated in the laboratory for 92 days. Both systems were identical except δD of water initially added to system 2 was 118% heavier than system 1. Between days 39 and 72 the systems were resupplemented with butyrate. Production of CH₄ and CO₂ and changes in volatile fatty acid concentration confirmed that active methanogenic populations had been established. CH₄ became ¹³C enriched in both incubations with time. Interpreting changes in acetate, butyrate, and propionate concentration during incubation is complicated, but these observations and other information suggest that the dominant methanogenic substrate changed front CO₂/H₂ to acetate as the experiment progressed. This is also consistent with the observed ¹³C enrichment of CH₄, as ¹³C discrimination during methane production from acetate is less than from CO₂. In contrast, δD(CH₄) remained relatively constant, suggesting that this measurement may not provide a reliable basis for distinguishing between CH₄ from CO₂ reduction and acetoclastic methanogenesis, as has previously been suggested.     

Winter precipitation and snow accumulation drive the methane sink or source strength of Arctic tussock tundra

Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH₄ emissions may represent a significant climate forcing feedback, long‐term impacts of changes in snow accumulation on CH₄ fluxes remain uncertain. We measured ecosystem CH₄ fluxes and soil CH₄ and CO₂ concentrations and ¹³C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH₄ flux over the growing season (Jun–Aug) after 18 years of experimental snow depth increases and decreases. Deeper snow increased soil wetness and warming, reducing soil %O₂ levels and increasing thaw depth. Soil moisture, through changes in soil %O₂ saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH₄ sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH₄ (Fox) by 75–120% compared to Ambient, switching the system from a small source to a net CH₄ sink (21 ± 2 and ?31 ± 1 mg CH₄ m^?2 season^?1 at Ambient and RS). Deeper snow reduced Fox by 35–40% and 90–100% in medium‐ (MS) and high‐ (HS) snow additions relative to Ambient, contributing to increasing the CH₄ source strength of moist acidic tundra (464 ± 15 and 3561 ± 97 mg CH₄ m^?2 season^?1 at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant‐mediated CH₄ transport associated with the expansion of tall graminoids. Deeper snow enhanced CH₄ production within newly thawed soils, responding mainly to soil warming rather than to increases in acetate fermentation expected from thaw‐induced increases in SOC availability. Our results suggest that increased winter precipitation will increase the CH₄ source strength of Arctic tundra, but the resulting positive feedback on climate change will depend on the balance between areas with more or less snow accumulation than they are currently facing.     

Methane emissions from tropical freshwater wetlands located in different climatic zones of Costa Rica

Wetlands are the largest natural source of the greenhouse gas methane to the atmosphere. Despite the fact that a large percentage of wetlands occur in tropical latitudes, methane emissions from natural tropical wetlands have not been extensively studied. The objective this research was to compare methane emissions from three natural tropical wetlands located in different climatic and ecological areas of Costa Rica. Each wetland was within a distinct ecosystem: (1) a humid flow‐through wetland slough with high mean annual temperatures (25.9 °C) and precipitation (3700 mm yr^?1); (2) a stagnant rainforest wetland with high mean annual temperatures (24.9 °C) and precipitation (4400 mm yr^?1); or (3) a seasonally wet riverine wetland with very high mean annual temperatures (28.2 °C) and lower mean annual precipitation (1800 mm yr^?1). Methane emission rates were measured from sequential gas samples using nonsteady state plastic chambers during six sampling periods over a 29‐month period from 2006 to 2009. Methane emissions were higher than most rates previously reported for tropical wetlands with means (medians) of 91 (52), 601 (79), and 719 (257) mg CH₄‐C m^?2 day^?1 for the three sites, with highest rates seen at the seasonally flooded wetland site. Methane emissions were statistically higher at the seasonally wet site than at the humid sites (P<0.001). Highest methane emissions occurred when surface water levels were between 30 and 50 cm. The interaction of soil temperature, water depth, and seasonal flooding most likely affected methanogenesis in these tropical sites. We estimate that Costa Rican wetlands produce about 0.80 Tg yr^?1 of methane, or approximately 0.6% of global tropical wetland emissions. Elevated methane emissions at the seasonally wet/warmer wetland site suggest that some current humid tropical freshwater wetlands of Central America could emit more methane if temperatures increase and precipitation becomes more seasonal with climate change.     

In Situ Cultivation of Subsurface Microorganisms in a Deep Mafic Sill: Implications for SLiMEs

An in situ culturing device was incubated within a flowing borehole in a mafic sill at 1.474 km depth in Evander Au mine, South Africa. The device was designed to enrich methanogenic, Fe^{3 +}-reducing and SO₄ ^{2 ?}-reducing microorganisms using acetate, formate, methanol, Fe^{3 +}-citrate and SO₄ ^{2 ?} enriched agar and sand cartridges. At the end of the 33 day incubation geochemical analyses detected elevated H₂, acetate, CH₄ and Fe concentrations and depleted SO₄ ^{2 ?} concentrations. 16S rDNA sequences and PLFA analyses revealed that the microbial community composition of the substrate-bearing cartridges were distinct from that of the original borehole water and the non-substrate-bearing control cartridge. 16S rDNA and dissimilatory sulfite reductase, dsrAB, gene sequences indicated the device successfully targeted SO₄ ^{2 ?} reducing bacteria (SRB), which were not detected in the original borehole water. 16S rDNA sequences also revealed a shift in the microbial community from one relying on H₂ based methanogenesis to one suggestive of H₂ based acetogenesis supporting aceticlastic methanogenesis and SO₄ ^{2 ?} reduction compatible with the subsurface lithoautotrophic hypothesis.     

Methanogenesis produces strong 13C enrichment in stromatolites of Lagoa Salgada,Brazil: a modern analogue for Palaeo‐/Neoproterozoic stromatolites?

Holocene stromatolites characterized by unusually positive inorganic δ¹³C_PDB values (i.e. up to +16‰) are present in Lagoa Salgada, a seasonally brackish to hypersaline lagoon near Rio de Janeiro (Brazil). Such positive values cannot be explained by phototrophic fixation of CO₂ alone, and they suggest that methanogenesis was a dominating process during the growth of the stromatolites. Indeed, up to 5 mm methane was measured in the porewater. The archaeal membrane lipid archaeol showing δ¹³C values between ?15 and 0‰ suggests that archaea are present and producing methane in the modern lagoon sediment. Moreover, ¹³C‐depleted hopanoids diplopterol and 3β‐methylated C₃₂ 17β(H),21β(H)‐hopanoic acid (both ?40‰) are preserved in lagoon sediments and are most likely derived from aerobic methanotrophic bacteria thriving in the methane‐enriched water column. Loss of isotopically light methane through the water column would explain the residual ¹³C‐enriched pool of dissolved inorganic carbon from where the carbonate constituting the stromatolites precipitated. The predominance of methanogenic archaea in the lagoon is most likely a result of sulphate limitation, suppressing the activity of sulphate‐reducing bacteria under brackish conditions in a seasonally humid tropical environment. Indeed, sulphate‐reduction activity is very low in the modern sediments. In absence of an efficient carbonate‐inducing metabolic process, we propose that stromatolite formation in Lagoa Salgada was abiotically induced, while the ¹³C‐enriched organic and inorganic carbon pools are due to methanogenesis. Unusually, ¹³C‐enriched stromatolitic deposits also appear in the geological record of prolonged periods in the Palaeo‐ and Neoproterozoic. Lagoa Salgada represents a possible modern analogue to conditions that may have been widespread in the Proterozoic, at times when low sulphate concentrations in sea water allowed methanogens to prevail over sulphate‐reducing bacteria.     

Evidence for Anaerobic CH 4 Oxidation in Freshwater Peatlands

This study involved in vitro assays of peat soil to investigate the occurrence, importance and potential mechanism(s) of anaerobic methane oxidation (AOM) in several northern peatlands ranging from ombrotrophic bog to minerotrophic fen. Although strong evidence suggests that AOM is linked to sulfate reduction in marine sediments, very little is known about AOM in freshwater systems such as northern peatlands, which have large methane (CH ₄ ) production and are a significant source of atmospheric CH ₄ . Our results showed a mean net AOM rate of 17 ± 2.6 nmol kg ^{? 1} s ^{? 1} with a maximum rate of 176 nmol kg ^{? 1} s ^{? 1} for a minerotrophic fen in central New York. AOM was demonstrated with three independent methods to verify our results: (a) additions of methanogenic inhibitors, (b) stable isotope enrichment ( ¹³ C-CH ₄ ), and (c) natural abundance stable isotope analysis ( ¹³ C-CH ₄ ). These experiments confirmed that AOM occurs simultaneously with methanogenesis, consumes a significant portion of gross CH ₄ production, and significantly fractionates C isotopes (～ ?127‰). Experiments using a variety of potential electron acceptors demonstrated that Fe(III) and SO₄ ^{2 ?} are not quantitatively important, while the role of NO ₃ ^? is uncertain and deserves more attention. The exact mechanism(s) for AOM in peat soils remains unclear; however the AOM rates reported in this study are similar to those reported for CH ₄ production and aerobic CH ₄ oxidation in northern peatlands, suggesting that AOM may be an important control on CH ₄ fluxes in northern peatland ecosystems.     

Microbial processes influencing methane emission from rice fields

Irrigated rice fields are an important source of atmospheric methane. In order to improve our understanding of the controlling processes, we measured in situ CH₄ emission and CH₄ oxidation in an Italian rice field in 1998 and 1999, and studied CH₄ production in soil and root samples. The CH₄ emission rates were correlated with diurnal temperature variations and showed pronounced seasonal and interannual variations. The contribution of CH₄ oxidation to total CH₄ flux, determined by specific inhibition with difluoromethane, decreased from 40% at the beginning to zero at the end of the season. The stable carbon isotopic composition of the emitted CH₄ also decreased. The CH₄‐oxidizing bacteria probably became limited by nitrogen as indicated by the seasonal decrease of NH₄⁺. Thus, CH₄ oxidation had little effect on CH₄ emission. Methane production on rice roots was relatively constant over the season. Methane production in soil slowly increased after flooding and was highest in the middle of the season. Pore water concentrations of CH₄ showed a similar seasonal pattern. In 1999, CH₄ production increased later in the season and reached lower rates than in 1998. An additional drainage in 1999 resulted in higher ferric iron concentrations, higher soil redox potentials and lower acetate concentrations. As a result, acetate‐utilizing methanogens were probably out‐competed by iron‐reducers so that a larger percentage of [2–¹⁴C]acetate was converted to ¹⁴CO₂ instead of ¹⁴CH₄. The residual CH₄ production was relatively low and was mainly due to H₂/CO₂‐dependent methanogenesis. Experiments with radioactive bicarbonate and with methyl fluoride as specific inhibitor showed that the theoretical ratio of 7:3 of methanogenesis from acetate vs. H₂/CO₂ was only reached later in the season when total CH₄ production was at the maximum. In conclusion, our results give a mechanistic explanation for the intraseasonal and interannual differences in CH₄ emission.     

Recovery of Methanogenesis Following Summer Drought in Soils from Two Cool Temperate Peatlands,New York State,USA

Following a summer drought, intact cores of peat soil from two cool temperate peatlands (a rain-fed bog and a groundwater-fed swamp) were exposed experimentally to three different water table levels. The goal was to examine recovery of anaerobic methanogenesis and to evaluate peat soil decomposition to methane (CH₄), carbon dioxide (CO₂), and dissolved organic carbon (DOC) upon rewetting. Methane emission from soils to the atmosphere was greatest (mean = 80 μmol m^?2 s^?1) when the entire peat core was rewetted quickly; emission was negligible at low water level and when peat cores were rewetted gradually. Rates of CO₂ emission (mean = 1.0 μmol m^?2 s^?1) were relatively insensitive to water level. Concentrations of CH₄ in soil air spaces suggest that onset of methanogenesis induces, but later represses, aerobic oxidation of CH₄ above the water table. Concentrations of CO₂ suggest production at the soil surface of swamp peat versus at greater depths in bog peat. Portions of peat soil incubated in vitro without oxygen (O₂) exhibited a lag before the onset of methanogenesis, and the lag time was less in peat from the cores rewetted quickly. The inhibition of methanogenesis by the selective inhibitor 2-bromoethanesulfonic acid (BES) decreased CO₂ production by 20 to 30% but resulted in an increase in concentrations of DOC by 2 to 5 times. The results show that methanogens in peat soils tolerate moderate drought, and recovery varies among different peat types. In peat soils, the inhibition of methanogenesis might enhance DOC availability.     

Effects of Temperature on Methanogenesis in a Thermophilic (58°C) Anaerobic Digestor

The short-term effects of temperature on methanogenesis from acetate or CO₂ in a thermophilic (58°C) anaerobic digestor were studied by incubating digestor sludge at different temperatures with ¹⁴C-labeled methane precursors (¹⁴CH₃COO⁻ or ¹⁴CO₂). During a period when Methanosarcina sp. was numerous in the sludge, methanogenesis from acetate was optimal at 55 to 60°C and was completely inhibited at 65°C. A Methanosarcina culture isolated from the digestor grew optimally on acetate at 55 to 58°C and did not grow or produce methane at 65°C. An accidental shift of digestor temperature from 58 to 64°C during this period caused a sharp decrease in gas production and a large increase in acetate concentration within 24 h, indicating that the aceticlastic methanogens in the digestor were the population most susceptible to this temperature increase. During a later period when Methanothrix sp. was numerous in the digestor, methanogenesis from ¹⁴CH₃COO⁻ was optimal at 65°C and completely inhibited at 75°C. A partially purified Methanothrix enrichment culture derived from the digestor had a maximum growth temperature near 70°C. Methanogenesis from ¹⁴CO₂ in the sludge was optimal at 65°C and still proceeded at 75°C. A CO₂-reducing Methanobacterium sp. isolated from the digestor was capable of methanogenesis at 75°C. During the period when Methanothix sp. was apparently dominant, sludge incubated for 24 h at 65°C produced more methane than sludge incubated at 60°C, and no acetate accumulated at 65°C. Methanogenesis was severely inhibited in sludge incubated at 70°C, but since neither acetate nor H₂ accumulated, production of these methanogenic substrates by fermentative bacteria was probably the most temperature-sensitive process. Thus, there was a correlation between digestor performance at different temperatures and responses to temperature by cultures of methanogens believed to play important roles in the digestor.     

Evidence for iron‐mediated anaerobic methane oxidation in a crude oil‐contaminated aquifer

In a methanogenic crude oil contaminated aquifer near Bemidji, Minnesota, the decrease in dissolved CH₄ concentrations along the groundwater flow path, along with the positive shift in δ¹³C_CH4 and negative shift in δ¹³C_DIC, is indicative of microbially mediated CH₄ oxidation. Calculations of electron acceptor transport across the water table, through diffusion, recharge, and the entrapment and release of gas bubbles, suggest that these processes can account for at most 15% of the observed total reduced carbon oxidation, including CH₄. In the anaerobic plume, the characteristic Fe(III)‐reducing genus Geobacter was the most abundant of the microbial groups tested, and depletion of labile sediment iron is observed over time, confirming that reduced carbon oxidation coupled to iron reduction is an important process. Electron mass balance calculations suggest that organic carbon sources in the aquifer, BTEX and non‐volatile dissolved organic carbon, are insufficient to account for the loss in sediment Fe(III), implying that CH₄ oxidation may also be related to Fe(III) reduction. The results support a hypothesis of Fe(III)‐mediated CH₄ oxidation in the contaminated aquifer.     

Competition between Fe(III)-Reducing and Methanogenic Bacteria for Acetate in Iron-Rich Freshwater Sediments

The kinetics of acetate uptake and the depth distribution of [2-¹⁴C]acetate metabolism were examined in iron-rich sediments from a beaver impoundment in northcentral Alabama. The half-saturation constant (K_m) determined for acetate uptake in slurries of Fe(III)-reducing sediment (0.8 µM) was more than 10-fold lower than that measured in methanogenic slurries (12 µM) which supported comparable rates of bulk organic carbon metabolism and V_max values for acetate uptake. The endogenous acetate concentration (S _n) was also substantially lower (1.7 µM) in Fe(III)-reducing vs methanogenic (9.0 µM) slurries. The proportion of [2-¹⁴C]acetate converted to ¹⁴CH₄ increased with depth from ca 0.1 in the upper 0.5 cm to ca 0.8 below 2 cm and was inversely correlated (r² = 0.99) to a decline in amorphous Fe(III) oxide concentration. The results of the acetate uptake kinetics experiments suggest that differences in the affinity of Fe(III)-reducing bacteria vs methanogens for acetate can account for the preferential conversion of [2-¹⁴C]acetate to ¹⁴CO₂ in Fe(III) oxide-rich surface sediments, and that the downcore increase in conversion of [2-¹⁴C]acetate to ¹⁴CH₄ can be attributed to progressive liberation of methanogens from competition with Fe(III) reducers as Fe(III) oxides are depleted with depth.     

Methane emission from feather moss stands

Data from remote sensing and Eddy towers indicate that forests are not always net sinks for atmospheric CH₄. However, studies describing specific sources within forests and functional analysis of microorganisms on sites with CH₄ turnover are scarce. Feather moss stands were considered to be net sinks for carbon dioxide, but received little attention to their role in CH₄ cycling. Therefore, we investigated methanogenic rates and pathways together with the methanogenic microbial community composition in feather moss stands from temperate and boreal forests. Potential rates of CH₄ emission from intact moss stands (n = 60) under aerobic conditions ranged between 19 and 133 pmol CH₄ h^?1 gdw^?1. Temperature and water content positively influenced CH₄ emission. Methanogenic potentials determined under N₂ atmosphere in darkness ranged between 22 and 157 pmol CH₄ h^?1 gdw^?1. Methane production was strongly inhibited by bromoethane sulfonate or chloroform, showing that CH₄ was of microbial origin. The moss samples tested contained fluorescent microbial cells and between 10⁴ and 10⁵ copies per gram dry weight moss of the mcrA gene coding for a subunit of the methyl CoM reductase. Archaeal 16S rRNA and mcrA gene sequences in the moss stands were characteristic for the archaeal families Methanobacteriaceae and Methanosarcinaceae. The potential methanogenic rates were similar in incubations with and without methyl fluoride, indicating that the CH₄ was produced by the hydrogenotrophic rather than aceticlastic pathway. Consistently, the CH₄ produced was depleted in ¹³C in comparison with the moss biomass carbon and acetate accumulated to rather high concentrations (3–62 mM). The δ¹³C of acetate was similar to that of the moss biomass, indicating acetate production by fermentation. Our study showed that the feather moss stands contained active methanogenic microbial communities producing CH₄ by hydrogenotrophic methanogenesis and causing net emission of CH₄ under ambient conditions, albeit at low rates.     

Limited reduction of ferrihydrite encrusted by goethite in freshwater sediment

Many physical and chemical processes control the extent of Fe(III) oxyhydroxide reduction by dissimilatory Fe(III)‐reducing bacteria. The surface precipitation of secondary Fe minerals on Fe(III) oxyhydroxides limits the extent of microbial Fe(III) reduction, but this phenomenon has not yet been observed in nature. This paper reports the observation of secondary Fe‐mineral (goethite) encrustation on ferrihydrite surface within freshwater sediment up to 10 cm deep. The sediment surface was characterized by the predominance of ferrihydrites with biogenic stalks and sheaths. An Fe(II)‐oxidizing bacterium (Gallionellaceae) was detected by 16S rRNA gene analysis at sediment depths of 1 and 2 cm. Fe²⁺ concentration in the sediment pore water was relatively higher at 2–4 cm depths. The 16S rRNA genes affiliated with dissimilatory Fe(III)‐reducing bacteria were detected at 1, 2, and 4 cm depths. The results of the Fe K‐edge extended X‐ray absorption fine structure (EXAFS) analysis suggested the presence of goethite and siderite at depths below 3 cm. However, the change in the Fe‐mineral composition was restricted to sediment depths between 3 and 4 cm, despite the presence of abundant ferrihydrite at depths below 4 cm. An increase in CH₄ concentration was observed at deeper than 6 cm. Stable isotopic analysis of CH₄ in the pore water indicated that acetoclastic CH₄ occurred at depths below 7 cm. Transmission electron microscope observations suggested the presence of goethite and siderite on stalks and sheaths at depths below 3 cm. Results from conversion electron yield EXAFS analysis suggested that goethite dominated at 10 cm depth, thereby indicating that ferrihydrite was encrusted by goethite at this depth. Moreover, the incomplete reduction of ferrihydrite below depths of 4 cm was not due to the lack of organic carbon, but was possibly due to the surface encrustation of goethite on ferrihydrite.