Diurnal Oscillations of Gas Production and Effluxes (CO2 and CH4) in Cores from a Peat Bog

Peat cores (15 cm diam X 30 cm deep) from Ellergower Moss, New Galloway, Scotland were kept and monitored at constant temperature (10 ± 0.1ºC) for gas production using a 1.6 mm diam stainless steel probe fitted with a membrane inlet and connected to a quadrupole mass spectrometer. In the headspace, O₂, CO₂ and CH₄ (measured at m/z values 32, 44 and 15 respectively) showed diurnal fluctuations in low-intensity natural daylight and under a light-dark (LD, 12:12) regime. Over the first few cycles O₂ and CO₂ increased together in the dark and decreased in the light, whereas CH₄ showed variations in antiphase with the other two gases. CO₂ and CH₄ also showed diurnal oscillations at 15 cm depth, but these decreased together in the light whereas argon (m/z = 40) was not varying. A highly-damped free-run of the oscillations in gas concentrations at 15cm depth was evident for only 3 cycles in complete darkness and at constant temperature. This might suggest desynchronization between individual plants with different free-running periods. A hydrocarbon signal (m/z = 26) at 15 cm depth also showed diurnal cycles but out of phase with CO 2 and CH 4. We postulate a circadian control of microbiological activities imposed by the vascular plants (Carex, Eriophorum, Molinia, Calluna, Erica). Under natural conditions the pronounced temperature sensitivity of CO₂ and CH₄ emission results in entrainment to daily temperature cycles. The amplitudes of the rhythms are greatest when temperature and light intensity changes are most pronounced, i.e. when the fluctuations in environmental factors are most potent as synchronizers (zeitgebers) and as masking factors.     

Diurnal Oscillations in Gas Production (O2, CO2, CH4, and N2) in Soil Monoliths

Soil cores (35 cm long, 7 cm diameter) from the Macaulay Land Use Research Institute's Sourhope Research Station in the Scottish Borders were kept and monitored at constant temperature (18± 1°C) for gas production using a 1.6 mm diameter stainless steel probe fitted with a membrane inlet and connected to a quadrupole mass spectrometer. This provided a novel method for on-line, real time monitoring of soil gas dynamics. In closed-system headspace experiments, O₂ and CO₂ (measured at m/z values 32 and 44, respectively) showed anti-phase diurnal fluctuations in low-intensity simulated daylight and under a light-dark (LD, 12:12 h) regime. O₂ increased during periods of illumination and decreased in the dark. The inverse was true for CO₂ production. Ar (m/z = 40) concentration and temperature (°C) remained constant throughout the experiments. The same phase-related oscillations, in CO₂ and O₂ concentrations, were observed at 2 and 5 cm depth in soil cores. The O₂ concentration did not oscillate diurnally at 10 cm depth. In below-ground experiments, CH₄ (m/z = 15) concentration showed diurnal cycles at 2, 5 and 10 cm depth. The CH₄ production had the same diurnal phase cycle as CO₂ but with lower amplitude. Evidence of below-ground diurnal oscillations in N₂ (m/z = 28) concentration was provided at 5 cm depth. The scale of production and consumption of gases associated with soil-atmosphere interactions and below-ground processes, are shown to be a multifaceted output of several variables. These include light, circadian-controlled physiological rhythms of plants and microbes, and the interactions between these organisms.     

Diurnal Oscillations in Gas Production (O2, CO2, CH4, and N2) in Soil Monoliths

Soil cores (35 cm long, 7 cm diameter) from the Macaulay Land Use Research Institute's Sourhope Research Station in the Scottish Borders were kept and monitored at constant temperature (18± 1°C) for gas production using a 1.6 mm diameter stainless steel probe fitted with a membrane inlet and connected to a quadrupole mass spectrometer. This provided a novel method for on-line, real time monitoring of soil gas dynamics. In closed-system headspace experiments, O₂ and CO₂ (measured at m/z values 32 and 44, respectively) showed anti-phase diurnal fluctuations in low-intensity simulated daylight and under a light-dark (LD, 12:12 h) regime. O₂ increased during periods of illumination and decreased in the dark. The inverse was true for CO₂ production. Ar (m/z = 40) concentration and temperature (°C) remained constant throughout the experiments. The same phase-related oscillations, in CO₂ and O₂ concentrations, were observed at 2 and 5 cm depth in soil cores. The O₂ concentration did not oscillate diurnally at 10 cm depth. In below-ground experiments, CH₄ (m/z = 15) concentration showed diurnal cycles at 2, 5 and 10 cm depth. The CH₄ production had the same diurnal phase cycle as CO₂ but with lower amplitude. Evidence of below-ground diurnal oscillations in N₂ (m/z = 28) concentration was provided at 5 cm depth. The scale of production and consumption of gases associated with soil-atmosphere interactions and below-ground processes, are shown to be a multifaceted output of several variables. These include light, circadian-controlled physiological rhythms of plants and microbes, and the interactions between these organisms.     

Nitrogen-Fixing Activity in Peat Soils from a Raised Bog

The nitrogenase (acetylene reductase) activity in monolithic and minced peat samples was found to be low, no more than 0.014–0.022 mg N/(kg h). Incorporation of the ¹⁵N₂ isotope into organic compounds of peat soil was 2.71–8.13 mg N/kg over 15 days. The nitrogen-fixing activity was the highest in a 10- to 20-cm layer of soil and much lower in the upper (under green moss) and deeper (20- to 30-cm) layers. The addition of glucose to soil samples stimulated nitrogen fixation considerably after 18–26 h. The maximum nitrogenase activity (3.5–3.8 mg N/(kg h)), observed after 60–70 h, coincided with the peak of respiratory activity. A repeated addition of glucose after its exhaustion increased nitrogenase activity, without a lag period, to 8.5 mg N/(kg h). Investigation of the effect of environmental factors (temperature, pH, aeration, and light intensity) on potential nitrogen-fixing activity in peat samples revealed that nitrogen fixation could proceed in a wide range of pH values (from 3.0 to 7.5) and temperatures (from 5 to 35°C). The nitrogen-fixing bacteria belonging to different trophic groups were enumerated by using nitrogen-free media with pH values and mineralization levels close to those in situ. In samples of peat soil, diazotrophic methanol-utilizing bacteria prevailed (2.0–2.5 × 10⁶ cells/g); the second largest group was facultatively anaerobic bacteria of the family Enterobacteriaceae.     

Control of carbon mineralization to CH4 and CO2 in anaerobic,Sphagnum-derived peat from Big Run Bog,West Virginia

The mineralization of organic carbon to CH₄ and CO₂ inSphagnum-derived peat from Big Run Bog, West Virginia, was measured at 4 times in the year (February, May, September, and November) using anaerobic, peat-slurry incubations. Rates of both CH₄ production and CO₂ production changed seasonally in surface peat (0–25 cm depth), but were the same on each collection date in deep peat (30–45 cm depth). Methane production in surface peat ranged from 0.2 to 18.8 mol mol(C)^–1 hr^–1 (or 0.07 to 10.4 g(CH₄) g^–1 hr^–1) between the February and September collections, respectively, and was approximately 1 mol mol(C)^–1 hr^–1 in deep peat. Carbon dioxide production in surface peat ranged from 3.2 to 20 mol mol(C)^–1 hr^–1 (or 4.8 to 30.3 g(CO₂) g^–1 hr^–1) between the February and September collections, respectively, and was about 4 mol mol(C)^–1 hr^–1 in deep peat. In surface peat, temperature the master variable controlling the seasonal pattern in CO₂ production, but the rate of CH₄ production still had the lowest values in the February collection even when the peat was incubated at 19°C. The addition of glucose, acetate, and H₂ to the peat-slurry did not stimulate CH₄ production in surface peat, indicating that CH₄ production in the winter was limited by factors other than glucose degradation products. The low rate of carbon mineralization in deep peat was due, in part, to poor chemical quality of the peat, because adding glucose and hydrogen directly stimulated CH₄ production, and CO₂ production to a lesser extent. Acetate was utilized in the peat by methanogens, but became a toxin at low pH values. The addition of SO₄ ^2– to the peat-slurry inhibited CH4 production in surface peat, as expected, but surprisingly increased carbon mineralization through CH4 production in deep peat. Carbon mineralization under anaerobic conditions is of sufficient magnitude to have a major influence on peat accumulation and helps to explain the thin (< 2 m deep), old (> 13,000 yr) peat deposit found in Big Run Bog.     

Oscillations of the internal CO(2) concentration in tobacco leaves transferred to low CO(2)

Measurement of the internal CO(2) concentration (Ci) in tobacco leaves using a fast-response CO(2) exchange system showed that in the light, switching from 350 microLL(-1) to a low CO(2) concentration of 36.5 microLL(-1) (promoting high photorespiration) resulted in the Ci oscillating near the value of CO(2) compensation point (Gamma*). The oscillations are highly irregular, the range of Ci varying by 2-4 microLL(-1) in substomatal cavities with a period of a few seconds. The statistical properties of the time series became stationary after a transient of approximately 100s following transfer to low CO(2). Attractor reconstruction shows that the observed oscillations are not chaotic but exhibit stochastic behavior. The period of oscillations is consistent with the duration of photorespiratory post-illumination burst (PIB). We suggest that the observed oscillations may be due to a similar mechanism to that which leads to PIB, and may play a role in switching mitochondrial operation between oxidation of the photorespiratory glycine and of the tricarboxylic acid cycle substrates.     

Enhanced CH4 emission from a wetland soil exposed to Elevated CO2

Methane emissions from wetland soils are generally a positive function ofplant size and primary productivity, and may be expected to increase dueto enhanced rates of plant growth in a future atmosphere of elevatedCO₂. We performed two experiments with Orontium aquaticum, acommon emergent aquatic macrophyte in temperate and sub-tropical wetlands, todetermine if enhanced rates of photosynthesis in elevated CO₂atmospheres would increase CH₄ emissions from wetland soils.O. aquaticum was grown from seed in soil cores under ambient and elevated(ca. 2-times ambient) concentrations of CO₂ in an initialglasshouse study lasting 3 months and then a growth chamber study lasting 6months. Photosynthetic rates were 54 to 71% higher underelevated CO₂ than ambient CO₂, but plantbiomass was not significantly different at the end of the experiment. Ineach case, CH₄ emissions were higher under elevated thanambient CO₂ levels after 2 to 4 months of treatment, suggestinga close coupling between photosynthesis and methanogenesis in our plant-soilsystem. Methane emissions in the growth chamber study increased by 136%. We observed a significant decrease in transpirationrates under elevated CO₂ in the growth chamber study, andspeculate that elevated CO₂ may also stimulate CH₄ emissions by increasing the extent and duration offlooding in some wetland ecosystems. Elevated CO₂ maydramatically increase CH₄ emissions from wetlands, a sourcethat currently accounts for 40% of global emissions.     

Partitioning of CH4 and CO2 Production Originating from Rice Straw,Soil and Root Organic Carbon in Rice Microcosms

Flooded rice fields are an important source of the greenhouse gas CH₄. Possible carbon sources for CH₄ and CO₂ production in rice fields are soil organic matter (SOM), root organic carbon (ROC) and rice straw (RS), but partitioning of the flux between the different carbon sources is difficult. We conducted greenhouse experiments using soil microcosms planted with rice. The soil was amended with and without ¹³C-labeled RS, using two ¹³C-labeled RS treatments with equal RS (5 g kg⁻¹ soil) but different δ¹³C of RS. This procedure allowed to determine the carbon flux from each of the three sources (SOM, ROC, RS) by determining the δ¹³C of CH₄ and CO₂ in the different incubations and from the δ¹³C of RS. Partitioning of carbon flux indicated that the contribution of ROC to CH₄ production was 41% at tillering stage, increased with rice growth and was about 60% from the booting stage onwards. The contribution of ROC to CO₂ was 43% at tillering stage, increased to around 70% at booting stage and stayed relatively constant afterwards. The contribution of RS was determined to be in a range of 12–24% for CH₄ production and 11–31% for CO₂ production; while the contribution of SOM was calculated to be 23–35% for CH₄ production and 13–26% for CO₂ production. The results indicate that ROC was the major source of CH₄ though RS application greatly enhanced production and emission of CH₄ in rice field soil. Our results also suggest that data of CH₄ dissolved in rice field could be used as a proxy for the produced CH₄ after tillering stage.     

Measurements of CO2 and CH4 evasion from UK peatland headwater streams

Peatland headwater streams are consistently supersaturated with respect to gaseous C and are known to degas CO₂ and CH₄ directly to the atmosphere. Using a combination of injection of a purposeful gas tracer (propane) and a soluble tracer (NaCl) we carried out 49 measurements of the gas transfer coefficient on 12 representative stream reaches to quantify the gas transfer rates of CO₂ and CH₄ in headwater (1st–3rd order) streams draining six UK peatlands. These were compared to measured stream reach physical variables, such as discharge and water travel time. Whilst we found that evasion rates were highly variable in space and time, $ {\text{K}}_{{{\text{CO}}_{2} }} $ (gas transfer coefficient of CO₂) was positively related to discharge. Individual study sites showed a high degree of variability in gas transfer rates; at all 49 sites median/mean values for $ {\text{K}}_{{{\text{CO}}_{2} }} $ were 0.087/0.157 and $ {\text{K}}_{{{\text{CH}}_{4} }} $ 0.092/0.176 min^?1. Median/mean instantaneous CO₂ and CH₄ evasion rates were 133/367 and 0.22/1.45 μg C m^?2 s^?1, respectively. Methane evasion rates were therefore more than two orders of magnitude lower than CO₂, with CH₄ invasion (rather than evasion) measured on 37 % of occasions. Our gas flux measurements from peatland headwater streams are higher than values previously used to estimate landscape scale fluxes and emphasise the importance of the evasion flux term in the overall carbon balance.     

Methanogenesis in McLean Bog,an Acidic Peat Bog in Upstate New York: Stimulation by H2/CO2 in the Presence of Rifampicin,or by Low Concentrations of Acetate

Acidic peat bog soils produce CH₄ and although molecular biological studies have demonstrated the presence of diverse methano-genic populations in them, few studies have sustained methanogenesis by adding the CH₄ precursors H₂/CO₂ or acetate, and few indigenous methanogens have been cultured. McLean Bog is a small (ca. 70 m across), acidic (pH 3.4–4.3) Sphagnum -dominated bog in upstate New York. Although addition of H₂/CO₂ or 10 mM acetate stimulated methanogenesis in soils from a nearby circumneutral-pH fen, neither of these substrates led to sustained methanogenesis in McLean Bog soil slurries. After a brief period of stimulation by H₂/CO₂, methanogenesis in McLean Bog soil declined, which could be attributed to buildup of large amounts of acetic acid produced from the H₂/CO₂ by acetogens. Addition of the antibiotic rifampicin inhibited acetogenesis (carried out by Bacteria) and allowed methanogenesis (carried out by Archaea) to continue. Using rifampicin, we were able to study effects of temperature, pH, and salts on methanogenesis from H₂/CO₂ in McLean Bog soil samples. The enriched H₂/CO₂-utilizing methanogens showed an optimum for activity near pH 5, and a temperature optimum near 35°C. Methanogenesis was not stimulated by addition of 10 mM acetate, but it was stimulated by 1 mM acetate, and multiple additions were consumed at increasing rates and nearly stoichiometrically converted to CH₄. In conclusion, we have found that both hydrogentrophic and aceticlastic methanogens are present in McLean Bog soils, and that methanogenic activity can be stimulated using H₂/CO₂ in the presence of rifampicin, or using low concentrations of acetate.     

Prebiotic synthesis in atmospheres containing CH4, CO,and CO2

The prebiotic synthesis of organic compounds using a spark discharge on various simulated primitive earth atmospheres at 25 degrees C has been studied. Methane mixtures contained H2 + CH4 + H2O + N2 + NH3 with H2/CH4 molar ratios from 0 to 4 and pNH3 = 0.1 torr. A similar set of experiments without added NH3 was performed. The yields of amino acids (1.2 to 4.7% based on the carbon) are approximately independent of the H2/CH4 ratio and whether NH3 was present, and a wide variety of amino acids are obtained. Mixtures of H2 + CO + H2O + N2 and H2 + CO2 + H2O + N2, with and without added NH3, all gave about 2% yields of amino acids at H2/CO and H2/CO2 ratios of 2 to 4. For a H2/CO2 ratio of 0, the yield of amino acids is extremely low (10(-3)%). Glycine is almost the only amino acid produced from CO and CO2 model atmospheres. These results show that the maximum yield is about the same for the three carbon sources at high H2/carbon ratios, but that CH4 is superior at low H2/carbon ratios. In addition, CH4 gives a much greater variety of amino acids than either CO or CO2. If it is assumed that an abundance of amino acids more complex than glycine was required for the origin of life, then these results indicate the requirement for CH4 in the primitive atmosphere.     

Molecular Ecological Analysis of Methanogens and Methanotrophs in Blanket Bog Peat

Abstract Methane production and methane oxidation potential were measured in a 30 cm peat core from the Moorhouse Nature Reserve, UK. The distribution of known groups of methanogens and methane oxidizing bacteria throughout this peat core was assessed. Using 16S rRNA gene retrieval and functional gene probing with genes encoding key proteins in methane oxidation and methanogenesis, several major groups of microorganisms were detected. Methane production and oxidation was detected in all depths of the peat core. PCR amplification and oligonucleotide probing experiments using DNA isolated from all sections of the peat core detected methanotrophs from the groups Methylosinus and Methylococcus and methanogens from the groups Methanosarcinaceae, Methanococcaceae, and Methanobacteriaceae. 16S rDNA sequences amplified with the Methylosinus-specific primer were shown to have a high degree of identity with 16S rDNA sequences previously detected in acidic environments. However, no methanogen sequences were detected by the probes available in this study in the sections of the peat core (above 7 cm) where the majority of methanogenesis occurred, either because of low methanogen numbers or because of the presence of novel methanogen sequences. Received: 9 March 1999; Accepted: 21 June 1999     

The flux of CO2 and CH4 from lakes and rivers in arctic Alaska

Partial pressures of CO₂ and CH₄ were measured directly or calculated from pH and alkalinity or DIC measurements for 25 lakes and 4 rivers on the North Slope of Alaska. Nearly all waters were super-saturated with respect to atmospheric pressures of CO₂ and CH₄. Gas fluxes to the atmosphere ranged from −6.5 to 59.8 mmol m⁻² d⁻¹ for CO₂ and from 0.08 to 1.02 mmol m⁻² d⁻¹ for CH₄, and were uncorrelated with latitude or lake morphology. Seasonal trends include a buildup of CO₂ and CH₄ under ice during winter, and often an increased CO₂ flux rate in August due to partial lake turnover. Nutrient fertilization experiments resulted in decreased CO₂ release from a lake due to photosynthetic uptake, but no change in CO₂ release from a river due to the much faster water renewal time. In lakes and rivers the groundwater input of dissolved CO₂ and CH₄ is supplemented by in-lake respiration of dissolved and particulate carbon washed in from land. The release of carbon from aquatic systems to the atmosphere averaged 24 g C m⁻² y⁻¹, and in coastal areas where up to 50% of the surface area is water, this loss equals frac 1/5 to 1/2 of the net carbon accumulation rates estimated for tundra.     

Release of CO2 and CH4 from lakes and drainage ditches in temperate wetlands

Shallow fresh water bodies in peat areas are important contributors to greenhouse gas fluxes to the atmosphere. In this study we determined the magnitude of CH₄ and CO₂ fluxes from 12 water bodies in Dutch wetlands during the summer season and studied the factors that might regulate emissions of CH₄ and CO₂ from these lakes and ditches. The lakes and ditches acted as CO₂ and CH₄ sources of emissions to the atmosphere; the fluxes from the ditches were significantly larger than the fluxes from the lakes. The mean greenhouse gas flux from ditches and lakes amounted to 129.1 ± 8.2 (mean ± SE) and 61.5 ± 7.1 mg m^?2 h^?1 for CO₂ and 33.7 ± 9.3 and 3.9 ± 1.6 mg m^?2 h^?1 for CH₄, respectively. In most water bodies CH₄ was the dominant greenhouse gas in terms of warming potential. Trophic status of the water and the sediment was an important factor regulating emissions. By using multiple linear regression 87% of the variation in CH₄ could be explained by PO₄ ^3? concentration in the sediment and Fe²⁺ concentration in the water, and 89% of the CO₂ flux could be explained by depth, EC and pH of the water. Decreasing the nutrient loads and input of organic substrates to ditches and lakes by for example reducing application of fertilizers and manure within the catchments and decreasing upward seepage of nutrient rich water from the surrounding area will likely reduce summer emissions of CO₂ and CH₄ from these water bodies.     

Bioconversion of organic carbon to CH4 and CO2

The activities of populations in complex anaerobic microbial communities that perform complete bioconversion of organic matter to CH₄ and CO₂ are reviewed. Species of eubacteria produce acetate, H₂, and CO₂ from organic substrates, and methanogenic species of archaebacteria transform the acetate, H₂, and CO₂ to CH₄. The characteristics and activities of the methanogenic bacteria are described. The impact of the use of H₂ by methanogens on the fermentations that produce acetate, H₂, and CO₂ and the importance of syntrophy in complete bioconversion are discussed.     

Linking Belowground Carbon Allocation to Anaerobic CH4 and CO2 Production in a Forested Peatland,New York State

Heterotrophic soil microorganisms rely on carbon (C) allocated belowground in plant production, but belowground C allocation (BCA) by plants is a poorly quantified part of ecosystem C cycling, especially, in peat soil. We applied a C balance approach to quantify BCA in a mixed conifer-red maple (Acer rubrum) forest on deep peat soil. Direct measurements of CH₄ and CO₂ fluxes across the soil surface (soil respiration), production of fine and small plant roots, and aboveground litterfall were used to estimate respiration by roots, by mycorrhizae and by free-living soil microorganisms. Measurements occurred in two consecutive years. Soil respiration rates averaged 1.2 bm μmol m^{? 2} s^{? 1} for CO₂ and 0.58 nmol m^{? 2} s^{? 1} for CH₄ (371 to 403 g C m^{? 2} year^{? 1}). Carbon in aboveground litter (144 g C m^{? 2} year^{? 1}) was 84% greater than C in root production (78 g C m^{? 2} year^{? 1}). Complementary in vitro assays located high rates of anaerobic microbial activity, including methanogenesis, in a dense layer of roots overlying the peat soil and in large-sized fragments within the peat matrix. Large-sized fragments were decomposing roots and aboveground leaf and twig litter, indicating that relatively fresh plant production supported most of the anaerobic microbial activity. Respiration by free-living soil microorganisms in deep peat accounted for, at most, 29 to 38 g C m^{? 2} year^{? 1}. These data emphasize the close coupling between plant production, ecosystem-level C cycling and soil microbial ecology, which BCA can help reveal.     

Calibration of Infra-red CO(2) Gas Analyzers

Precision gas mixing pumps produce CO₂ gas mixtures for the calibration of infra-red CO₂ gas analyzers equivalent in accuracy to the standard CO₂ gas mixtures (± 1%) supplied by the National Bureau of Standards, Washington, D. C.     

Contribution of subsurface peat to CO2 and CH4 fluxes in a neotropical peatland

Tropical peatlands play an important role in the global carbon cycling but little is known about factors regulating carbon dioxide (CO₂) and methane (CH₄) fluxes from these ecosystems. Here, we test the hypotheses that (i) CO₂ and CH₄ are produced mainly from surface peat and (ii) that the contribution of subsurface peat to net C emissions is governed by substrate availability. To achieve this, in situ and ex situ CO₂ and CH₄ fluxes were determined throughout the peat profiles under three vegetation types along a nutrient gradient in a tropical ombrotrophic peatland in Panama. The peat was also characterized with respect to its organic composition using ¹³C solid state cross‐polarization magic‐angle spinning nuclear magnetic resonance spectroscopy. Deep peat contributed substantially to CO₂ effluxes both with respect to actual in situ and potential ex situ fluxes. CH₄ was produced throughout the peat profile with distinct subsurface peaks, but net emission was limited by oxidation in the surface layers. CO₂ and CH₄ production were strongly substrate‐limited and a large proportion of the variance in their production (30% and 63%, respectively) was related to the quantity of carbohydrates in the peat. Furthermore, CO₂ and CH₄ production differed between vegetation types, suggesting that the quality of plant‐derived carbon inputs is an important driver of trace gas production throughout the peat profile. We conclude that the production of both CO₂ and CH₄ from subsurface peat is a substantial component of the net efflux of these gases, but that gas production through the peat profile is regulated in part by the degree of decomposition of the peat.     

CH4 production from H2 and CO2 byMethanobacterium thermoautotrophicum cells fixed on hollow fibers

Summary Methane was produced from H₂ and CO₂ byMethanobacterium thermoautotrophicum cells fixed on the surface of hollow fibers. The mineral solution permeated through the inside of fibers was consumed by the cells, while the gaseous substrate flowing outside the fibers was directly metabolized to methane. Methane production was proportional to hollow fiber length i.e., contact area between cell layer and gas phase. In repeated batch cultures, the production rates of methane and cell mass were 33.1 L/L reactor/day and 1.75 g cells/L reactor/day, respectively with 90% conversion rate.