Search

Serpentinization is the hydration and oxidation of ultramafic rock, which occurs as oceanic lithosphere is emplaced onto continental margins (ophiolites), and along the seafloor as faulting exposes this mantle-derived material to circulating hydrothermal fluids. This process leads to distinctive fluid chemistries as molecular hydrogen (H2 ) and hydroxyl ions (OH- ) are produced and reduced carbon compounds are mobilized. Serpentinizing ophiolites also serve as a vector to transport sulfur compounds from the seafloor onto the continents. We investigated hyperalkaline, sulfur-rich, brackish groundwater in a serpentinizing continental ophiolite to elucidate the role of sulfur compounds in fuelling in situ microbial activities. Here we illustrate that key sulfur-cycling taxa, including Dethiobacter, Desulfitispora and 'Desulforudis', persist throughout this extreme environment. Biologically catalysed redox reactions involving sulfate, sulfide and intermediate sulfur compounds are thermodynamically favourable in the groundwater, which indicates they may be vital to sustaining life in these characteristically oxidant- and energy-limited systems. Furthermore, metagenomic and metatranscriptomic analyses reveal a complex network involving sulfate reduction, sulfide oxidation and thiosulfate reactions. Our findings highlight the importance of the complete inorganic sulfur cycle in serpentinizing fluids and suggest sulfur biogeochemistry provides a key link between terrestrial serpentinizing ecosystems and their submarine heritage.

Serpentinization of peridotites in Earth’s mantle is associated with the generation of hydrogen and low molecular weight organics that could support subsurface life. Studies of microbial metabolisms in peridotite-hosted environments have focused primarily on methanogenesis, yet DNA sequences, isotopic composition of sulfides and thermodynamic calculations suggest there is potential for microbial sulfate reduction too. Here, we use a sulfate radiotracer-based method to quantify microbial sulfate reduction rates in serpentinization fluids recovered from boreholes in the Samail Ophiolite, Oman and the California Coast Range Ophiolite, USA. We find that low levels of sulfate reduction occur at pH up to 12.3. These low levels could not be stimulated by addition of hydrogen, methane or small organic acids, which indicates that this metabolism is limited by factors other than substrate availability. Cellular activity drops at pH > 10.5 which suggests that high fluid pH exerts a strong control on sulfate-reducing organisms in peridotites., Communications Earth & Environment, 2, ISSN:2662-4435

The Cassini mission to the Saturn system discovered a plume of ice grains and water vapor erupting from cracks on the icy surface of the satellite Enceladus. This moon has a global ocean in contact with a rocky core beneath its icy exterior, making it a promising location to search for evidence of extraterrestrial life in the solar system. The previous detection of H$_2$ in the plume indicates that there is free energy available for methanogenesis, the metabolic reaction of H$_2$ with CO$_2$ to form methane and water. Additional metabolic pathways could provide sources of energy in Enceladus' ocean, but require the use of other oxidants that have not been detected in the plume. Here, we perform chemical modeling to determine how the production of radiolytic O$_2$ and H$_2$O$_2$, and abiotic redox chemistry in the ocean and rocky core, contribute to chemical disequilibria that could support metabolic processes in Enceladus' ocean. We consider three possible cases for ocean redox chemistry: Case I in which reductants are not present in appreciable amounts and oxidants accumulate over time, and Cases II and III in which aqueous reductants or seafloor minerals, respectively, convert O$_2$ and H$_2$O$_2$ to SO$_4^{2-}$ and ferric oxyhydroxides. We calculate the upper limits on the concentrations of oxidants and chemical energy available for metabolic reactions in all three cases, neglecting additional abiotic reactions. For all three cases, we find that many aerobic and anaerobic metabolic reactions used by microbes on Earth could meet the minimum free energy threshold required for terrestrial life to convert ADP to ATP, as well as sustain positive cell density values within the Enceladus seafloor and/or ocean. These findings indicate that oxidant production and oxidation chemistry could contribute to supporting possible life and a metabolically diverse microbial community on Enceladus., Comment: Accepted to Icarus 2 December 2020

The geochemistry of hot springs and the availability of oxidants capable of supporting microbial metabolisms are influenced by subsurface processes including the separation of hydrothermal fluids into vapor and liquid phases. Here, we characterized the influence of geochemical variation and oxidant availability on the abundance, composition, and activity of hydrogen (H2 )-dependent chemoautotrophs along the outflow channels of two-paired hot springs in Yellowstone National Park. The hydrothermal fluid at Roadside East (RSE; 82.4°C, pH 3.0) is acidic due to vapor-phase input while the fluid at Roadside West (RSW; 68.1°C, pH 7.0) is circumneutral due to liquid-phase input. Most chemotrophic communities exhibited net rates of H2 oxidation, consistent with H2 support of primary productivity, with one chemotrophic community exhibiting a net rate of H2 production. Abundant H2 -oxidizing chemoautotrophs were supported by reduction in oxygen, elemental sulfur, sulfate, and nitrate in RSW and oxygen and ferric iron in RSE; O2 utilizing hydrogenotrophs increased in abundance down both outflow channels. Sequencing of 16S rRNA transcripts or genes from native sediments and dilution series incubations, respectively, suggests that members of the archaeal orders Sulfolobales, Desulfurococcales, and Thermoproteales are likely responsible for H2 oxidation in RSE, whereas members of the bacterial order Thermoflexales and the archaeal order Thermoproteales are likely responsible for H2 oxidation in RSW. These observations suggest that subsurface processes strongly influence spring chemistry and oxidant availability, which in turn select for unique assemblages of H2 oxidizing microorganisms. Therefore, these data point to the role of oxidant availability in shaping the ecology and evolution of hydrogenotrophic organisms.

To determine if microbial species play an active role in the development of calcium carbonate (CaCO 3 ) deposits (speleothems) in cave environments, we isolated 51 culturable bacteria from a coralloid speleothem and tested their ability to dissolve and precipitate CaCO 3 . The majority of these isolates could precipitate CaCO 3 minerals; scanning electron microscopy and X-ray diffractrometry demonstrated that aragonite, calcite and vaterite were produced in this process. Due to the inability of dead cells to precipitate these minerals, this suggested that calcification requires metabolic activity. Given growth of these species on calcium acetate, but the toxicity of Ca 2+ ions to bacteria, we created a loss-of-function gene knock-out in the Ca 2+ ion efflux protein ChaA. The loss of this protein inhibited growth on media containing calcium, suggesting that the need to remove Ca 2+ ions from the cell may drive calcification. With no carbonate in the media used in the calcification studies, we used stable i...

Recent studies employing novel analytical tools provide detailed, microscopic portraits of archaea/sulfate-reducing bacteria aggregates in sediments from methane seep and vent sites. One of the most striking features of these aggregates is that lipid and cell carbon are highly depleted in 13C (δ13C We derive general equations based on isotope mass-balance and calibrated with laboratory and field data to predict the isotopic composition of archaeal cell carbon and lipids derived from autotrophic methanogenesis and anaerobic methane oxidation. The calculations show that observed δ13C values for archaeal biomass and lipids at methane seep and vent sites are readily accounted for by isotope fractionation during methane production from CO2, and that biomass produced during anaerobic methane oxidation is only slightly depleted in 13C relative to methane unless the enzymatic isotope effect associated with the anabolic arm of the assimilation-dissimilation branch point is considerably larger than the isotope effect associated with the catabolic arm. We also apply an isotope diffusion-reaction model to demonstrate that micro-gradients in δ13C-CO2 cannot be maintained within archaea/SRB aggregates. However, 13C-depleted carbon in SRB members of the aggregate is readily explained by autotrophic sulfate-reduction with bulk porewater CO2 as carbon source. These results illustrate that 13C-depleted biomass and lipids observed in sediments from methane seep and vent sites may be derived from CO2-reducing archaea and autotrophic sulfate-reducing bacteria. The inference of anaerobic methanotrophy based on 13C depletion in archaeal and sulfate-reducing bacterial cell carbon and/or lipids should be considered tentative unless corroborated by independent, concordant evidence of net methane consumption.

Aggregates of archaea and sulfate-reducing bacteria (SRB) recently discovered in methane-seep sediments are widely assumed to engage in anaerobic methane oxidation (AMO), but the reaction mechanism remains poorly understood. We used a spherical diffusion-reaction model that incorporates thermodynamic controls, realistic aggregate morphology, and essential elements of cell structure to quantify maximum reaction rates and energy yields for competing mechanisms, to determine how cellular energy yields are affected by aggregate size and morphology, and to investigate the impact of organic-matter remineralization on archaea and SRB in the aggregate. The model provides the following insights: (a) syntrophic AMO is thermodynamically and physically possible for a variety of intermediate compounds (including H 2 , formate, and acetate); (b) the energy yield for syntrophic AMO is low but compatible with the maintenance needs of non- or slowly-growing cells; (c) archaea and SRB engaged in syntrophic AMO face a substantial energetic cost for aggregating; (d) direct contact between archaea and SRB provides only a modest energetic advantage compared to a loose association; and (e) sulfidogenic-methanogenic aggregates that take advantage of fermentation products released during organic-matter decay have a substantial energetic advantage over aggregates that rely exclusively on syntrophic AMO. Moreover, the model calls attention to a discrepancy between the observed sulfate-reduction rate at a well-characterized methane-seep site and the theoretical upper-limit rate of syntrophic AMO by a mechanism involving interspecies transfer of H 2 , formate, acetate, or other chemical intermediates. An analysis of possible errors, ambiguities, and artifacts in modeling and experimental techniques leads us to a surprising conclusion: that archaea/SRB aggregates in methane-seep sediments may be methanogenic rather than methanotrophic. In contrast, AMO in non-seep (diffusion-dominated) sediments is best explained by a consortium involving methanogenic archaea (that oxidize methane and release H 2 ) and hydrogenotrophic SRB.

Substantial isotopic fractionations are associated with many microbial sulfur metabolisms and measurements of the bulk δ^(34)S isotopic composition of sulfur species (predominantly sulfates and/or sulfides) have been a key component in developing our understanding of both modern and ancient biogeochemical cycling. However, the interpretations of bulk δ^(34)S measurements are often non-unique, making reconstructions of paleoenvironmental conditions or microbial ecology challenging. In particular, the link between the μm-scale microbial activity that generates isotopic signatures and their eventual preservation as a bulk rock value in the geologic record has remained elusive, in large part because of the difficulty of extracting sufficient material at small scales. Here we investigate the potential for small-scale (~100 μm–1 cm) δ^(34)S variability to provide additional constraints for environmental and/or ecological reconstructions. We have investigated the impact of sulfate concentrations (0.2, 1, and 80 mM SO_4) on the δ^(34)S composition of hydrogen sulfide produced over the diurnal (day/night) cycle in cyanobacterial mats from Guerrero Negro, Baja California Sur, Mexico. Sulfide was captured as silver sulfide on the surface of a 2.5 cm metallic silver disk partially submerged beneath the mat surface. Subsequent analyses were conducted on a Cameca 7f-GEO secondary ion mass spectrometer (SIMS) to record spatial δ^(34)S variability within the mats under different environmental conditions. Isotope measurements were made in a 2-dimensional grid for each incubation, documenting both lateral and vertical isotopic variation within the mats. Typical grids consisted of ~400–800 individual measurements covering a lateral distance of ~1 mm and a vertical depth of ~5–15 mm. There is a large isotopic enrichment (10–20‰) in the uppermost mm of sulfide in those mats where [SO_4] was non-limiting (field and lab incubations at 80 mM). This is attributed to rapid recycling of sulfur (elevated sulfate reduction rates and extensive sulfide oxidation) at and above the chemocline. This isotopic gradient is observed in both day and night enrichments and suggests that, despite the close physical association between cyanobacteria and select sulfate-reducing bacteria, photosynthetic forcing has no substantive impact on δ^(34)S in these cyanobacterial mats. Perhaps equally surprising, large, spatially-coherent δ^(34)S oscillations (~20–30‰ over 1 mm) occurred at depths up to ~1.5 cm below the mat surface. These gradients must arise in situ from differential microbial metabolic activity and fractionation during sulfide production at depth. Sulfate concentrations were the dominant control on the spatial variability of sulfide δ^(34)S. Decreased sulfate concentrations diminished both vertical and lateral δ^(34)S variability, suggesting that small-scale variations of δ^(34)S can be diagnostic for reconstructing past sulfate concentrations, even when original sulfate δ^(34)S is unknown.

Hydrogen produced by serpentinization has the potential to fuel subsurface microbial metabolisms. In the serpentinizing subsurface, the solids comprise ultramafic parent rocks derived from the Earth's mantle, serpentine minerals, veins of hydroxides, and accessory magnetite and/or other metal-rich grains. Fluid that occurs with these solids is altered seawater and/or meteoric water and is predicted to be reducing. Hydrogen, a powerful reducing agent, is generated when Fe2+ in Fe(OH)2 is oxidized to magnetite, coupled to the reduction of water. Theoretical considerations and experimental work suggest that serpentinization may generate fluid H2 concentrations as high as ≈75 millimolar, and that related seeps on land should have ≈300 micromolar. Field observations have shown that submarine serpentinizing seeps contain fluid H2 concentrations of 1 to 15 millimolar H2, subseafloor sediments have ≈7–100 nanomolar H2, and thermal springs have ≈13 nanomolar H2. Fluid H2 has the potential to drive a varie...

Liquid water is essential to life as we know it on Earth; therefore, the search for water on Mars is a critical component of the search for life. Olivine, a mineral identified as present on Mars, has been proposed as an indicator of the duration and characteristics of water because it dissolves quickly, particularly under low-pH conditions. The duration of olivine persistence relative to glass under conditions of aqueous alteration reflects the pH and temperature of the reacting fluids. In this paper, we investigate the utility of 3 methodologies to detect silicate weathering in a Mars analog environment (Sverrefjell volcano, Svalbard). CheMin, a miniature X-ray diffraction instrument developed for flight on NASA's upcoming Mars Science Laboratory, was deployed on Svalbard and was successful in detecting olivine and weathering products. The persistence of olivine and glass in Svalbard rocks was also investigated via laboratory observations of weathered hand samples as well as an in situ burial experiment. Observations of hand samples are consistent with the inference that olivine persists longer than glass at near-zero temperatures in the presence of solutions at pH approximately 7-9 on Svalbard, whereas in hydrothermally altered zones, glass has persisted longer than olivine in the presence of fluids at similar pH at approximately 50 degrees C. Analysis of the surfaces of olivine and glass samples, which were buried on Sverrefjell for 1 year and then retrieved, documented only minor incipient weathering, though these results suggest the importance of biological impacts. The 3 types of observations (CheMin, laboratory observations of hand samples, burial experiments) of weathering of olivine and glass at Svalbard show promise for interpretation of weathering on Mars. Furthermore, the weathering relationships observed on Svalbard are consistent with laboratory-measured dissolution rates, which suggests that relative mineral dissolution rates in the laboratory, in concert with field observations, can be used to yield valuable information regarding the pH and temperature of reacting martian fluids.

Rates of dissolved inorganic carbon (DIC), formate, and acetate mineralization and/or assimilation were determined in 13 high-temperature (>73°C) hot springs in Yellowstone National Park (YNP), Wyoming, in order to evaluate the relative importance of these substrates in supporting microbial metabolism. While 9 of the hot spring communities exhibited rates of DIC assimilation that were greater than those of formate and acetate assimilation, 2 exhibited rates of formate and/or acetate assimilation that exceeded those of DIC assimilation. Overall rates of DIC, formate, and acetate mineralization and assimilation were positively correlated with spring pH but showed little correlation with temperature. Communities sampled from hot springs with similar geochemistries generally exhibited similar rates of substrate transformation, as well as similar community compositions, as revealed by 16S rRNA gene-tagged sequencing. Amendment of microcosms with small (micromolar) amounts of formate suppressed DIC assimilation in short-term ( K m ) for formate transformation, as determined by community kinetic assays. These results suggest that dominant chemoautotrophs in high-temperature communities are facultatively autotrophic or mixotrophic, are adapted to fluctuating nutrient availabilities, and are capable of taking advantage of energy-rich organic substrates when they become available.

Methane is a key component in the global carbon cycle with a wide range of anthropogenic and natural sources. Although isotopic compositions of methane have traditionally aided source identification, the abundance of its multiply-substituted “clumped” isotopologues, e.g., 13CH3D, has recently emerged as a proxy for determining methane-formation temperatures; however, the impact of biological processes on methane’s clumped isotopologue signature is poorly constrained. We show that methanogenesis proceeding at relatively high rates in cattle, surface environments, and laboratory cultures exerts kinetic control on 13CH3D abundances and results in anomalously elevated formation temperature estimates. We demonstrate quantitatively that H2 availability accounts for this effect. Clumped methane thermometry can therefore provide constraints on the generation of methane in diverse settings, including continental serpentinization sites and ancient, deep groundwaters., National Science Foundation (U.S.) (EAR-1250394), National Science Foundation (U.S.) (EAR-1322805), Deep Carbon Observatory (Program), Natural Sciences and Engineering Research Council of Canada, Deutsche Forschungsgemeinschaft (Gottfried Wilhelm Leibniz Program), United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship), Neil & Anna Rasmussen Foundation, Grayce B. Kerr Fund, Inc. (Fellowship), MIT Energy Initiative (Shell-MITEI Graduate Fellowship), Shell International Exploration and Production B.V. (N. Braunsdorf and D. Smit of Shell PTI/EG grant)

Cyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulfate concentrations ([SO4]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm paleoclemates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulfate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [SO4=] (ca. 70 mM) and "Archean" [SO4] (less than 0.2 mM). At low [SO4], primary production in the mats was essentially unaffected, while rates of sulfate reduction decreased by a factor of three, and methane fluxes increased by up to ten-fold. However, remineralization by methanogenesis still amounted to less than 0.4 % of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats w-ere probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low- environmental levels oxygen and sulfate.

The simple biochemistry of H2 is critical to a large number of microbial processes, affecting the interaction of organisms with each other and with the environment. The sensitivity of each of these processes to H2 can be described collectively, through the quantitative language of thermodynamics. A necessary prerequisite is to understand the factors that, in turn, control H2 partial pressures. These factors are assessed for two distinctly different ecosystems. In anoxic sediments from Cape Lookout Bight (North Carolina, USA), H2 partial pressures are strictly maintained at low, steady-state levels by H2-consuming organisms, in a fashion that can be quantitatively predicted by simple thermodynamic calculations. In phototrophic microbial mats from Baja California (Mexico), H2 partial pressures are controlled by the activity of light-sensitive H2-producing organisms, and consequently fluctuate over orders of magnitude on a daily basis. The differences in H2 cycling can subsequently impact any of the H2-sensitive microbial processes in these systems. In one example, methanogenesis in Cape Lookout Bight sediments is completely suppressed through the efficient consumption of H2 by sulfate-reducing bacteria; in contrast, elevated levels of H2 prevail in the producer-controlled phototrophic system, and methanogenesis occurs readily in the presence of 40 mM sulfate.

Geochemical reactions associated with serpentinization alter the composition of dissolved organic compounds in circulating fluids and potentially liberate mantle-derived carbon and reducing power to support subsurface microbial communities. Previous studies have identified Betaproteobacteria from the order Burkholderiales and bacteria from the order Clostridiales as key components of the serpentinite–hosted microbiome, however there is limited knowledge of their metabolic capabilities or growth characteristics. In an effort to better characterize microbial communities, their metabolism, and factors limiting their activities, microcosm experiments were designed with fluids collected from several monitoring wells at the Coast Range Ophiolite Microbial Observatory (CROMO) in northern California during expeditions in March and August 2013. The incubations were initiated with a hydrogen atmosphere and a variety of carbon sources (carbon dioxide, methane, acetate and formate), with and without the addition of nutrients and electron acceptors. Growth was monitored by direct microscopic counts; DNA yield and community composition was assessed at the end of the three month incubation. For the most part, results indicate that bacterial growth was favored by the addition of acetate and methane, and that the addition of nutrients and electron acceptors had no significant effect on microbial growth, suggesting no nutrient- or oxidant-limitation. However the addition of sulfur amendments led to different community compositions. The dominant organisms at the end of the incubations were closely related to Dethiobacter sp. and to the family Comamonadaceae, which are also prominent in culture-independent gene sequencing surveys. These experiments provide one of first insights into the biogeochemical dynamics of the serpentinite subsurface environment and will facilitate experiments to trace microbial activities in serpentinizing ecosystems.

Modern laminated photosynthetic microbial mats are ideal environments to study how microbial activity creates and modifies carbon and sulfur isotopic signatures prior to lithification. Laminated microbial mats from a hypersaline lagoon (Guerrero Negro, Baja California, Mexico) maintained in a flume in a greenhouse at NASA Ames Research Center were sampled for δ^(13)C of organic material and carbonate to assess the impact of carbon fixation (e.g., photosynthesis) and decomposition (e.g., bacterial respiration) on δ^(13)C signatures. In the photic zone, the δ^(13)C_(org) signature records a complex relationship between the activities of cyanobacteria under variable conditions of CO_2 limitation with a significant contribution from green sulfur bacteria using the reductive TCA cycle for carbon fixation. Carbonate is present in some layers of the mat, associated with high concentrations of bacteriochlorophyll e (characteristic of green sulfur bacteria) and exhibits δ^(13)C signatures similar to DIC in the overlying water column (−2.0‰), with small but variable decreases consistent with localized heterotrophic activity from sulfate-reducing bacteria (SRB). Model results indicate respiration rates in the upper 12 mm of the mat alter in situ pH and HCO_3^- concentrations to create both phototrophic CO_2 limitation and carbonate supersaturation, leading to local precipitation of carbonate minerals. The measured activity of SRB with depth suggests they variably contribute to decomposition in the mat dependent on organic substrate concentrations. Millimeter-scale variability in the δ^(13)C_(org) signature beneath the photic zone in the mat is a result of shifting dominance between cyanobacteria and green sulfur bacteria with the aggregate signature overprinted by heterotrophic reworking by SRB and methanogens. These observations highlight the impact of sedimentary microbial processes on δ^(13)C_(org) signatures; these processes need to be considered when attempting to relate observed isotopic signatures in ancient sedimentary strata to conditions in the overlying water column at the time of deposition and associated inferences about carbon cycling.

A combination of radiotracer and pore-water concentration measurements provide evidence for the occurrence of acetogenesis from CO2 in anoxic marine sediments that are ordinarily dominated by sulfate reduction and methanogenesis. In a month-long incubation experiment using sediments from Cape Lookout Bight, North Carolina, we measured H2 and acetate concentrations and monitored the incorporation of 14 CO2 into 14 CH4 and 14 C-acetate. Depletion of pore-water sulfate resulted in a period of elevated H 2 concentrations that made acetogenic CO2 reduction thermodynamically favorable. During this period, 14 C-acetate was produced from 14 CO2 at rates comparable to those of methanogenesis or sulfate reduction during their respective periods of dominance in the incubation. Maintenance of elevated but constant H2 concentrations immediately following sulfate depletion likely reflects control by acetogenic bacteria, suggesting they were the dominant consumers of H2 during this period.

Molecular hydrogen plays a central role in bacterially mediated anoxic sediment chemistry, as both an important electron transfer agent and a key thermodynamic control. We studied the response of hydrogen concentrations to changes in temperature, specific electron acceptor, sulfate concentration, and pH in a series of laboratory experiments using sediments from Cape Lookout Bight, North Carolina. Hydrogen concentrations were found to depend significantly on each of these factors in a fashion that suggests thermodynamic control. In general, the change in hydrogen concentrations was apparently driven by a necessity to maintain a constant ΔG for the predominant terminal electron-accepting process. We hypothesize this situation derives from highly competetive conditions that force terminal metabolic bacteria to operate right at their thermodynamic limits. The response of hydrogen to individual controls in the laboratory experiments was reflected in relatively quantitiative fashion in down-core, seasonal, and inter-environmental variations observed in sediment cores from Cape Lookout Bight and the White Oak River, NC.

Field and laboratory studies of anoxic sediments from Cape Lookout Bight, North Carolina, suggest that anaerobic methane oxidation is mediated by a consortium of methanogenic and sulfate-reducing bacteria. A seasonal survey of methane oxidation and CO2 reduction rates indicates that methane production was confined to sulfate-depleted sediments at all times of year, while methane oxidation occurred in two modes. In the summer, methane oxidation was confined to sulfate-depleted sediments and occurred at rates lower than those of CO2 reduction. In the winter, net methane oxidation occurred in an interval at the base of the sulfate-containing zone. Sediment incubation experiments suggest both methanogens and sulfate reducers were responsible for the observed methane oxidation. In one incubation experiment both modes of oxidation were partially inhibited by 2-bromoethanesulfonic acid (a specific inhibitor of methanogens). This evidence, along with the apparent confinement of methane oxidation to sulfate-depleted sediments in the summer, indicates that methanogenic bacteria are involved in methane oxidation. In a second incubation experiment, net methane oxidation was induced by adding sulfate to homogenized methanogenic sediments, suggesting that sulfate reducers also play a role in the process. We hypothesize that methanogens oxidize methane and produce hydrogen via a reversal of CO2 reduction. The hydrogen is efficiently removed and maintained at low concentrations by sulfate reducers. Pore water H2 concentrations in the sediment incubation experiments (while net methane oxidation was occurring) were low enough that methanogenic bacteria could derive sufficient energy for growth from the oxidation of methane. The methanogen-sulfate reducer consortium is consistent not only with the results of this study, but may also be a feasible mechanism for previously documented anaerobic methane oxidation in both freshwater and marine environments.

Methane is a potent greenhouse gas with many times the global warming potential of carbon dioxide, so understanding how its emissions might change with increasing temperature is important for climate predictions. These authors conduct a meta-analysis of the temperature dependence of methane emissions from studies of laboratory cultures, environmental samples and whole ecosystems. They find that methane emissions increase with temperature at a greater rate than two other key rate processes in the carbon cycle, respiration and photosynthesis. The effect scales identically from cultures of individual methanogens in the lab up to the whole ecosystems.

The carbon isotopic composition of methane produced in anoxic marine sediment is controlled by four factors: (1) the pathway of methane formation, (2) the isotopic composition of the methanogenic precursors, (3) the isotope fractionation factors for methane production, and (4) the isotope fractionation associated with methane oxidation. The importance of each factor was evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Methane did not accumulate during the initial 42-day period when sediment contained sulfate, indicating little methane production from “noncompetitive” substrates. Following sulfate depletion, methane accumulation proceeded in three distinct phases. First, CO2 reduction was the dominant methanogenic pathway and the isotopic composition of the methane produced ranged from −80 to −94‰. The acetate concentration increased during this phase, suggesting that acetoclastic methanogenic bacteria were unable to keep pace with acetate production. Second, acetate fermentation became the dominant methanogenic pathway as bacteria responded to elevated acetate concentrations. The methane produced during this phase was progressively enriched in 13C, reaching a maximum δ13C value of −42‰. Third, the acetate pool experienced a precipitous decline from >5 mM to

The effect of variations in H2 concentrations on methanogenesis from the non-competitive substrates methanol and methylamine (used by methanogens but not by sulfate reducers) was investigated in methanogenic marine sediments. Imposed variations in sulfate concentration and temperature were used to drive systematic variations in pore water H2 concentrations. Specifically, increasing sulfate concentrations and decreasing temperatures both resulted in decreasing H2 concentrations. The ratio of CO2 and CH4 produced from 14C-labelled methylamine and methanol showed a direct correlation with the H2 concentration, independent of the treatment, with lower H2 concentrations resulting in a shift towards CO2. We conclude that this correlation is driven by production of H2 by methylotrophic methanogens, followed by loss to the environment with a magnitude dependent on the extracellular H2 concentrations maintained by hydrogenotrophic methanogens (in the case of the temperature experiment) or sulfate reducers (in the case of the sulfate experiment). Under sulfate-free conditions, the loss of reducing power as H2 flux out of the cell represents a loss of energy for the methylotrophic methanogens while, in the presence of sulfate, it results in a favourable free energy yield. Thus, hydrogen leakage might conceivably be beneficial for methanogens in marine sediments dominated by sulfate reduction. In low-sulfate systems such as methanogenic marine or freshwater sediments it is clearly detrimental - an adverse consequence of possessing a hydrogenase that is subject to externally imposed control by pore water H2 concentrations. H2 leakage in methanogens may explain the apparent exclusion of acetoclastic methanogenesis in sediments dominated by sulfate reduction

Each year, ocean sediments produce a quantity of methane equivalent to about half of the methane emitted to the atmosphere from all natural sources ( 1 ). Very little of the methane produced below the sea floor, however, reaches the ocean or the atmosphere; most is consumed by anaerobic microorganisms as it diffuses up through oxygen-poor (anoxic) sediments. Researchers recognized this process, known as anaerobic methane oxidation (AMO), nearly 35 years ago ( 2 ), but it remains poorly understood. Investigators have not been able to firmly establish the reaction mechanism, fully understand the factors that control oxidation rates, or isolate the responsible organisms. This represents a gaping hole in our understanding of one of Earth's primary sinks for methane. Recent studies of a rare but intriguing sedimentary environment–sea-floor seeps of methane-rich fluids–have provided new insights into the microorganisms that inhabit methane-rich sediments, but raised new questions regarding reaction mechanisms.

The factors that contribute to variability in the isotopic composition of methane produced by anoxic sediment were evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Anoxic marine sediment from Cape Lookout Bight, North Carolina, was placed in a 2-L pyrex incubation syringe for 114 d. Sediment samples were removed at weekly intervals and analyzed for sulfate, acetate, and methane concentrations as well as13C/12C ratios in the dissolved CO2 and methane pools. In addition, rates of sulfate reduction, methane production from acetate, and methane production from CO2 were measured by radiotracer techniques. The isotopic composition of the methane pool varied by more than 40%0 over the course of the experiment. The changes appear to be the result of at least two factors: (1) changes in the isotopic composition of the dissolved CO2 pool and (2) changes in the fraction of the methane production derived from acetate. A comparison of the laboratory experiment with field data suggests that similar biogeochemical processes operate to control carbon isotope ratios of methane in both undisturbed and laboratory-incubated sediment.