Μmol CH4 Research Articles

Simulation-based optimization of cycle timing for CO2 capture and hydrogenation with dual function catalyst

CO2 methanation could play a significant role in the future energy system. The excess of renewable electric energy can be transformed into storable methane to balance the energy demand when required. Moreover, the CO2 methanation can be performed alternating steps of CO2 storage and reduction, avoiding expensive CO2 purification steps. In this work, we will use a previously developed and validated model to optimize by simulation the CO2 adsorption and hydrogenation cycles timing (tCO2/tH2). The performance of the catalyst is quantified by the CO2 conversion (XCO2, %), H2 conversion (XH2, %) and CH4 production (YCH4, mmol g−1 cycle−1). Long adsorption and hydrogenation times result in high CH4 productions per cycle, however, low CO2 and H2 conversion. Therefore, adsorption times close to the catalyst saturation (tCO2=60 s) and moderate hydrogenation times are preferable. To better select the optimal hydrogenation time, a new catalytic parameter is set, the average formation rate of CH4 (r¯CH4, μmol g−1 s−1). The optimal hydrogenation time is set at 120 s. In addition to having a high average formation rate of CH4, tCO2/tH2= 60/120 cycle timing would allow to work with three identical beds in parallel, one in adsorption mode and two in regenerating mode. With the optimum cycle timing of 60/120 the production of CH4 results in 148 μmol g−1 cycle−1 (1.2 μmol CH4 g−1 s−1) and a CO2 and H2 conversion of 25% and 43%, respectively

High methane emissions from thermokarst lakes on the Tibetan Plateau are largely attributed to ebullition fluxes

Ebullition has been shown to be an important pathway for methane (CH4) emissions from inland waters. However, the CH4 fluxes and their magnitudes in thermokarst lakes remain unclear due to limited research data, especially on the Tibetan Plateau (TP). The magnitude and regulation of two CH4 pathways, ebullition and diffusion, were investigated in 32 thermokarst lakes on the TP during the summer of 2020. CH4 emissions from thermokarst lakes on the TP showed significant spatiotemporal heterogeneity. Diffusion fluxes in lakes averaged 2.6 mmol m−2 d−1 (ranging from 0.003 to 48.4 mmol m−2 d−1), and ebullition fluxes in lakes averaged 6.6 mmol CH4 m−2 d−1 (ranging from 0.002 to 140.0 mmol m−2 d−1). Together, these ebullition fluxes contributed 66.1 ± 24.9% (ranging 5.4 to 100.0%) to the total (diffusion + ebullition) CH4 emissions, indicating the importance of ebullition as a major CH4 transport mechanism on the TP. In general, thermokarst lakes with higher CH4 diffusion fluxes and ebullition fluxes occurred in alpine meadows (2.5 ± 5.3 mmol m−2 d−1; 8.2 ± 20.6 mmol m−2 d−1), followed by alpine steppes (0.6 ± 5.3 mmol m−2 d−1; 0.7 ± 10.8 mmol m−2 d−1) and desert steppes (0.2 ± 0.2 mmol m−2 d−1; 0.6 ± 0.8 mmol m−2 d−1). The organic matter contents in water and sediment were found to be important factors influencing the seasonal variations in CH4 diffusion fluxes. However, the ebullition CH4 fluxes did not show a clear seasonal variation pattern. Our findings highlight the importance of considering the large spatiotemporal variations in ebullition CH4 fluxes to improve the accuracy of large-scale estimations of CH4 fluxes in thermokarst lakes on the TP. Greater insight into these aspects will increase the understanding of CH4 dynamics in thermokarst lakes on the TP, which is essential for forecasting and climate impact assessments and to better constrain feedback to climate warming.

Large Seasonal and Habitat Differences in Methane Ebullition on the Amazon Floodplain

AbstractTropical floodplains are an important source of methane (CH4) to the atmosphere, and ebullitive fluxes are likely to be important. We report direct measurements of CH4 ebullition in common habitats on the Amazon floodplain over two years based on floating chambers that allowed detection of bubbles, and submerged bubble traps. Ebullition was highly variable in space and time. Of the 840 floating chamber measurements (equivalent to 8,690 min of 10‐min deployments), 22% captured bubbles. Ebullitive CH4 fluxes, measured using bubble traps deployed for a total of approximately 230 days, ranged from 0 to 109 mmol CH4 m−2 d−1, with a mean of 4.4 mmol CH4 m−2 d−1. During falling water, a hydroacoustic echosounder detected bubbles in 24% of the 70‐m segments over 34 km. Ebullitive flux increased as the water level fell faster during falling water periods. In flooded forests, highest ebullitive fluxes occurred during falling water, while in open water and herbaceous plant habitats, higher ebullitive fluxes were measured during low water periods. The contribution of diffusive plus ebullitive CH4 flux represented by ebullition varied from 1% (high and rising water in open water of the lake) to 93% (falling water in flooded forests) based on bubble traps. Combining ebullitive and diffusive fluxes among habitats in relation to variations in water depth and areal coverage of aquatic habitats provides the basis for improved floodplain‐wide estimates of CH4 evasion.

Deciphering the role of Ni particle size and nickel-ceria interfacial perimeter in the low-temperature CO2 methanation reaction over remarkably active Ni/CeO2 nanorods

The structure sensitivity of CO2 methanation was explored over nickel particles (10–25 nm) supported on CeO2 nanorods. An optimum Ni particle size of 20 nm was revealed, with the corresponding sample demonstrating remarkable activity, i.e., 187 μmol CH4 g−1 s−1 and 92 % CH4 yield at 275 °C, which is among the highest ever reported. Notably, the intrinsic activity on the basis of the exposed Ni sites or Ni-ceria perimeter is largely independent of the Ni size, showcasing that neither the exposed Ni sites nor the Ni-ceria interface can be employed as activity descriptors. A compromise between the length of the metal-support perimeter and the competitive presence of larger Ni particles is necessary for the optimum activity. On the grounds of a structure-sensitivity analysis, the superior activity of larger Ni particles could be attributed to the presence of under-coordinated step and kink sites, instead of largely inactive terrace sites.

The water column of the Yamal tundra lakes as a microbial filter preventing methane emission

Abstract. Microbiological, molecular ecological, biogeochemical, and isotope geochemical research was carried out in four lakes of the central part of the Yamal Peninsula in the area of continuous permafrost. Two of them were large (73.6 and 118.6 ha) and deep (up to 10.6 and 12.3 m) mature lakes embedded into all geomorphological levels of the peninsula, and two others were smaller (3.2 and 4.2 ha) shallow (2.3 and 1.8 m) lakes which were formed as a result of thermokarst on constitutional (segregated) ground ice. Samples were collected in August 2019. The Yamal tundra lakes were found to exhibit high phytoplankton production (340–1200 mg C m−2 d−1) during the short summer season. Allochthonous and autochthonous, particulate and dissolved organic matter was deposited onto the bottom sediments, where methane was the main product of anaerobic degradation, and its content was 33–990 µmol CH4 dm−3. The rates of hydrogenotrophic methanogenesis appeared to be higher in the sediments of deep lakes than in those of the shallow ones. In the sediments of all lakes, Methanoregula and Methanosaeta were predominant components of the archaeal methanogenic community. Methane oxidation (1.4–9.9 µmol dm−3 d−1) occurred in the upper sediment layers simultaneously with methanogenesis. Methylobacter tundripaludum (family Methylococcaceae) predominated in the methanotrophic community of the sediments and the water column. The activity of methanotrophic bacteria in deep mature lakes resulted in a decrease in the dissolved methane concentration in lake water from 0.8–4.1 to 0.4 µmol CH4 L−1 d−1, while in shallow thermokarst lakes the geochemical effect of methanotrophs was much less pronounced. Thus, only small, shallow Yamal lakes may contribute significantly to the overall diffusive methane emissions from the water surface during the warm summer season. The water column of large, deep lakes on Yamal acts, however, as a microbial filter preventing methane emission into the atmosphere. It can be assumed that climate warming will lead to an increase in the total area of thermokarst lakes, which will enhance the effect of methane release into the atmosphere.

The role of termite CH<sub>4</sub> emissions on the ecosystem scale: a case study in the Amazon rainforest

Abstract. The magnitude of termite methane (CH4) emissions is still an uncertain part of the global CH4 budget and current emission estimates are based on limited field studies. We present in situ CH4 emission measurements of termite mounds and termite mound subsamples performed in the Amazon rainforest. Emissions from five termite mounds of the species Neocapritermes brasiliensis were measured by use of a large flux chamber connected to a portable gas analyser measuring CH4 and CO2. In addition, the emissions of mound subsamples were measured, after which the termites were counted so that a termite CH4 and CO2 emission factor could be determined. Mound emissions were found to range between 17.0 and 34.8 nmol mound−1 s−1 for CH4 and between 1.1 and 13.0 µmol mound−1 s−1 for CO2. A termite emission factor of 0.35 µmol CH4 gtermite-1 h−1 was found, which is almost twice as high as the only other reported value for the Amazon. By combining mound emission measurements with the termite emission factor, colony sizes could be estimated, which were found to range between 55–125 thousand individuals. Estimates were similar to literature values, and we therefore propose that this method can be used as a quick non-intrusive method to estimate termite colony size in the field. The role of termites in the ecosystem's CH4 budget was evaluated by use of two approaches. Termite mound emission values were combined with local mound density numbers, leading to an estimate of 0.15–0.71 nmol CH4 m−2 s−1, on average, emitted by termite mounds. In addition, the termite CH4 emission factor from this study was combined with termite biomass numbers, resulting in an estimate of termite-emitted CH4 of ∼1.0 nmol m−2 s−1. Considering the relatively low net CH4 emissions previously measured at this ecosystem, we expect that termites play an important role in the CH4 budget of this terra firme ecosystem.

Continuous Dynamics of Dissolved Methane Over 2 Years and its Carbon Isotopes (δ13C, Δ14C) in a Small Arctic Lake in the Mackenzie Delta

AbstractSeasonally ice‐covered permafrost lakes in the Arctic emit methane to the atmosphere during periods of open‐water. However, processes contributing to methane cycling under‐ice have not been thoroughly addressed despite the potential for significant methane emission to the atmosphere at ice‐out. We studied annual dissolved methane dynamics within a small (0.2 ha) Mackenzie River Delta lake using sensor and water sampling packages that autonomously and continuously collected lake water samples, respectively, for two years at multiple water column depths. Lake physical and biogeochemical properties (temperature; light; concentrations of dissolved oxygen, manganese, iron, and dissolved methane, including stable carbon, and radiocarbon isotopes) revealed annual patterns. Dissolved methane concentrations increase under‐ice after electron acceptors (oxygen, manganese, and iron oxides) are depleted or inaccessible from the water column. The radiocarbon age of dissolved methane suggests a source from recently decomposed carbon as opposed to thawed ancient permafrost. Sources of dissolved methane under‐ice include a diffusive flux from the sediments and may include water column methanogenesis and/or under‐ice hydrodynamic controls. Following ice‐out, the water column only partially mixes allowing half of the winter‐derived dissolved methane to be microbially oxidized. Despite oxidation at depth, surface water was a source of methane to the atmosphere. The greatest diffusive fluxes to the atmosphere occurred following ice‐out (75 mmol CH4 m−2 d−1) and during a mixing episode in mid‐July, likely driven by a storm event. This study demonstrates the importance of fine‐scale temporal sampling to understand dissolved methane processes in seasonally ice‐covered lakes.

Study of the Photothermal Catalytic Mechanism of CO2 Reduction to CH4 by Ruthenium Nanoparticles Supported on Titanate Nanotubes.

The Sabatier reaction could be a key tool for the future of the renewable energy field due to the potential of this reaction to produce either fuels or to stabilize H2 in the form of stable chemicals. For this purpose, a new composite made of ruthenium oxide nanoparticles (NPs) deposited on titanate nanotubes (TiNTs) was tested. Titanate nanotubes are a robust semiconductor with a one-dimensional (1D) morphology that results in a high contact area making this material suitable for photocatalysis. Small ruthenium nanoparticles (1.5 nm) were deposited on TiNTs at different ratios by Na+-to-Ru3+ ion exchanges followed by calcination. These samples were tested varying light power and temperature conditions to study the reaction mechanism during catalysis. Methanation of CO2 catalyzed by Ru/TiNT composite exhibit photonic and thermic contributions, and their ratios vary with temperature and light intensity. The synthesized composite achieved a production rate of 12.4 mmol CH4·gcat−1·h−1 equivalent to 110.7 mmol of CH4·gRu−1·h−1 under 150 mW/cm2 simulated sunlight irradiation at 210 °C. It was found that photo-response derives either from Ru nanoparticle excitation in the visible (VIS) and near-infrared (NIR) region (photothermal and plasmon excitation mechanism) or from TiNT excitation in the ultraviolet (UV) region leading to electron–hole separation and photoinduced electron transfer.

Molten salt bubble columns for low-carbon hydrogen from CH4 pyrolysis: Mass transfer and carbon formation mechanisms

• Varied CH 4 bubble SA:V ratios in molten salt bubble columns to elucidate mass-transfer mechanisms. • A specific surface area dependent reaction rate of 0.64 mmol CH 4 m −2 s −1 was identified. • Particulate suspensions in molten salts highlight a mass transfer limitation at the bubble surface. • Reaction kinetics and mechanisms of pyrolytic carbon formed in molten salts are evaluated. • Insights are key to designing industrial-scale CH 4 pyrolysis processes and to tailoring carbon. Methane pyrolysis in molten salts could potentially provide a large-scale, low-cost and low-CO 2 production route of H 2 from CH 4 . The interplay of reaction and transport phenomena in molten pyrolysis reactors are currently poorly characterised and further understanding of the mass transfer and mechanistic pathways elucidated here may prove key to designing separation strategies that favour minimum carbon contamination and maximised production rates. Kinetic experiments were carried out at 850–1000 °C in a 590 mm bubble column of a eutectic mixture of molten NaBr and KBr. The use of four different injector sizes allowed for the variation of the mean surface area of the CH 4 bubbles in the range of ~600–1200 m 2 m −3 (0.77–2.9 cm 2 per bubble). For the first time, a clear decoupling of bubble specific surface area and volume-based CH 4 reaction rates in molten pyrolysis systems was demonstrated. These phenomena become more complex with catalyst particles present. Tests with γ-Al 2 O 3 particles of varying sizes and with varying particle loadings were undertaken. The carbon depositing on the particles has a high catalytic activity and the material is thus a promising material for structured packings. Two clearly distinguishable kinetic regimes were identified, which transitioned at a γ-Al 2 O 3 loading of ~1.25 wt%. We propose that the reaction is primarily enhanced by a shuttling mechanism of γ-Al 2 O 3 adsorbing hydrocarbons (HC) in the HC-rich liquid-side film surrounding the bubble and desorbing it in the HC-lean liquid bulk. The findings provide a basis for (i) further models to extract kinetic parameters from CH 4 pyrolysis in molten salt particle suspensions and (ii) for reaction rate modelling of industrial-scale pyrolysis reactors.

Organic matter and sediment properties determine in-lake variability of sediment CO<sub>2</sub> and CH<sub>4</sub> production and emissions of a small and shallow lake

Abstract. Inland waters, particularly small and shallow lakes, are significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. However, the spatial in-lake heterogeneity of CO2 and CH4 production processes and their drivers in the sediment remain poorly studied. We measured potential CO2 and CH4 production in slurry incubations from 12 sites within the small and shallow crater lake Windsborn in Germany, as well as fluxes at the water–atmosphere interface of intact sediment core incubations from four sites. Production rates were highly variable and ranged from 7.2 to 38.5 µmol CO2 gC−1 d−1 and from 5.4 to 33.5 µmol CH4 gC−1 d−1. Fluxes ranged from 4.5 to 26.9 mmol CO2 m−2 d−1 and from 0 to 9.8 mmol CH4 m−2 d−1. Both CO2 and CH4 production rates and the CH4 fluxes exhibited a significant and negative correlation (p<0.05, ρ<−0.6) with a prevalence of recalcitrant organic matter (OM) compounds in the sediment as identified by Fourier-transformed infrared spectroscopy. The carbon / nitrogen ratio exhibited a significant negative correlation (p<0.01, ρ=-0.88) with CH4 fluxes but not with production rates or CO2 fluxes. The availability of inorganic (nitrate, sulfate, ferric iron) and organic (humic acids) electron acceptors failed to explain differences in CH4 production rates, assuming a competitive suppression, but observed non-methanogenic CO2 production could be explained up to 91 % by prevalent electron acceptors. We did not find clear relationships between OM quality, the thermodynamics of methanogenic pathways (acetoclastic vs. hydrogenotrophic) and electron-accepting capacity of the OM. Differences in CH4 fluxes were interestingly to a large part explained by grain size distribution (p<0.05, ρ=±0.65). Surprisingly though, sediment gas storage, potential production rates and water–atmosphere fluxes were decoupled from each other and did not show any correlations. Our results show that within a small lake, sediment CO2 and CH4 production shows significant spatial variability which is mainly driven by spatial differences in the degradability of the sediment OM. We highlight that studies on production rates and sediment quality need to be interpreted with care, though, in terms of deducing emission rates and patterns as approaches based on production rates only neglect physical sediment properties and production and oxidation processes in the water column as major controls on actual emissions.

Groundwater‐Driven Methane Export Reduces Salt Marsh Blue Carbon Potential

AbstractThe burial of “blue carbon” in coastal marsh soils is partially offset by marsh‐atmosphere methane (CH4) fluxes, but this offset may be greater if other pathways of CH4 export exist. Here we report that salt marshes also export dissolved CH4 via submarine groundwater discharge (SGD). The volumetric fluxes of salt marsh groundwater into adjacent tidal creeks were calculated from mass balances of the conservative tracer 226Ra at four study sites in coastal Georgia, USA. Over the 2‐year study period, volumetric groundwater fluxes across all sites ranged between 1,700 and 105,000 m3 day−1. Dissolved CH4 fluxes of 27–1,200 μmol CH4 m−2 day−1 were calculated by multiplying the volumetric groundwater flux by the groundwater CH4 concentration and normalizing to the intertidal salt marsh area estimated from satellite images. On a mass basis, the cross‐site range in CH4 fluxes was 1.3–5.5 g CH4 m−2 year−1 with a cross‐site mean of 2.8 g CH4 m−2 year−1. This is equivalent to 125 (56–245) g CO2 m−2 year−1 assuming that CH4 is 45 times more potent than CO2 as a greenhouse gas over a 100‐year time frame. This sustained‐flux global warming potential is similar to the 138 (1.1–260) g CO2 m−2 year−1 average calculated across other studies of the direct marsh soil to atmosphere CH4 flux. Therefore, SGD drives an effective doubling of salt marsh CH4 export that offsets a combined total of ~30% of the global cooling potential derived from soil carbon sequestration.

Coupled reduction of bicarbonate to methane and generation/ sequestration of carbon dioxide in natural basaltic- and clay- matrices at 25 °C ≤ T ≤ 100 °C

In nature, methane (CH4) is produced via both biotic and abiotic processes, yet little is known on the abiotic mechanism(s) on the hydrogenation of abiotic carbon (C) to produce CH4 at low temperatures. This study reports on the abiotic generation of CH4via Fischer-Tropsch type reactions at room temperature. Natural sources of mineral Fe2+ were reacted with aqueous bicarbonate under sterile and non-sterile conditions. The selected sources of mineral Fe2+ were well-characterized Ediacaran basalts and green clay veins in basalts from the Volyn-Brest continental flood basalt province and hectorite SHCa-1 from Hector, California. Quantitative X-ray powder diffraction results and the Mössbauer characteristics of these materials from the literature were supplemented with specific surface area values determined by N2 physisorption, high-resolution transmission electron microscopy and nanodiffraction analysis. The headspace concentration of the gases in the dispersions was related to the presence of bicarbonate and the mineral composition. Evidence presented herein confirmed that wüstite occurs naturally in SHCa-1, which is unprecedented. Of all mineral dispersions those containing wüstite (basalt MIB and SHCa-1) showed the highest concentration of CH4 and CO2, ranging from 1457 to 6329 μmol CH4 g solid−1 and from 2100 to 8771 μmol CO2 g solid−1, respectively. The production of CO2 was also registered in dispersions containing the sources of mineral Fe(II) chlorite and saponite, which ranged from 375 to 8180 μmol CO2 g solid−1 and from 480 to 1230 μmol CO2 g solid−1, respectively. Registered mineral transformations i) wüstite → hematite and ii) chlorite or saponite → hematite, and iii) wüstite → pyrite → jarosite were explained because bicarbonate and wüstite (chlorite or saponite) reduction and oxidation to CH4 and hematite; and wustite dissolution releasing Fe2+ that reacted with sulfide ions.

Methane and Primary Productivity in Lakes: Divergence of Temporal and Spatial Relationships

AbstractIn lakes, the production and emission of methane (CH4) have been linked to lake trophic status. However, few studies have quantified the temporal response of lake CH4 dynamics to primary productivity at the ecosystem scale or considered how the response may vary across lakes. Here, we investigate relationships between lake CH4 dynamics and ecosystem primary productivity across both space and time using data from five lakes in northern Wisconsin, USA. From 2014 to 2019, we estimated hypolimnetic CH4 storage rates for each lake using timeseries of hypolimnetic CH4 concentration through the summer season. Across all lakes and years, hypolimnetic CH4 storage ranged from <0.001 to 7.6 mmol CH4 m−2 d−1 and was positively related to the mean summer rate of gross primary productivity (GPP). However, within‐lake temporal responses to GPP diverged from the spatial relationship, and GPP was not a significant predictor of interannual variability in CH4 storage at the lake scale. Using these data, we consider how and why temporal responses may differ from spatial patterns and demonstrate how extrapolating cross‐lake relationships for prediction at the lake scale may substantially overestimate the rate of change of CH4 dynamics in response to lake primary productivity. We conclude that future predictions of lake‐mediated climate feedbacks in response to a shifting distribution of trophic status should incorporate both varying lake responses and the temporal scale of change.

High contribution of methane in greenhouse gas emissions from a eutrophic lake: a mass balance synthesis

ABSTRACT Lakes are ‘hotspots’ for greenhouse gas (GHG) emissions, primarily carbon dioxide (CO2) and methane (CH4). Understanding the processes regulating GHG emissions from lakes, and their temporal variability, is essential for more accurately quantifying the role of lakes in global GHG cycles. In this study, we identified the processes that affect CO2 and CH4 concentrations in a small (0.3 km2) eutrophic monomictic lake (Okaro, New Zealand). A mass balance model was used to calculate changes in CO2 and CH4 storage in the lake as a result of internal cycling and atmospheric fluxes. To support model computations, CO2 and CH4 concentrations profiles were measured monthly over a one-year period, in addition to temperature, dissolved oxygen and chlorophyll a. Annually, Lake Okaro acted as a sink of CO2 from the atmosphere (425.4 mmol CO2 m−2 y−1) and a source of CH4 (553.4 mmol CH4 m−2 y−1) equating to a net GHG emission (diffusive fluxes of CO2 and CH4 combined) of 0.22 kg CO2-eq m−2 y−1. Although it may be viewed as conservative and applies only to diffusive fluxes, our study indicates that eutrophic lakes with high rates of primary production may act as a net source of GHGs.

Microbial electrolysis cell produced biogas as sustainable electron donor for microbial chromate reduction

Microbial electrolysis cell (MEC) is an attractive technology to produce biogas containing hydrogen (H2) and methane (CH4). Both gaseous products are effective electron donors for chromate [Cr(VI)] bioreduction when either is applied individually. However, microbial Cr(VI) reduction driven by H2 and CH4 mixture remains poorly understood. This study investigates Cr(VI) biotransformation supported by fluctuant MEC-produced biogas. Synergy between H2 and CH4 promoted Cr(VI) detoxification. Maximum Cr(VI) removal rate reached 1.81 ± 0.17 mg/L·d with 200.5 ± 13.6 μmol H2 and 203.3 ± 9.0 μmol CH4 produced by MEC at 0.6 V in a 4-d cycle. Biogas yield and H2/CH4 mole ratio were mainly regulated by Geobacter and Methanobacterium. The process in which Cr(VI) reduced to insoluble Cr(III) was proposed in two major pathways. A single autohydrogenotrophic genus (e.g., Hydrogenophaga, Thiobacillus) oxidized H2 independently coupled with Cr(VI) reduction. Cr(VI) reduction was also achieved through CH4-metabolizing microorganisms (e.g., Methanobacterium, Methanosaeta), which convert CH4 into organic metabolites to donate electrons for heterotrophic Cr(VI) reducers (e.g., Geobacter, Anaerolineaceae). Genes relevant to Cr(VI) reduction (yieF) and CH4 oxidation (mcrA) increased significantly. Electrons were effectively transferred through extracellular cytochrome c and intracellular nicotinamide adenine dinucleotide. This study offers an innovative route to sustainable bioremediation of Cr(VI)-polluted groundwater and promising direct utilization of mixed biogas from MEC.

Assessing the potential for non-turbulent methane escape from the East Siberian Arctic Shelf

Abstract. The East Siberian Arctic Shelf (ESAS) hosts large yet poorly quantified reservoirs of subsea permafrost and associated gas hydrates. It has been suggested that the global-warming induced thawing and dissociation of these reservoirs is currently releasing methane (CH4) to the shallow coastal ocean and ultimately the atmosphere. However, a major unknown in assessing the contribution of this CH4 flux to the global CH4 cycle and its climate feedbacks is the fate of CH4 as it migrates towards the sediment–water interface. In marine sediments, (an)aerobic oxidation reactions generally act as a very efficient methane sink. However, a number of environmental conditions can reduce the efficiency of this biofilter. Here, we used a reaction-transport model to assess the efficiency of the benthic methane filter and, thus, the potential for benthic methane escape across a wide range of environmental conditions that could be encountered on the East Siberian Arctic Shelf. Results show that, under steady-state conditions, anaerobic oxidation of methane (AOM) acts as an efficient biofilter. However, high CH4 escape is simulated for rapidly accumulating and/or active sediments and can be further enhanced by the presence of organic matter with intermediate reactivity and/or intense local transport processes, such as bioirrigation. In addition, in active settings, the sudden onset of CH4 flux triggered by, for instance, permafrost thaw or hydrate destabilization can also drive a high non-turbulent methane escape of up to 19 µmol CH4 cm−2 yr−1 during a transient, multi-decadal period. This “window of opportunity” arises due to delayed response of the resident microbial community to suddenly changing CH4 fluxes. A first-order estimate of non-turbulent, benthic methane efflux from the Laptev Sea is derived as well. We find that, under present-day conditions, non-turbulent methane efflux from Laptev Sea sediments does not exceed 1 Gg CH4 yr−1. As a consequence, we conclude that previously published estimates of ocean–atmosphere CH4 fluxes from the ESAS cannot be supported by non-turbulent, benthic methane escape.

Hydrogen utilization by Methylocystis sp. strain SC2 expands the known metabolic versatility of type IIa methanotrophs

Methane, a non-expensive natural substrate, is used by Methylocystis spp. as a sole source of carbon and energy. Here, we assessed whether Methylocystis sp. strain SC2 is able to also utilize hydrogen as an energy source. The addition of 2% H2 to the culture headspace had the most significant positive effect on the growth yield under CH4 (6%) and O2 (3%) limited conditions. The SC2 biomass yield doubled from 6.41 (±0.52) to 13.82 (±0.69) mg cell dry weight per mmol CH4, while CH4 consumption was significantly reduced. Regardless of H2 addition, CH4 utilization was increasingly redirected from respiration to fermentation-based pathways with decreasing O2/CH4 mixing ratios. Theoretical thermodynamic calculations confirmed that hydrogen utilization under oxygen-limited conditions doubles the maximum biomass yield compared to fully aerobic conditions without H2 addition. Hydrogen utilization was linked to significant changes in the SC2 proteome. In addition to hydrogenase accessory proteins, the production of Group 1d and Group 2b hydrogenases was significantly increased in both short- and long-term incubations. Both long-term incubation with H2 (37 d) and treatments with chemical inhibitors revealed that SC2 growth under hydrogen-utilizing conditions does not require the activity of complex I. Apparently, strain SC2 has the metabolic capacity to channel hydrogen-derived electrons into the quinone pool, which provides a link between hydrogen oxidation and energy production. In summary, H2 may be a promising alternative energy source in biotechnologically oriented methanotroph projects that aim to maximize biomass yield from CH4, such as the production of high-quality feed protein.

Anaerobic microbial communities and their potential for bioenergy production in heavily biodegraded petroleum reservoirs.

Most of the oil in low temperature, non-uplifted reservoirs is biodegraded due to millions of years of microbial activity, including via methanogenesis from crude oil. To evaluate stimulating additional methanogenesis in already heavily biodegraded oil reservoirs, oil sands samples were amended with nutrients and electron acceptors, but oil sands bitumen was the only organic substrate. Methane production was monitored for over 3000 days. Methanogenesis was observed in duplicate microcosms that were unamended, amended with sulfate or that were initially oxic, however methanogenesis was not observed in nitrate-amended controls. The highest rate of methane production was 0.15 μmol CH4 g-1 oil d-1 , orders of magnitude lower than other reports of methanogenesis from lighter crude oils. Methanogenic Archaea and several potential syntrophic bacterial partners were detected following the incubations. GC-MS and FTICR-MS revealed no significant bitumen alteration for any specific compound or compound class, suggesting that the very slow methanogenesis observed was coupled to bitumen biodegradation in an unspecific manner. After 3000 days, methanogenic communities were amended with benzoate resulting in methanogenesis rates that were 110-fold greater. This suggests that oil-to-methane conversion is limited by the recalcitrant nature of oil sands bitumen, not the microbial communities resident in heavy oil reservoirs.

Warming enhances carbon dioxide and methane fluxes from Red Sea seagrass (<i>Halophila stipulacea</i>) sediments

Abstract. Seagrass meadows are autotrophic ecosystems acting as carbon sinks, but they have also been shown to be sources of carbon dioxide (CO2) and methane (CH4). Seagrasses can be negatively affected by increasing seawater temperatures, but the effects of warming on CO2 and CH4 fluxes in seagrass meadows have not yet been reported. Here, we examine the effect of two disturbances on air–seawater fluxes of CO2 and CH4 in Red Sea Halophila stipulacea communities compared to adjacent unvegetated sediments using cavity ring-down spectroscopy. We first characterized CO2 and CH4 fluxes in vegetated and adjacent unvegetated sediments, and then experimentally examined their response, along with that of the carbon (C) isotopic signature of CO2 and CH4, to gradual warming from 25 ∘C (winter seawater temperature) to 37 ∘C, 2 ∘C above current maximum temperature. In addition, we assessed the response to prolonged darkness, thereby providing insights into the possible role of suppressing plant photosynthesis in supporting CO2 and CH4 fluxes. We detected 6-fold-higher CO2 fluxes in vegetated compared to bare sediments, as well as 10- to 100-fold-higher CH4 fluxes. Warming led to an increase in net CO2 and CH4 fluxes, reaching average fluxes of 10 422.18 ± 2570.12 µmol CO2 m−2 d−1 and 88.11±15.19 µmol CH4 m−2 d−1, while CO2 and CH4 fluxes decreased over time in sediments maintained at 25 ∘C. Prolonged darkness led to an increase in CO2 fluxes but a decrease in CH4 fluxes in vegetated sediments. These results add to previous research identifying Red Sea seagrass meadows as a significant source of CH4, while also indicating that sublethal warming may lead to increased emissions of greenhouse gases from seagrass meadows, providing a feedback mechanism that may contribute to further enhancing global warming.

A synthesis of methane emissions from shallow vegetated coastal ecosystems.

Vegetated coastal ecosystems (VCEs; i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cycling, storing 10× more C than temperate forests. Methane (CH4 ), a potent greenhouse gas, can form in the sediments of these ecosystems. Currently, CH4 emissions are a missing component of VCE C budgets. This review summarizes 97 studies describing CH4 fluxes from mangrove, salt marsh, and seagrass ecosystems and discusses factors controlling CH4 flux in these systems. CH4 fluxes from these ecosystems were highly variable yet they all act as net methane sources (median, range; mangrove: 279.17, -67.33 to 72,867.83; salt marsh: 224.44, -92.60 to 94,129.68; seagrass: 64.80, 1.25-401.50µmol CH4 m-2 day-1 ). Together CH4 emissions from mangrove, salt marsh, and seagrass ecosystems are about 0.33-0.39 Tmol CH4 -C/year-an addition that increases the current global marine CH4 budget by more than 60%. The majority (~45%) of this increase is driven by mangrove CH4 fluxes. While organic matter content and quality were commonly reported in individual studies as the most important environmental factors driving CH4 flux, they were not significant predictors of CH4 flux when data were combined across studies. Salinity was negatively correlated with CH4 emissions from salt marshes, but not seagrasses and mangroves. Thus the available data suggest that other environmental drivers are important for predicting CH4 emissions in vegetated coastal systems. Finally, we examine stressor effects on CH4 emissions from VCEs and we hypothesize that future changes in temperature and other anthropogenic activites (e.g., nitrogen loading) will likely increase CH4 emissions from these ecosystems. Overall, this review highlights the current and growing importance of VCEs in the global marine CH4 budget.