Atmospheric Methane Research Articles

Evidence of cryptic methane cycling and non-methanogenic methylamine consumption in the sulfate-reducing zone of sediment in the Santa Barbara Basin, California

Abstract. The recently discovered cryptic methane cycle in the sulfate-reducing zone of marine and wetland sediment couples methylotrophic methanogenesis to anaerobic oxidation of methane (AOM). Here we present evidence of cryptic methane cycling activity within the upper regions of the sulfate-reducing zone, along a depth transect within the Santa Barbara Basin, off the coast of California, USA. The top 0–20 cm of sediment from each station was subjected to geochemical analyses and radiotracer incubations using 35S–SO42-, 14C–mono-methylamine, and 14C–CH4 to find evidence of cryptic methane cycling. Methane concentrations were consistently low (3 to 16 µM) across the depth transect, despite AOM rates increasing with decreasing water depth (from max 0.05 nmol cm−3 d−1 at the deepest station to max 1.8 nmol cm−3 d−1 at the shallowest station). Porewater sulfate concentrations remained high (23 to 29 mM), despite the detection of sulfate reduction activity from 35S–SO42- incubations with rates up to 134 nmol cm−3 d−1. Metabolomic analysis showed that substrates for methanogenesis (i.e., acetate, methanol and methylamines) were mostly below the detection limit in the porewater, but some samples from the 1–2 cm depth section showed non-quantifiable evidence of these substrates, indicating their rapid turnover. Estimated methanogenesis from mono-methylamine ranged from 0.2 to 0.5 nmol cm−3 d−1. Discrepancies between the rate constants (k) of methanogenesis (from 14C–mono-methylamine) and AOM (from either 14C–mono-methylamine-derived 14C–CH4 or from directly injected 14C–CH4) suggest the activity of a separate, concurrent metabolic process directly metabolizing mono-methylamine to inorganic carbon. We conclude that the results presented in this work show strong evidence of cryptic methane cycling occurring within the top 20 cm of sediment in the Santa Barbara Basin. The rapid cycling of carbon between methanogenesis and methanotropy likely prevents major build-up of methane in the sulfate-reducing zone. Furthermore, our data suggest that methylamine is utilized by both methanogenic archaea capable of methylotrophic methanogenesis and non-methanogenic microbial groups. We hypothesize that sulfate reduction is responsible for the additional methylamine turnover, but further investigation is needed to elucidate this metabolic activity.

Single-Molecule Methane Sensing Using Palladium-Functionalized nIR Fluorescent Single-Walled Carbon Nanotubes.

There has been considerable interest in detecting atmospheric and process-associated methane (CH4) at low concentrations due to its potency as a greenhouse gas. Nanosensor technology, particularly fluorescent single-walled carbon nanotube (SWCNT) arrays, is promising for such applications because of their chemical sensitivities at single-molecule detection limits. However, the methodologies for connecting the stochastic molecular fluctuations from gas impingement on such sensors require further development. In this work, we synthesize Pd-conjugated ss(GT)15-DNA-wrapped SWCNTas near-infrared (nIR) fluorescent, single-molecule sensors of CH4. The complexes are characterized using X-ray photoelectron spectroscopy (XPS) and spectrophotometry, demonstrating spectral changes between the Pd2+ and Pd0 oxidation states. The nIR fluctuations generated upon exposure from 8 to 26 ppb of CH4 were separated into high- and low-frequency components. Aggregating the low-frequency components for an array of sensors showed the most consistent levels of detection with a limit of 0.7 ppb. These results advance the hardware and computational methods necessary to apply this approach to the challenge of environmental methane sensing.

Avoiding methane emission rate underestimates when using the divergence method

Methane is a powerful greenhouse gas, and a primary target for mitigating climate change in the short-term future due to its relatively short atmospheric lifetime and greater ability to trap heat in Earth’s atmosphere compared to carbon dioxide. Top-down observations of atmospheric methane are possible via drone and aircraft surveys as well as satellites such as the TROPOspheric Monitoring Instrument (TROPOMI). Recent work has begun to apply the divergence method to produce regional methane emission rate estimates. Here we show that when the divergence method is applied to spatially incomplete observations of methane, it can result in negatively biased time-averaged regional emission rates. We show that this effect can be counteracted by adopting a procedure in which daily advective fluxes of methane are time-averaged before the divergence method is applied. Using such a procedure with TROPOMI methane observations, we calculate yearly Permian emission rates of 3.1, 2.4 and 2.7 million tones per year for the years 2019 through 2021. We also show that highly-resolved plumes of methane can have negatively biased estimated emission rates by the divergence method due to the presence of turbulent diffusion in the plume, but this is unlikely to affect regional methane emission budgets constructed from TROPOMI observations of methane. The results from this work are expected to provide useful guidance for future implementations of the divergence method for emission rate estimation from satellite data—be it for methane or other gaseous species in the atmosphere.

Successful Integration of UAV Aeromagnetic Mapping with Terrestrial Methane Emissions Surveys in Orphaned Well Remediation

Orphaned oil and gas wells represent an important environmental and economic development concern in areas where historical energy exploration and production activity pre-dated regulations on well abandonment documentation practices. From an economic development perspective, these wells pose a subsurface risk to infrastructure development, while the environmental impact of orphaned wells is largely associated with uncontrolled emissions of both fluids and gasses, most notably atmospheric methane. Often neglected in regulatory oversight, methane emissions from orphaned wells contribute to global greenhouse gas concentrations and without proper mitigation, these emissions contribute to and further exacerbate global climate change processes. An inherent challenge of remediation efforts targeting orphaned wells is the consistent observation that only a fraction of located and identified wells produce the majority of methane emissions, yet no methodology exists to effectively separate out heavy emitters without visiting and assessing each and every well. We demonstrate that emitting wells have certain defined characteristics largely pertaining to the presence and integrity of metal casing to the surface, which can be distinguished as an organized high intensity magnetic anomaly in low-altitude UAV-based aeromagnetic surveys. In this paper, we present a database of identified and ground-truthed wellsites correlated to their magnetic signatures and provide a roadmap to initial prioritization of wellsites for subsequent remediation activities that can be implemented in complex environments where other survey options are not feasible. In a regulatory environment where resources dedicated to wellsite remediation are limited, we propose implementing this methodology as a key element of a targeted approach to wellsite prioritization for subsequent remediation activity.

Identifying Putative Subsurface Microbial Drivers of Methane Flux on Earth and Mars

On Earth microorganisms are critical drivers of the methane cycle, both producing and consuming methane (Boetius et al. 2000, Knittel and Boetius 2009, Orphan et al. 2001). Molecular and isotopic-based investigations of archaeal-bacterial consortia catalyzing the anaerobic oxidation of methane (AOM) in marine methane seeps identified the pivotal role of these microorganisms in mitigating the release of methane into the atmosphere (Knittel and Boetius 2009, Orphan et al. 2001). In the marine environment, AOM is predominantly carried out by closely associated consortia of methanotrophic archaea (ANME) and sulfate reducing bacteria (SRB) coupling methane oxidation to sulfate reduction in the absence of oxygen. Wolf Spring (WS), Axel Heiberg Island, Nunavut is a hypersaline cold spring methane seep and the only known terrestrial permafrost hosted methane seep known to host ANME-1 archaea associated with AOM (Niederberger et al. 2010, Magnuson et al. 2022). Wolf Spring is an unparalleled analogue for putative subsurface brines and sites of methane release on Mars. Enigmatic observations of methane in the near-surface Martian atmosphere remain a tantalizing potential biosignature. The combination of field site characterization, microbial microcosm experiments, and in situ methane monitoring represents a coordinated interdisciplinary effort to identify methane driven microbial metabolisms not only critical to understanding methane flux in the Arctic, but also as possible drivers to the methane cycle on Mars. Detailed microbial characterization of these springs has identified a chemotrophic community dominated by sulfur cycling (Altshuler et al. 2022, Niederberger et al. 2010). To date, microbial and geochemical characterization has been carried out on sediment samples to a few centimeters depth. This study expands on these initial studies, with the successful collection and analysis of deeper sediment cores at WS focusing on AOM activity to better understand the microorganisms involved and the methane cycling capacity at depth. Two decades of observing methane on Mars (Mumma et al. 2009) have generated data indicative of a dynamic, geochemical system characterized by a profile similar to the release of methane from seeps on Earth (Etiope and Oehler 2019) producing both distinct pulses known as plumes and slow background seepage. These observations suggest as of yet unknown geochemical and potentially geobiological methane sources and sinks. While methane can be produced abiotically (Etiope and Lollar 2013), on Earth most methane is biogenic. Determining the biogenicity of CH4 is non-trivial and requires a correlated approach including determination of carbon isotopes. In terrestrial systems, biogenic CH4 is 13C depleted. To characterize methane sources and sinks on Mars, near surface measurements at a frequency not possible with existing instrumentation are required. We are currently developing off-axis integrated cavity-enhanced output (OA-ICOS) spectrometry as a portable trace gas analyzer capable of obtaining high frequency measurements of methane at the sub-ppb level (Sapers et al. 2021). Optimizing OA-ICOS trace methane measurements at WS will help refine sensitivity and measurement cadence in a Mars-like environment as well as providing new remote methane monitoring capabilities for Arctic methane emissions. We are currently developing in situ 12CH4:13CH4 capabilities using OA-ICOS technology. The importance of δ 13C as a biosignature is summarized in Fig. 1.

Updates on microbial Iodine Cycling in snotty Biofilms of a prealpine Mineral Spring Cavern

We have previously described the massive, methane-oxidizing microbial biofilms discovered in the cavern of an iodine-rich former medicinal spring in prealpine southern Germany (Karwautz et al. 2017). Next to up to 3000 ppm of methane in the cavern atmosphere, the mineral spring water can contain up to 23 mM of iodine, ~thousand-fold higher than in natural freshwaters. Since reactive iodine species can be toxic for microbes, the massive microbial growth in this cave is a fascinating phenomenon. We postulate that microbes capable of utilizing different iodine species should be prevalent in the cavern. Here, we present our recent work investigating the possible involvement of biofilm microbiota in either oxidative or reductive iodine cycling. Gradient tubes set up with iodide and oxygen as redox partners showed ample microbial growth and the formation of elemental iodine. Amplicon sequencing suggested different Alpha- (Magnetospirillum spp.) and Gammaproteobacteria (Aeromonas spp.) to be capable of iodide oxidation. Moreover, we address a possible iodate-dependent methane oxidation hosted within biofilm microbiota. Metagenomes allowed to assemble the MAGs of a novel member of the recently discovered anaerobic Methylomirabilota methanotrophs, Candidatus Methylomirabilis iodofontis. Its genetic repertoire included not only known markers of oxygenic denitrification and aerobic methane oxidation, but also of iodate respiration (Zhu et al. 2022). Our ongoing work will provide further evidence of the still largely uncharted iodine-cycling ecophysiologies of the biofilm microbiota of this unique microbe-dominated subsurface ecosystem.

Anaerobic Isoprene-Degrading Microorganisms and their Impact on Microbial Methane Dynamics in Deep-Sea Carbonates and Eucalyptus-Leaf Sediments

Isoprene, a highly abundant biogenic volatile organic compound, has emerged as a crucial yet overlooked factor in addressing climate change. Despite its widespread production in all forms of life, comprehensive data on its global biogeochemical cycle remain scarce. Isoprene's reactivity in the atmosphere influences methane concentrations, with detrimental implications for climate, air quality, and health. Conversely, methane is abundant in marine and terrestrial subsurface environments, where deep-sea carbonates serve as hotspots for microorganisms performing anaerobic methane oxidation—an essential process in long-term methane storage and removal from the marine carbon cycle. Recent studies by Beckmann et al. (2020), Prouty et al. (2020) have shed light on novel metabolic pathways utilized by microbial communities in carbonates for methane oxidation. Similarly, in terrestrial environments, eucalyptus trees, as the highest emitters of isoprene, may harbor microorganisms capable of metabolizing isoprene alongside methane-oxidation and formation in eukalyptus leaf detritus. However, little is known about the fate of isoprene and the potential microbial communities involved in its metabolism within deep-sea carbonates and eucalyptus-leaf sediments, potentially impacting methane metabolism. Our study aims to unravel the anaerobic pathways of microbial isoprene degradation and investigate the effects of isoprene abundance and degradation on microbial methane production and oxidation. We detected anaerobic isoprene-degradation in deep-sea carbonates and terrestrial eukalyptus-leave sediments, where methane oxidation and methane formation was pronounced. Surprisingly, the presence of isoprene inhibited mcirobial methane-oxidation as well as methane-formation, suggesting an substantial impact of the presence of isoprene on methane cycling and storage. We are currently characterizing and isolating the microorganisms involved in the isoprene and methane metabolism in these enrichment cultures.

Clumped and conventional isotopes of natural gas reveal basin burial, denudation, and biodegradation history

Formation and post-genetic alteration of hydrocarbons provide insights into the dynamic and complex geologic, hydrologic, and microbial history of shallow crustal environments. Clumped isotopologues of methane (e.g., Δ13CH3D) have emerged as a proxy for constraining methane formation temperatures in sedimentary basins. However, unrealistically high apparent temperatures and microbial cycling of methane necessitate further investigation into how the generation and biodegradation of hydrocarbons may modify methane clumped isotopologue signatures. This study analyzed and modeled the clumped isotopologues of methane, in addition to traditional gas isotopes, to provide new insights into the origin, thermal maturity, migration, and biodegradation histories of hydrocarbons in the Paradox Basin in the Colorado Plateau. The basin was deeply buried in the geologic past and has been recently incised, leading to rapid denudation, enhanced meteoric circulation, and microbial activity. δ13CCH4 and CH4/ΣC2+ ratios suggest that most natural gases in various reservoirs throughout the basin are thermogenic in origin with variable thermal maturities. However, signatures suggestive of anaerobic oxidation of ethane and propane, and secondary microbial methane generation, exist. In the northeastern part of the basin, Δ13CH3D values in reservoirs above and below the Paradox Formation source rocks are consistent with thermodynamic equilibrium, indicating that the thermally mature hydrocarbons equilibrated at ≥160 °C during maximum burial over 30–80 Ma. Disequilibrium Δ13CH3D values of natural gas in Paradox Formation reservoirs along the southwestern margin of the basin suggest the presence of low-maturity hydrocarbons consistent with the region’s shallower burial history. Models of Δ13CH3D values based on the exchange rate of hydrogen isotopes between methane and water and the basin thermal history support that meteoric recharge and microbial activity, following incision/denudation over the past few million years, promoted anaerobic oxidation of hydrocarbons (particularly ethane and propane), biodegradation of crude oil, and generation of secondary microbial methane in shallow reservoirs.

Global termite methane emissions have been affected by climate and land-use changes

Termites with symbiotic methanogens are a known source of atmospheric methane (CH4), but large uncertainties remain regarding the flux magnitude. This study estimated global termite CH4 emissions using a framework similar to previous studies but with contemporary datasets and a biogeochemical model. The global termite emission in 2020 was estimated as 14.8 ± 6.7 Tg CH4 year−1, mainly from tropical and subtropical ecosystems, indicating a major natural source from upland regions. Uncertainties associated with estimation methods were assessed. The emission during the historical period 1901–2021 was estimated to have increased gradually (+ 0.7 Tg CH4 year−1) as a result of combined influences of elevated CO2 (via vegetation productivity), climatic warming, and land-use change. Future projections using climate and land-use scenarios (shared socioeconomic pathways [ssp] 126 and 585) also showed increasing trends (+ 0.5 to 5.9 Tg CH4 year−1 by 2100). These results suggest the importance of termite emissions in the global CH4 budget and, thus, in climatic prediction and mitigation.

Correction to: 'Is the destruction or removal of atmospheric methane a worthwhile option?' (2021) by Nisbet-Jones etal.

Correction to: 'Is the destruction or removal of atmospheric methane a worthwhile option?' (2021) by Nisbet-Jones etal.

Higher rice grain yield and lower methane emission achieved by alternate wetting and drying in central Vietnam

Rice is the main staple food for more than half of the world’s population, but rice cultivation is a significant source of atmospheric methane (CH4). Alternate wetting and drying (AWD) reduces CH4 emission from paddy field, but the effect on rice yield remains unclear. Organic soil amendment increases grain yield but simultaneously increases CH4 emission. Therefore, the combination of AWD and organic amendment may compensate for each other’s shortcomings. The objective of this study was to assess whether AWD and organic amendment can increase rice yield while mitigating CH4 emission. The experiments were conducted in a farmer’s paddy field in Thua Thien Hue province, Vietnam, during five consecutive rice growing seasons in 2019–2021. Two water management practices, continuous flooding (CF) and AWD, with (+O) or without (−O) the application of a commercially available organic fertilizer were examined under normal cultivation conditions. Compared with CF, AWD significantly reduced CH4 emission by 34 %, increased nitrous oxide (N2O) by 46 %, and increased grain yield by 4.4 %. The +O treatment significantly increased the yield by 3.7 % relative to −O. Relative to CF−O by season, AWD+O significantly increased the yield. The integrated global warming potential of CH4 and N2O emissions was decreased by 33 % and irrigation water use was reduced by 33 % in AWD plots relative to that in CF plots. These results indicate that AWD by itself has the potential to increase rice yield as well as reduce CH4 emission, and the combination of AWD with organic amendment would ensure yield increase.

Land use drives large CH4 fluxes from a highly urbanized Indian estuary

There is growing awareness of the need to better constrain the contribution of atmospheric methane (CH4) fluxes from urbanized estuaries due to the high global warming potential of CH4 and the accelerating growth of urban expansion. This study undertook seasonal sampling campaigns to understand the impact of urbanization on atmospheric CH4 fluxes and their drivers in a large, tropical estuary in India. Overall, the study found that the Cochin estuary emitted large amounts of CH4 (398.8 ± 141.6 μmolm−2d−1) to the atmosphere with CH4 hotspots reaching up to 939.7 μmolm−2d−1 were identified. The strongest drivers of CH4 dynamics in different anthropogenically impacted zones were traced. The source of organic matter for CH4 production was revealed to be terrestrial C3 plants, autochthonous production, marine phytoplankton, and sewage inputs. The study suggests that monsoonal urbanized tropical estuaries may be an important but under-recognized element of the global CH4 budget.

The Transient Outgassed Atmosphere of 55 Cancri e

T​​​​​​he enigmatic nature of 55 Cancri e has defied theoretical explanation. Any explanation needs to account for the observed variability of its secondary eclipse depth, which is at times consistent with zero in the visible/optical range of wavelengths—a phenomenon that does not occur with its also variable infrared eclipses. Yet despite this variability, its transit depth remains somewhat constant in time and is inconsistent with opaque material filling its Hill sphere. The current study explores the possibility of a thin, transient, secondary atmosphere on 55 Cancri e that is sourced by geochemical outgassing. Its transient nature derives from the inability of outgassing to be balanced by atmospheric escape. As the outgassed atmosphere escapes and is replenished, it rapidly adjusts to radiative equilibrium and the temperature fluctuations cause the infrared eclipse depths to vary. Atmospheres of pure carbon dioxide or carbon monoxide produce sufficient Rayleigh scattering to explain the observed optical/visible eclipse depths, which vanish in the absence of an atmosphere and the presence of a dark rocky surface. Atmospheres of pure methane are ruled out, because they produce insufficient Rayleigh scattering. Upcoming observations by the James Webb Space Telescope will potentially allow the atmospheric temperature and surface pressure, as well as the surface temperature, to be measured.

Multi-heme cytochrome-mediated extracellular electron transfer by the anaerobic methanotroph ‘Candidatus Methanoperedens nitroreducens’

Anaerobic methanotrophic archaea (ANME) carry out anaerobic oxidation of methane, thus playing a crucial role in the methane cycle. Previous genomic evidence indicates that multi-heme c-type cytochromes (MHCs) may facilitate the extracellular electron transfer (EET) from ANME to different electron sinks. Here, we provide experimental evidence supporting cytochrome-mediated EET for the reduction of metals and electrodes by ‘Candidatus Methanoperedens nitroreducens’, an ANME acclimated to nitrate reduction. Ferrous iron-targeted fluorescent assays, metatranscriptomics, and single-cell imaging suggest that ‘Ca. M. nitroreducens’ uses surface-localized redox-active cytochromes for metal reduction. Electrochemical and Raman spectroscopic analyses also support the involvement of c-type cytochrome-mediated EET for electrode reduction. Furthermore, several genes encoding menaquinone cytochrome type-c oxidoreductases and extracellular MHCs are differentially expressed when different electron acceptors are used.

Significant methane ebullition from large shallow eutrophic lakes of the semi-arid region of northern China

Eutrophic lakes are a major source of the atmospheric greenhouse gas methane (CH4), and CH4 ebullition emissions from inland lakes have important implications for the carbon cycle. However, the spatio-temporal heterogeneity of CH4 ebullition emission and its influencing factors in shallow eutrophic lakes of arid and semi-arid regions remain unclear. This study aimed to determine the mechanism of CH4 emission via eutrophication in Lake Ulansuhai, a large shallow eutrophic lake in a semi-arid region of China.To this end, monthly field surveys were conducted from May to October 2021, and gas chromatography was applied using the headspace equilibrium technique with an inverted funnel arrangement. The total CH4 fluxes ranged from 0.102 mmol m−2 d−1 to 59.296 mmol m−2 d−1 with an average value of 4.984 ± 1.82 mmol m−2 d−1. CH4 ebullition emissions showed significant temporal and spatial variations. The highest CH4 ebullition emission was observed in July with a grand mean of 9.299 mmol m−2 d−1, and the lowest CH4 ebullition emissions occurred in October with an average of 0.235 mmol m−2 d−1. Among seven sites (S1–S7), the maximum (3.657 mmol m−2 d−1) and minimum (1.297 mmol m−2 d−1). CH4 ebullition emissions were observed at S2 and S7, respectively. As the main route of CH4 emission to the atmosphere in Lake Ulansuhai, the CH4 ebullition flux during May to October accounted for 69% of the total CH4 flux. Statistical analysis showed that CH4 ebullition was positively correlated with temperature (R = 0.391, P < 0.01) and negatively correlated with air pressure (R = 0.286, P < 0.00). Temperature and air pressure were found to strongly regulate the production and oxidation of CH4. Moreover, nutritional status indicators such as TP and NH4+-N significantly affect CH4 ebullition emissions (R = 0.232, P < 0.01; R = −0.241, P < 0.01). This study reveals the influencing factors of CH4 ebullition emission in Lake Ulansuhai, and provides theoretical reference and data support for carbon emission from eutrophic lakes. Nevertheless, research on eutrophic shallow lakes needs to be further strengthened. Future research should incorporate improved flux measurement techniques with process-based models to improve the accuracy from regional to large-scale estimation of CH4 emissions and clarify the carbon budget of aquatic ecosystems. In this manner, the understanding and predictability of CH4 ebullition emission from shallow lakes can be improved.

Aquatic plant wax hydrogen and carbon isotopes in Greenland lakes record shifts in methane cycling during past Holocene warming.

Predicting changes to methane cycling in Arctic lakes is of global concern in a warming world but records constraining lake methane dynamics with past warming are rare. Here, we demonstrate that the hydrogen isotopic composition (δ2H) of mid-chain waxes derived from aquatic moss clearly decouples from precipitation during past Holocene warmth and instead records incorporation of methane in plant biomass. Trends in δ2Hmoss and δ13Cmoss values point to widespread Middle Holocene (11,700 to 4200 years ago) shifts in lake methane cycling across Greenland during millennia of elevated summer temperatures, heightened productivity, and lowered hypolimnetic oxygen. These data reveal ongoing warming may lead to increases in methane-derived C in many Arctic lakes, including lakes where methane is not a major component of the C cycle today. This work highlights a previously unrecognized mechanism influencing δ2H values of mid-chain wax and draws attention to the unquantified role of common aquatic mosses as a potentially important sink of lake methane across the Arctic.

Changes in stoichiometric ratio of carbon and sulfate affect methanogenesis pathways in sulfate-rich sewers

Methane emissions from sewer systems have been a major concern in terms of posing safety risks and aggravating the greenhouse effect. Discharging a large amount of sulfate-rich wastewater produced by industrial manufacture into municipal sewers breaks the balance between organic matter and sulfate, and has a significant influence on methanogenesis. However, the direct impact of sulfate-rich wastewater on the interactions between methane and sulfate metabolism in sewers remains unclear. In this study, methane production, microbial community and functional genes of the sewers with different chemical oxygen demand/sulfate ratios (C/S) were studied by 33 sampling sites collected in full-scale field investigation. The concentrations of chemical oxygen demand in the sewers were 10.0–378.0 mg/L, while the concentrations of sulfate fluctuated in a wider range of 4.1–758.4 mg/L, which distinguished the sewers into different C/S of 0.3, 2.5 and 5.8. With the average sulfate concentrations of 37.9 mg/L, the methane-production capabilities of sewers were reduced as the decreased C/S from 5.8 to 2.5. In contrast, the sewers with low C/S of 0.3 and high sulfate concentration of 368.2 mg/L exhibited twice as much methane concentrations (1.2 mL/L) than the sewers (0.6 mL/L) with C/S of 2.5 and 5.8. Genera showing a positive correlation (Enterobacter, Prevotella_9 and Acinetobacter) and a negative correlation (Bacteroidetes vadinHA17_norank, Smithella and Thiobacillus) with methanogenesis contributed significant variations of methane in the sewers with different C/S. The results of functional gene prediction suggested that the main pathways of methanogenesis in sewers with C/S of 5.8 and 2.5 were acetoclastic (ACSS and ackA) and hydrogenotrophic methanogenesis (mtrA, mtrH and K00400), whereas methylotrophic methanogenesis (mttB) was dominant in sewers with C/S of 0.3. The genes expressed by methylotrophic methanogens in sewer with low C/S and higher sulfate concentration refrained from the competitive inhibition by SRB, which contributed to the superior methane-production of the sewers. These results bridge the knowledge gap of methanogenesis with different sulfate levels and improve the understanding of methane cycle driven by microorganisms, facilitating more accurate prediction and assessment of methane in sewers.

Dual sources and consumption of methane in shallow sediments of southern SCS: Insight from porewater composition and numerical modeling

Methane seepages have significant impacts on marine environments and ecosystems. Deep understanding of that is critical for the resources exploration and the climate change. Numerous methane seepages have been reported worldwide, and mostly investigated on the slope and margin, with less attention paid to the basin. Here presents the determination of three sediment cores (H59, H79, and H55Y) retrieved in the Beikang Basin in the southern of the South China Sea (SCS), an important hydrocarbons and gas hydrates potential domain.A preliminary evaluation of the methane-related geochemical processes was obtained based on the compositions of porewater, and a reactive transport model was further developed to quantitively investigate the corresponding processes. The model results matched well the measurements. Both porewater compositions and model results indicated that anaerobic oxidation of methane dominantly consumed the sulfate at sites H55Y and H59, while sulfate was mainly consumed by organoclastic sulfate reduction at site H79. The shallow depths of SMI (3.8–7.8 mbsf), benthic methane fluxes (0.5–1.5 mmol m−2 yr−1) and high AOM rates (37–103 mmol m−2 yr−1) indicated the methane seepages at H55Y and H59. Model and sensitivity tests revealed further that the consumed methane was mainly supplied by external source, which ascended from deep reservoir, while in-situ methanogenesis contributed only a negligible fraction. The deduced δ13C of the external methane of H55Y demonstrated it was also microbial origin, which was probably contributed by the biogenic source rocks and/or dissociation of gas hydrates.The comprehensive regional research of the source and consumption of methane determined different seepage scenarios in the basin of southern SCS and enhanced our understanding of methane cycling in the marine basins.

Short-term variation in Mars atmospheric methane concentrations driven by barometric pumping

Measurements of atmospheric methane by the Curiosity rover's SAM-TLS instrument are providing evidence of seasonality and diurnal variation in concentration. Given methane's short atmospheric lifetime relative to geological timescales, its presence implies a replenishing source, and the observed variation demands the proposition of a modulation mechanism. This paper focuses on the modulation mechanism on a diurnal scale, extending earlier modeling of seasonal variation. Our modeling shows that barometric pumping driven by both diurnal and seasonal variation of atmospheric pressure, along with possible adsorption and desorption of methane in the shallow subsurface driven by temperature and pressure change, can explain the variation in methane concentration. In the model, an active, continuous, steady-state deep source of methane is assumed, and carbon dioxide serves as the carrier gas for drawing methane and possible trace gaseous constituents into the atmosphere.

Water depth and productivity regulate methane (CH4) emissions from temperate cascade reservoirs in northern China

Reservoirs are important sources of emissions of the greenhouse gas methane, but the temporal and spatial variability of methane emissions and the underlying mechanisms driving methane flux in cascade reservoirs remain poorly understood. In this study, we used floating chambers to study methane emission fluxes and their temporal and spatial variations in three temperate cascade reservoirs with different nutrient levels and morphologies in northern China. We measured the methane production rate, sediment properties and water quality to identify key factors driving methane emissions. The results showed that the three cascade reservoirs were persistent sources of atmospheric methane during the ice-free period, and the methane flux ranged from 1.01 to 107.17 µmol m-2h−1, with a mean of 11.17 µmol m-2h−1. The mean methane flux of the eutrophic reservoir was more than 2 and 4 times that of the two mesotrophic reservoirs. Overall, methane flux showed high spatial and temporal variability within and among reservoirs, and the three reservoirs showed different patterns of spatial and temporal variability. Statistical analysis showed that the three reservoirs had different key drivers, indicating that the methane emissions of the three reservoirs were regulated by different underlying mechanisms. When all three reservoirs were pooled, we found that Chl-a was the most important driver of methane emissions. Our results suggest that to obtain an accurate estimate of whole-reservoir methane flux, different reservoir zones and seasons must be considered. Our results also highlight that regional and global methane budget upscaling should include considerations of reservoir diversity (different climatic zones, morphology and trophic levels).