Anaerobic Oxidation Of Methane Rates Research Articles

Increasing temperature and sulfate enhances the efficiency of methane abatement in an anaerobic oxidation of methane bioreactor (AOMB) system

Controlling methane emission sources is important for the sustainable application of energy from the deep sea to alleviate methane-induced global warming. The anaerobic oxidation of methane (AOM) process coupled with sulfate reduction (SR) plays a key role in seafloor methane biofiltration. However, the mechanisms for the enhancement of the methane abatement efficiency in an AOM reaction system and the responses of temperature and sulfate concentration remain known. To cover the knowledge gap, this study investigated the enhancement of methane abatement in a self-manufactured AOMB system for 230 days via increasing temperatures and sulfate concentrations. The primary conclusions are as follows: (a) AOM and SR rates were significantly elevated at the conditions of temperature increase (8 to 15 °C) and sulfate addition (+15 mM). (b) Sulfate and temperature were key factors influencing the diversity of archaea and bacterial communities. (c) ANME-2c, ANME-1b, SEEP-SRB1, and Halodesulfovibrio were dominant genera of methane‑sulfur cycling in the system. (d) Functional gene prediction revealed that the coupling of methane oxidation and dissimilatory sulfate reduction was responsible for the methane abatement. Our findings provide new insights into the enhancement of methane abatement efficiency in the deep sea and the application of AOM reaction systems.

Vertical and temporal variations in activity, abundance, and composition of nitrite-driven anaerobic methanotrophs in a paddy field

Paddy soil is an important source of methane (CH4) emissions, and the nitrite-driven anaerobic oxidation of methane (AOM) mediated by Candidatus Methylomirabilis bacteria is a crucial process for CH4 mitigation. However, the vertical and temporal fluctuations in nitrite-driven AOM activity, as well as the abundance and composition of Methylomirabilis bacteria in paddy soil, remain poorly understood. We investigated the potential rate of nitrite-driven AOM and the composition of Methylomirabilis bacteria at varying depths (0–40 cm, at 10 cm intervals) within a paddy soil during different growth stages of rice, including tillering, jointing, flowering, and milky. The potential rate of nitrite-driven AOM ranged from 0.24 to 20.6 nmol CO2 g−1 (dry soil) d−1, and the abundance of Methylomirabilis bacterial 16S rRNA genes varied from 5.8 × 106 to 4.4 × 107 copies g−1 dry soil. The 10–20 cm soil depth showed higher potential activity and bacterial abundance compared to other depths, and the tillering and jointing stages exhibited greater activity and abundance. The composition of Methylomirabilis bacteria showed significant variation across different soil depths but remained stable throughout the growth stages of rice. Soil organic carbon content was positively correlated with potential nitrite-driven AOM rate, and nitrite content was negatively correlated with the abundance of Methylomirabilis bacteria. Soil organic carbon and ammonium content had a significant impact on the composition of Methylomirabilis bacteria. The results underscore the substantial influence of soil depth and the growth stage of rice on nitrite-driven AOM in paddy ecosystems.

Humic substances as electron acceptor for anaerobic oxidation of methane (AOM) and electron shuttle in Mn (IV)-dependent AOM

Anaerobic methanotrophic archaea (ANME) belonging to the family Methanoperedenaceae are crucial for the global carbon cycle and different biogeochemical processes, owing to their metabolic versatility to couple anaerobic oxidation of methane (AOM) with different electron acceptors. A universal feature of Methanoperedenaceae is the abundant genes encoded in their genomes associated with extracellular electron transfer (EET) pathways. Candidatus. ‘Methanoperedens manganicus’, an archaeon belonging to the family Methanoperedenaceae, was recently enriched in a bioreactor performing AOM coupled with Mn (IV) reduction. Using this EET-capable ANME, we tested the hypothesis in this study that ANME can catalyse the humic-dependent AOM for growth. A two-year incubation showed that AOM activity can be sustained by Ca. ‘M. manganicus’ consortium in a bioreactor fed only with humic acids and methane. An isotopic mass balance batch test confirmed that the observed AOM was coupled to the reduction of humic acids. The increase of relative abundance of Ca. ‘M. manganicus’, and the total archaea population in the microbial community suggested that Ca. ‘M. manganicus’ can grow on methane and humic acids. The observation of humic-dependent AOM led to a subsequent hypothesis that humic acids could be used as the electron shuttle to mediate the EET in dissimilatory Mn (IV) reduction by Ca. ‘M. manganicus’. We tested this hypothesis by adding humic acids to a Ca. ‘M. manganicus’ dominated-culture, which showed that the AOM rate was doubled by the addition of humic acids. X-ray photoelectron spectroscopy (XPS) showed that quinone moieties were consumed when humic acids worked as electron acceptors while remaining stable when functioning as a shuttle for electron transfer. The results of our study suggest that humic acids may serve as electron shuttles to allow ANME to access more electron acceptors through long-range EET.

Temperature sensitivity of methanogenesis and anaerobic methane oxidation in thermokarst lakes modulated by surrounding vegetation on the Qinghai-Tibet Plateau

Understanding the balance between methane (CH4) production (methanogenesis) and its oxidation is important for predicting carbon emissions from thermokarst lakes under global warming. However, the response of thermokarst lake methanogenesis and the anaerobic oxidation of methane (AOM) to warming, especially from Qinghai-Tibetan Plateau (QTP), is still not quantified. In this study, sediments were collected from 11 thermokarst lakes on the QTP. These lakes are surrounded with different vegetation types, including alpine desert (AD), alpine steppe (AS), alpine meadow (AM) and alpine swamp meadow (ASM). The results showed that methanogenesis and AOM rates exponentially increased with temperature, while the temperature sensitivity (Q10, average Q10 values of methanogenesis and AOM were 0.69–30 and 0.54–16.9 respectively) of methanogenesis were larger than AOM, but not significant, showing a similar temperature dependence of methanogenesis and AOM in thermokarst lake sediments. Thermokarst lake sediments in the ASM had higher methanogenesis and anaerobic oxidation potential, matching its higher NDVI and relative abundances of methanogens and SBM (syntrophic bacteria with methanogens). Although the thermokarst lake sediments AOM depleted 15 %–27.8 % of the total CH4 production, the AOM rate was lower than methanogenesis in thermokarst lake sediments, it did not offset increased CH4 production under anaerobic conditions. The increase in CH4 production in thermokarst lake sediments will likely lead to higher emissions within a warming world. These findings indicate that methanogenesis and AOM in thermokarst lake sediments are sensitive to climate change. Models should consider the Q10 values of methanogenesis and AOM and vegetation types when predicting carbon cycle in thermokarst lakes under global warming.

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.

Anaerobic oxidation of methane does not attenuate methane emissions from thermokarst lakes

AbstractThe ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene‐aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice‐rich permafrost thaw. In cores of surface (~ 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4°C) and high (16°C) temperatures. Methanogenesis rates were measured by the accumulation of methane over ~ 90 d, whereas AOM rates were measured by adding labeled‐13CH4 and measuring the produced dissolved inorganic 13C. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility (“back‐flux”) during CH4 production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1‐m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.

Anaerobic methane-oxidizing activity in a deep underground borehole dominantly colonized by Ca. Methanoperedenaceae.

The family Ca. Methanoperedenaceae archaea mediates the anaerobic oxidation of methane (AOM) in different terrestrial environments. Using a newly developed high-pressure laboratory incubation system, we investigated 214- and 249-m deep groundwater samples at Horonobe Underground Research Laboratory, Japan, where the high and low abundances of Ca. Methanoperedenaceae archaea have been shown by genome-resolved metagenomics, respectively. The groundwater samples amended with 13 C-labelled methane and amorphous Fe(III) were incubated at a pressure of 1.6MPa. After 3-7 days of incubation, the AOM rate was 45.8 ± 19.8nM/day in 214-m groundwater. However, almost no activity was detected from 249-m groundwater. Based on the results from 16S rRNA gene analysis, the abundance of Ca. Methanoperedenaceae archaea was high in the 214-m deep groundwater sample, whereas Ca. Methanoperedenaceae archaea was undetected in the 249-m deep groundwater sample. These results support the in situ AOM activity of Ca. Methanoperedenaceae archaea in the 214-m deep subsurface borehole interval. Although the presence of Fe-bearing phyllosilicates was demonstrated in the 214-m deep groundwater, it needs to be determined whether Ca. Methanoperedenaceae archaea use the Fe-bearing phyllosilicates as in situ electron acceptors by high-pressure incubation amended with the Fe-bearing phyllosilicates.

The role of anaerobic methane oxidation on the carbonate authigenesis in sediments of the subtropical Beibu Gulf, South China Sea: A reactive–transport modelling approach

The formation and burial of authigenic carbonate in marine sediment significantly affect the sedimentary carbon cycle and its isotopic mass balance in geological history. Anaerobic oxidation of methane (AOM) is the primary driver of authigenic carbonate precipitation within the sulfate-methane transition zone (SMTZ). Quantitative estimations of the role of AOM on the authigenic carbonate precipitation and its carbon isotope under non-steady-state processes (e.g., changes in methane fluxes at the bottom sediment, sedimentation rates or organic fluxes in the surface sediment), however, are still limited. In this study, we use geochemical data from porewater (e.g., the concentration of sulfate, calcium, magnesium, strontium, dissolved inorganic carbon, total alkalinity) and solid sediment (e.g., organic matter content, and carbonate content) in different depositional environments of the subtropical Beibu Gulf, South China Sea, combined with a diagenetic reactive-transport modelling approach, to estimate the mineralogy of authigenic carbonate, the relationship between AOM and authigenic carbonate precipitation, and the impact of AOM rate on carbon isotope of sediment carbonate (δ13CCar). The results show that high-Mg carbonates (high-Mg calcite and dolomite) are the main type of authigenic carbonate (∼80%) formed in the methane-bearing sediments, leading to higher porewater Sr2+/Ca2+ (>0.02) and Mg2+/Ca2+ (>20) within the SMTZ. Our modelling analysis highlights that the non-steady-state induced by increased methane flux from the underlying sediments can significantly accelerate the authigenic carbonates formation within the SMTZ. Using parametric sensitivity analysis, we observed that even a 1% increase in the authigenic carbonate fraction of sediment carbonates results in significant changes in δ13CCar within the SMTZ (from −1‰ to −2‰), mainly due to lighter carbon isotopes produced by more intensive AOM processes. Noteworthily, the terrestrial-to-marine transition was identified by the sediment and porewater geochemical profiles at site SO-8. Although lower authigenic carbonate precipitation occurs in terrestrial sedimentary environments, the proportion of authigenic carbonate in terrestrial environments (11%) is much higher than that in marine environments (1%), resulting in carbon isotopes of carbonate in terrestrial sediments becoming more negative (−5‰).

Isotopic Evidence for Anaerobic Oxidation of Methane in the Freshwater Sediments of Reservoirs: The Impact of Selected Environmental Factors

This paper presents the results of research conducted in 2018–2019 on the anaerobic oxidation of methane (AOM) in reservoir sediments. Located in SE Poland, Maziarnia, Nielisz and Rzeszów Reservoirs were selected for the purposes of the research. Rates of AOM were determined via 50-day incubation of sediment from the 0–5, 5–10 and 10–15 cm layers, to which a 13CH4 isotope tracer was added. The sediments had been collected from a single station at each reservoir in places that had earlier reported high levels of emission of CH4 to the atmosphere. Results demonstrate ongoing AOM processes in the kinds of freshwater ecosystem represented by reservoirs, further implying the existence of an important sink for CH4. More specifically, however, AOM rates were found to differ among both the reservoirs, and the layers of sediment, studied. Preliminary analysis of selected environmental factors capable of affecting AOM failed to suggest the availability of electron acceptors (NO3−, SO42−, Fe3+) as key controlling factors. Important factors also proved to be sediment pH, the quality of organic matter (especially the content of organic electron acceptors), the salinity of pore water, and—primarily—the presence of the microorganisms actually responsible for AOM. The results here are important, given the low level of knowledge of AOM process in reservoirs. They therefore help supply key information on the functioning of these ecosystems and the role in global climate change they play.

[Effect of gradual increase of atmospheric CO2 concentration on nitrate-dependent anaerobic methane oxidation in paddy soils].

Nitrate-dependent anaerobic oxidation of methane (AOM) is a new pathway to reduce methane emissions from paddy ecosystems. The elevated atmospheric CO2 concentration can affect methane emissions from paddy ecosystems, but its impact on the process of nitrate-dependent AOM is poorly known. Based on the automatic CO2 control platform with open top chambers and the 13CH4 stable isotope experiments, the responses of the activity of nitrate-dependent AOM, abundance and community composition of Candidatus Methanoperedens nitroreducens (M. nitroreducens)-like archaea to the gradual increase of CO2 concentration were investigated in paddy fields. We set up two CO2 concentration treatments, including an ambient CO2 and a gradual increase of CO2(increase of 40 μL·L-1 per year above ambient CO2 concentration until 160 μL·L-1). The results showed the nitrate-dependent AOM rate of 0.7-11.3 nmol CO2·g-1·d-1 in the studied paddy fields, and quantitative PCR showed the abundance of M. nitroreducens-like archaeal mcrA genes of 2.2×106-8.5×106 copies·g-1. Compared to the ambient CO2 treatment, the slow elevated CO2 treatment enhanced the nitrate-dependent AOM rate and stimulated the abundance of M. nitroreducens-like archaea, particularly in 5-10 cm soil layer. The gradual increased CO2 concentration treatment did not change the community composition of M. nitroreducens-like archaea, but significantly decreased their diversity. The soil organic carbon content was an important factor influencing the nitrate-dependent AOM process. Overall, our results showed that the gradual increase of CO2 concentration could promote the nitrate-dependent AOM, suggesting its positive role in mitigating methane emissions from paddy ecosystems under future climate change.

Characterization of anaerobic oxidation of methane and microbial community in landfills with aeration

Landfills are the third largest source of anthropogenic CH4 emissions. Anaerobic oxidation of methane (AOM) activity and communities of methane-oxidizing bacteria were investigated in three informal landfills in this study, namely, BJ, CH and SZ landfills, among which BJ and CH represent traditional anaerobic landfills, while the SZ landfill was subjected to aeration to accelerate waste stabilization. The AOM rates of the investigated landfilled wastes ranged from 3.66 to 23.91 nmol g−1 h−1. Among the three landfills, the AOM rate was highest in the SZ-1-Top sample, which was closest to the aeration pipe. Among the possible electron acceptors for AOM, including NO3−, NO2−, SO42− and Fe3+, the NO2−-N content was the only variable that was positively correlated with the AOM rate. Compared with α-Proteobacteria methanotrophs, γ-Proteobacteria methanotrophs were more abundant in the landfilled waste, especially Methylobacter, which was detected in nearly all samples. Members of the family Methylomirabilaceae, including Candidatus Methylomirabilis, were also detected in the SZ-1 and SZ-2-Bot samples. The relative abundance of the main methanotrophs in the families Methylomonadaceae, Methylococcaceae, Rokubacteriales and Methylomirabilaceae, the genus Methylocystis and the phylum NC10 were all positive correlations with the contents of NO2−-N in the landfilled waste samples. Additionally, significantly positive correlations were observed between the AOM rates and the relative abundance of the main methanotrophs except for the family Methylococcaceae. This indicated that aeration could enhance the conversion of nitrogen compounds in the landfilled waste, in which the high contents of NO2−-N could stimulate the growth of methanotrophs and increase AOM rate. These findings are helpful for understanding the mechanisms of CH4 oxidation in landfills and for taking effective measures to mitigate CH4 emissions from landfills.

An introduction to factors influencing anaerobic oxidation of methane

Methane affects global climate change 28 times more than carbon dioxide, which indicates the significance of the process of aerobic and anaerobic oxidation of methane (AOM) in reducing methane emissions from different ecosystems. The rate of AOM is usually slower than aerobic methane oxidation; however, AOM has higher carbon efficiency and can utilise methane as a carbon source, thus reducing methane release to the environment. This article provides an introduction to AOM dependent on different electron acceptors such as sulphate; nitrite/nitrate; iron; manganese, etc. The paper also has an overview of the effect of environmental factors on AOM rates as well as various microorganisms involved in AOM with a higher focus on AOM coupled to denitrification. Moreover, the application of isotope analysis is reviewed as a tool to measure the gross AOM rate in experimental studies. Finally, the available analytical models to estimate AOM rates are discussed.

Significance of anaerobic oxidation of methane (AOM) in mitigating methane emission from major natural and anthropogenic sources: a review of AOM rates in recent publications

AOM rates in literature were analyzed and anaerobic methanotrophs significantly cut methane emissions in oceans but not in wetlands, rice paddy, and fresh water. The trophic and metabolic patterns of microorganisms may be limiting the AOM rates.

The Baltic Sea methane pockmark microbiome: The new insights into the patterns of relative abundance and ANME niche separation

Pockmarks are important “pumps”, which are believed to play a significant role in the global methane cycling and harboring a unique assemblage of very diverse prokaryotes. This study reports the results of massive sequencing of the 16S rRNA gene V4 hypervariable regions for the samples from thirteen pockmark horizons (the Baltic Sea) collected at depths from 0 to 280 cm below seafloor (cmbsf) and the rates of microbially mediated anaerobic oxidation of methane (AOM) and sulfate reduction (SR). Altogether, 76 bacterial and 12 archaeal phyla were identified, 23 of which were candidate divisions. Of the total obtained in the pockmark sequences, 84.3% of them were classified as Bacteria and 12.4% as Archaea; 3.3% of the sequences were assigned to unknown operational taxonomic units (OTUs). Members of the phyla Planctomycetota, Chloroflexota, Desulfobacterota, Caldatribacteriota, Acidobacteriota and Proteobacteria predominated across all horizons, comprising 58.5% of the total prokaryotic community. These phyla showed different types of patterns of relative abundance. Analysis of AOM-SR-mediated prokaryotes abundance and biogeochemical measurements revealed that ANME-2a-2b subcluster was predominant in sulfate-rich upper horizons (including sulfate-methane transition zone (SMTZ)) and together with sulfate-reducing bacterial group SEEP-SRB1 had a primary role in AOM coupled to SR. At deeper sulfate-depleted horizons ANME-2a-2b shifted to ANME-1a and ANME-1b which alone mediated AOM or switch to methanogenic metabolism. Shifting of the ANME subclusters depending on depth reflect a tendency for niche separation in these groups. It was shown that the abundance of Caldatribacteriota and organohalide-respiring Dehalococcoidia (Chloroflexota) exhibited a strong correlation with AOM rates. This is the first detailed study of depth profiles of prokaryotic diversity, patterns of relative abundance, and ANME niche separation in the Baltic Sea pockmark microbiomes sheds light on assembly of prokaryotes in a pockmark.

Sulfate differentially stimulates but is not respired by diverse anaerobic methanotrophic archaea.

Sulfate-coupled anaerobic oxidation of methane (AOM) is a major methane sink in marine sediments. Multiple lineages of anaerobic methanotrophic archaea (ANME) often coexist in sediments and catalyze this process syntrophically with sulfate-reducing bacteria (SRB), but the potential differences in ANME ecophysiology and mechanisms of syntrophy remain unresolved. A humic acid analog, anthraquinone 2,6-disulfonate (AQDS), could decouple archaeal methanotrophy from bacterial sulfate reduction and serve as the terminal electron acceptor for AOM (AQDS-coupled AOM). Here in sediment microcosm experiments, we examined variations in physiological response between two co-occurring ANME-2 families (ANME-2a and ANME-2c) and tested the hypothesis of sulfate respiration by ANME-2. Sulfate concentrations as low as 100 µM increased AQDS-coupled AOM nearly 2-fold matching the rates of sulfate-coupled AOM. However, the SRB partners remained inactive in microcosms with sulfate and AQDS and neither ANME-2 families respired sulfate, as shown by their cellular sulfur contents and anabolic activities measured using nanoscale secondary ion mass spectrometry. ANME-2a anabolic activity was significantly higher than ANME-2c, suggesting that ANME-2a was primarily responsible for the observed sulfate stimulation of AQDS-coupled AOM. Comparative transcriptomics showed significant upregulation of ANME-2a transcripts linked to multiple ABC transporters and downregulation of central carbon metabolism during AQDS-coupled AOM compared to sulfate-coupled AOM. Surprisingly, genes involved in sulfur anabolism were not differentially expressed during AQDS-coupled AOM with and without sulfate amendment. Collectively, this data indicates that ANME-2 archaea are incapable of respiring sulfate, but sulfate availability differentially stimulates the growth and AOM activity of different ANME lineages.

Different impacts of an electron shuttle on nitrate- and nitrite-dependent anaerobic oxidation of methane in paddy soil

Quinones, redox-active functional groups in soil organic matter, can act as electron shuttles for microbial anaerobic transformation. Here, we used <sup>13</sup>CH<sub>4</sub> to trace <sup>13</sup>C conversion (<sup>13</sup>C-CO<sub>2</sub> + <sup>13</sup>C-SOC) to investigate the influence of an artificial electron shuttle (anthraquinone-2,6-disulfonate, AQDS) on denitrifying anaerobic methane oxidation (DAMO) in paddy soil. The results showed that AQDS could act as the terminal electron acceptor for the anaerobic oxidation of methane (AOM) in the paddy field. Moreover, AQDS significantly enhanced nitrate-dependent AOM rates and the amount of <sup>13</sup>C-CH<sub>4</sub> assimilation to soil organic carbon (SOC), whereas it was remarkably reduced nitrite-dependent AOM rates and <sup>13</sup>C assimilation. Ultimately, AQDS notably increased the total DAMO rates and <sup>13</sup>C assimilation to SOC. However, the electron shuttle did not change the percentage of <sup>13</sup>C-SOC in total <sup>13</sup>C-CH<sub>4</sub> conversion. These results suggest that electron shuttles in the natural organic matter might be able to offset methane emission by facilitating AOM coupled with the denitrification process.

Are seep carbonates quantitative proxies of CH4 leakage? Modeling the influence of sulfate reduction and anaerobic oxidation of methane on pH and carbonate precipitation

Seep carbonates tell us where and when CH4-charged fluids escaped from the subsurface, thus providing qualitative information to reconstruct the activity of petroleum systems. The potential of seep carbonates as quantitative proxies for the amount of CH4 leaked, however, remains largely unexplored, which limit their applicability as exploration tools. This paper tackles the quantification of the CH4 flux - seep carbonate relationship by simulating the coupled sedimentary carbon (C) – sulfur (S) cycles in a reaction-transport modeling (RTM) framework. We first establish a theoretical basis demonstrating that the stoichiometry of diagenetic reactions and the ambient pH of pore waters are the main drivers of the rate of change in the saturation state of carbonate minerals (ΩCal), while the concentrations of total dissolved inorganic carbon and sulfide are only of secondary importance. It results that anaerobic oxidation of methane (AOM) is the main driver of carbonate precipitation, while organoclastic sulfate reduction (SR) has a minor impact. We further show that SR mostly drives carbonate dissolution, but can also contribute to precipitation when pH is low (<7–7.1). The RTM simulations reveal that an increase in upward fluid flow triggers an intensification of peak AOM rates, associated to a shallowing and thinning of the zone of carbonate precipitation. Such behavior leads to an almost linear relationship between the amount of carbonate precipitated and flux of CH4 (nCH4 = 3.3–5.2 * nCaCO3), until, eventually, full cementation occurs. We thus define a “quantitative domain” at moderate fluid flow and a “threshold domain” at high fluid velocities, where full cementation solely provides a lower bound estimate of the amount of CH4 leaked. We also show that in contrast to a traditional view of seep carbonate formation mainly controlled by venting activity, sedimentation rate and water depth also play major roles, via their control on residence time and saturation concentration of CH4, respectively. The interpretation of vertical seep carbonate stacks should thus not solely focus on changes in fluid flow, but also consider changes in sedimentation rate and/or water depth.

Biomarker and Isotopic Composition of Seep Carbonates Record Environmental Conditions in Two Arctic Methane Seeps

Present-day activity of cold seeps in the ocean is evident from direct observations of methane emanating from the seafloor, the presence of chemosynthetic organisms, or the quantification of high gas concentrations in sediment pore waters and the water column. Verifying past cold seep activity and biogeochemical characteristics is more challenging but may be reconstructed from proxy records of authigenic seep carbonates. Here, we investigated the lipid-biomarker inventory, carbonate mineralogy, and stable carbon and oxygen isotope compositions of seep-associated carbonates from two active Arctic methane seeps, located to the northwest (Vestnesa Ridge; ∼1,200 m water depth) and south (Storfjordrenna; ∼380 m water depth) offshore Svalbard. The aragonite-dominated mineralogy of all but one carbonate sample indicate precipitation close to the seafloor in an environment characterized by high rates of sulfate-dependent anaerobic oxidation of methane (AOM). In contrast, Mg-calcite rich nodules sampled in sediments of Storfjordrenna appear to have formed at the sulfate-methane-transition zone deeper within the sediment at lower rates of AOM. AOM activity at the time of carbonate precipitation is indicated by the 13C-depleted isotope signature of the carbonates [−20 to −30‰ Vienna Pee Dee Belemnite (VPDB)], as well as high concentrations of 13C-depleted lipid biomarkers diagnostic for anaerobic methanotrophic archaea (archaeol and sn2-hydroxyarchaeol) and sulfate-reducing bacteria (iso and anteiso-C15:0 fatty acids) in the carbonates. We also found 13C-depleted lipid biomarkers (diploptene and a 4α-methyl sterol) that are diagnostic for bacteria mediating aerobic oxidation of methane (MOx). This suggests that the spatial separation between AOM and MOx zones was relatively narrow at the time of carbonate formation, as is typical for high methane-flux regimes. The seep-associated carbonates also displayed relatively high δ18O values (4.5–5‰ VPDB), indicating the presence of 18O-enriched fluids during precipitation, possibly derived from destabilized methane gas hydrates. Based on the combined isotopic evidence, we suggest that all the seep carbonates resulted from the anaerobic oxidation of methane during intense methane seepage. The seepage likely was associated to gas hydrates destabilization, which led to the methane ebullition from the seafloor into the water column.

Exogenous nitrogen addition inhibits sulfate-mediated anaerobic oxidation of methane in estuarine coastal sediments

Anaerobic oxidation of methane (AOM) plays an important role in regulating methane (CH4) emissions from coastal wetland ecosystems. In recent decades, coastal estuarine wetlands have been subjected to great environmental pressure associated with terrestrial nitrogen (N) input. Here, we used 13CH4 to quantify AOM rates in coastal wetland sediments with and without vegetation in the Yangtze River Estuary in East China. Impact of addition of inorganic N forms (NO3−, NO2− and NH4+) on AOM process was assessed by 13CH4 anaerobic oxidation into 13C-CO2 and 13C-SOC, as well as by community structure and quantitative characteristics of anaerobic methanotrophic (ANME) archaea. The results showed that AOM process occurred in the bare wetland (BW) and reed wetland (RW). Inorganic N addition, regardless of N forms, notably decreased AOM rates by 14–58% in the BW sediments and by 26–65% in the RW sediments. Rates of 13CH4 assimilation into SOC, which were similar to but faster than rates of AOM into CO2, were reduced by 12%–66% under inorganic N addition. After 120-day anaerobic incubation, the net changes in the relative abundance of ANME-2 and ANME-3 were significantly correlated with AOM rates in the RW and BW, respectively, thereby indicating their contribution to the variation in AOM under N addition. Our findings provide insight into the potential consequences of terrestrial N input; these may weaken CH4 consumption by anaerobic oxidation in the Yangtze River Estuary, and decrease carbon stabilization and storage in coastal wetland sediments.

Methyl-compounds driven benthic carbon cycling in the sulfate-reducing sediments of South China Sea.

Methane is a potent greenhouse gas; methane production and consumption within seafloor sediments has generated intense interest. Anaerobic oxidation of methane (AOM) and methanogenesis (MOG) primarily occur at the depth of the sulfate-methane transition zone or underlying sediment respectively. Methanogenesis can also occur in the sulfate-reducing sediments through the utilization of non-competitive methylated compounds; however, the occurrence and importance of this process are not fully understood. Here, we combined a variety of data, including geochemical measurements, rate measurements and molecular analyses to demonstrate the presence of a cryptic methane cycle in sulfate-reducing sediments from the continental shelf of the northern South China Sea. The abundance of methanogenic substrates as well as the high MOG rates from methylated compounds indicated that methylotrophic methanogenesis was the dominant methanogenic pathway; this conclusion was further supported by the presence of the methylotrophic genus Methanococcoides. High potential rates of AOM were observed in the sediments, indicating that methane produced in situ could be oxidized simultaneously by AOM, presumably by ANME-2a/b as indicated by 16S rRNA gene analysis. A significant correlation between the relative abundance of methanogens and methanotrophs was observed over sediment depth, indicating that methylotrophic methanogenesis could potentially fuel AOM in this environment. In addition, higher potential rates of AOM than sulfate reduction rates at in situ methane conditions were observed, making alternative electron acceptors important to support AOM in sulfate-reducing sediment. AOM rates were stimulated by the addition of Fe/Mn oxides, suggesting AOM could be partially coupled to metal oxide reduction. These results suggest that methyl-compounds driven methane production drives a cryptic methane cycling and fuels AOM coupled to the reduction of sulfate and other electron acceptors.