Anaerobic Oxidation Of Methane Activity Research Articles

Impact of electron acceptor availability on the anaerobic oxidation of methane in coastal freshwater and brackish wetland sediments

Methane, a powerful greenhouse gas, is both produced and consumed in anoxic coastal sediments via microbial processes. Although the anaerobic oxidation of methane (AOM) is almost certainly an important process in coastal freshwater and salt marsh sediments, the factors that control the rates and pathways of AOM in these habitats are poorly understood. Here, we present the first direct measurements of AOM activity in freshwater (0PSU) and brackish (25PSU) wetland sediments. Despite disparate sulfate concentrations, both environments supported substantial rates of AOM. Higher sulfate reduction (SR) rates were measured in the freshwater site and SR at both sites was of sufficient magnitude to support the observed AOM activity. Laboratory incubations of freshwater and brackish tidal, wetland sediments amended with either nothing [control], sulfate, nitrate, manganese oxide (birnessite) or iron oxide (ferrihydrite) and supplied with a methane headspace were used to evaluate the impact(s) of electron acceptor availability on potential AOM rates. Maximum AOM rates in brackish slurries occurred in the sulfate amendments. In contrast, addition of sulfate and several possible electron acceptors to the freshwater slurries decreased AOM rates relative to the control. High ratios of AOM activity relative to SR activity in the nitrate, birnessite, and ferrihydrite treatments of both the brackish and freshwater slurries provided evidence of AOM decoupled from SR. This study demonstrates that both freshwater and brackish coastal wetland sediments support considerable rates of anaerobic methanotrophy and provides evidence for sulfate-independent AOM that may be coupled to nitrate, iron, or manganese reduction in both environments.

Anaerobic oxidation of methane in an iron‐rich Danish freshwater lake sediment

Freshwater systems are identified as one of the main natural methane sources, but little is known about the importance of anaerobic oxidation of methane (AOM) in these systems. We investigated AOM in a lake sediment characterized by a high reactive iron content, normal sulfate concentrations in the bottom water (∼ 250 µmol L−1), and a relatively deep sulfate penetration of ∼ 14 cm, which facilitated the spatial resolution of the zones of methane production and consumption. Methane concentrations, δ13C methane profiles, and directly measured and modeled AOM rates all consistently demonstrated methane consumption throughout the anoxic, nitrate‐free, Fe(III)‐ and sulfate‐containing zone, oxidizing ∼ 90% of the diffusive methane flux. Thus, the concentration gradient of methane was steepest at the base of the Fe(III) and sulfate zone and decreased strongly toward the sediment surface; while δ13CH4 increased from < −80‰ in the methanogenic zone to −48‰ in the surface sediment. Direct measurements demonstrated AOM activity throughout the Fe(III) and sulfate zone. AOM rates peaked at sulfate concentrations below 3 µmol L−1, which suggests a possible coupling of AOM to the reduction of more crystalline Fe(III) oxides. Alternatively, AOM could be coupled to sulfate reduction, which was in turn supported by a cryptic sulfur cycle coupled to Fe(III) reduction. Our results show that AOM can substantially reduce methane emission from freshwater sediments, and the finding of AOM at sulfate concentrations < 3 µmol L−1 suggests that AOM could be of greater importance in freshwater systems, and in ancient low‐sulfate oceans, than was previously appreciated.

Anaerobic oxidation of methane in hypersaline cold seep sediments

Life in hypersaline environments is typically limited by bioenergetic constraints. Microbial activity at the thermodynamic edge, such as the anaerobic oxidation of methane (AOM) coupled to sulphate reduction (SR), is thus unlikely to thrive in these environments. In this study, carbon and sulphur cycling was investigated in the extremely hypersaline cold seep sediments of Mercator mud volcano. AOM activity was partially inhibited but still present at salinity levels of 292 g L(-1) (c. eightfold sea water concentration) with rates of 2.3 nmol cm(-3) day(-1) and was even detectable under saturated conditions. Methane and evaporite-derived sulphate comigrated in the ascending geofluids, which, in combination with a partial activity inhibition, resulted in AOM activity being spread over unusually wide depth intervals. Up to 79% of total cells in the AOM zone were identified by fluorescence in situ hybridization (FISH) as anaerobic methanotrophs of the ANME-1. Most ANME-1 cells formed monospecific chains without any attached partner. At all sites, AOM activity co-occurred with SR activity and sometimes significantly exceeded it. Possible causes of these unexpected results are discussed. This study demonstrates that in spite of a very low energy yield of AOM, microorganisms carrying this reaction can thrive in salinity up to halite saturation.

Biomarkers from individual carbonate phases of an Oligocene cold-seep deposit, Washington State, USA

An Oligocene cold-seep limestone (Lincoln Creek Formation, Washington State, USA) was studied for its lipid biomarker inventory. Biomarker analysis on minute amounts of sample (tens of mg) and complementary 13Ccarbonate analyses allowed us to link biogeochemical processes with individual, closely intertwined carbonate phases. The ancient seep deposit exhibits four major carbonate phases, according to the paragenetic sequence of (I) micrite, (II) yellow aragonite, (III) clear aragonite and (IV) equant calcite spar. For the micrite, varying but significant amounts of archaea-derived isoprenoids clearly indicate that the precipitation of this phase was induced by the microbial anaerobic oxidation of methane (AOM). However, water column-derived lipids present in this carbonate phase reflect the incorporation of organic matter from background sediment cemented by authigenic micrite. Yellow aragonite made up only a minor rock component (<10% vol.), but contained a major portion of lipid biomarkers indicative of AOM. Along with low δ13Ccarbonate values (less than −30‰ Pee Dee Belemnite), this points to an intimate spatial association of AOM consortia with the precipitation of yellow aragonite. Clear aragonite showed similar δ13Ccarbonate values but much lower, if any, contents of AOM biomarkers. This suggests that AOM-derived carbonate ions diffused over a greater distance to the site of precipitation compared with yellow aragonite. The latest phase, equant calcite spar, did not yield appreciable biomarkers, but showed a notable 13Ccarbonate-enrichment that is most likely caused by methanogenesis that prevailed in the sediments after AOM activity had ceased. A comparison of the ancient seep carbonates with modern counterparts from Hydrate Ridge (offshore Oregon, USA) revealed a remarkable coincidence of the respective mineral phases and their biomarker patterns. This suggests that the mechanisms of carbonate formation and the associated biogeochemical processes remained unchanged over geological times. □Biomarkers, carbonates, cold seeps, Hydrate Ridge, isotopes, Oligocene, Washington.

Thermophilic anaerobic oxidation of methane by marine microbial consortia

The anaerobic oxidation of methane (AOM) with sulfate controls the emission of the greenhouse gas methane from the ocean floor. AOM is performed by microbial consortia of archaea (ANME) associated with partners related to sulfate-reducing bacteria. In vitro enrichments of AOM were so far only successful at temperatures ≤25 °C; however, energy gain for growth by AOM with sulfate is in principle also possible at higher temperatures. Sequences of 16S rRNA genes and core lipids characteristic for ANME as well as hints of in situ AOM activity were indeed reported for geothermally heated marine environments, yet no direct evidence for thermophilic growth of marine ANME consortia was obtained to date. To study possible thermophilic AOM, we investigated hydrothermally influenced sediment from the Guaymas Basin. In vitro incubations showed activity of sulfate-dependent methane oxidation between 5 and 70 °C with an apparent optimum between 45 and 60 °C. AOM was absent at temperatures ≥75 °C. Long-term enrichment of AOM was fastest at 50 °C, yielding a 13-fold increase of methane-dependent sulfate reduction within 250 days, equivalent to an apparent doubling time of 68 days. The enrichments were dominated by novel ANME-1 consortia, mostly associated with bacterial partners of the deltaproteobacterial HotSeep-1 cluster, a deeply branching phylogenetic group previously found in a butane-amended 60 °C-enrichment culture of Guaymas sediments. The closest relatives (Desulfurella spp.; Hippea maritima) are moderately thermophilic sulfur reducers. Results indicate that AOM and ANME archaea could be of biogeochemical relevance not only in cold to moderate but also in hot marine habitats.

A distinct freshwater‐adapted subgroup of ANME‐1 dominates active archaeal communities in terrestrial subsurfaces in Japan

Anaerobic methane-oxidizing archaea (ANME) are known to play an important role in methane flux, especially in marine sediments. The 16S rRNA genes of ANME have been detected in terrestrial freshwater subsurfaces. However, it is unclear whether ANME are actively involved in methane oxidation in these environments. To address this issue, Holocene sediments in the subsurface of the Kanto Plain in Japan were collected for biogeochemical and molecular analysis. The potential activity of the anaerobic oxidation of methane (AOM) (0.38-3.54 nmol cm⁻³ day⁻¹) was detected in sediment slurry incubation experiments with a (13) CH(4) tracer. Higher AOM activity was observed in low-salinity treatment compared with high-salinity condition (20‰), which supports the adaptation of ANME in freshwater habitats. The 16S rRNA sequence analysis clearly revealed the presence of a distinct subgroup of ANME-1, designated ANME-1a-FW. Phylogenetic analysis of the mcrA genes also implied the presence of the distinct subgroup in ANME-1. ANME-1a-FW was found to be the most dominant active group in the archaeal communities on the basis of 16S rRNA analysis (75.0-93.8% of total archaeal 16S rRNA clones). Sulfate-reducing bacteria previously known as the syntrophic bacterial partners of ANME-1 was not detected. Our results showed that ANME-1a-FW is adapted to freshwater habitats and is responsible for AOM in terrestrial freshwater subsurface environments.

Bacterial enzymes for dissimilatory sulfate reduction in a marine microbial mat (Black Sea) mediating anaerobic oxidation of methane

Anaerobic oxidation of methane (AOM) with sulfate is catalysed by microbial consortia of archaea and bacteria affiliating with methanogens and sulfate-reducing Deltaproteobacteria respectively. There is evidence that methane oxidation is catalysed by enzymes related to those in methanogenesis, but the enzymes for sulfate reduction coupled to AOM have not been examined. We collected microbial mats with high AOM activity from a methane seep in the Black Sea. The mats consisted mainly of archaea of the ANME-2 group and bacteria of the Desulfosarcina-Desulfococcus group. Cell-free mat extract contained activities of enzymes involved in sulfate reduction to sulfide: ATP sulfurylase (adenylyl : sulfate transferase; Sat), APS reductase (Apr) and dissimilatory sulfite reductase (Dsr). We partially purified the enzymes by anion-exchange chromatography. The amounts obtained indicated that the enzymes are abundant in the mat, with Sat accounting for 2% of the soluble mat protein. N-terminal amino acid sequences of purified proteins suggested similarities to the corresponding enzymes of known species of sulfate-reducing bacteria. The deduced amino acid sequence of PCR-amplified genes of the Apr subunits is similar to that of Apr of the Desulfosarcina/Desulfococcus group. These results indicate that the major enzymes involved in sulfate reduction in the Back Sea microbial mats are of bacterial origin, most likely originating from the bacterial partner in the consortium.

Substantial 13C/12C and D/H fractionation during anaerobic oxidation of methane by marine consortia enriched in vitro

The anaerobic oxidation of methane (AOM) by methanotrophic archaea and sulfate-reducing bacteria is the major sink of methane formed in marine sediments. The study of AOM as well as of methanogenesis in different habitats is essentially connected with the in situ analysis of stable isotope ((13) C/(12) C, D/H) signatures (δ-values). For their kinetic interpretation, experimental (cultivation-based) isotope fractionation factors (α-values) are richly available in the case of methanogenesis, but are scarce in the case of AOM. Here we used batch enrichment cultures with high AOM activity and without background methanogenesis from detrital remnants to determine (13) C/(12) C and D/H fractionation factors. The enrichment cultures which originated from three marine habitats (Hydrate Ridge, NE Pacific; Amon Mud Volcano, Mediterranean Sea; NW shelf, Black Sea) were dominated by archaeal phylotypes of anaerobic methanotrophs (ANME-2 clade). Isotope fractionation factors calculated from the isotope signatures as a function of the residual proportion of methane were 1.012-1.039 for (13) CH4 /(12) CH4 and 1.109-1.315 for CDH3 /CH4 . The present values from in vitro experiments were significantly higher than values previously estimated from isotope signature distributions in marine sediment porewater, in agreement with the overlap of other processes with AOM in the natural habitat.

Subsurface microbiology and biogeochemistry of a deep, cold‐water carbonate mound from the Porcupine Seabight (IODP Expedition 307)

The Porcupine Seabight Challenger Mound is the first carbonate mound to be drilled (∼270 m) and analyzed in detail microbiologically and biogeochemically. Two mound sites and a non-mound Reference site were analyzed with a range of molecular techniques [catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH), quantitative PCR (16S rRNA and functional genes, dsrA and mcrA), and 16S rRNA gene PCR-DGGE] to assess prokaryotic diversity, and this was compared with the distribution of total and culturable cell counts, radiotracer activity measurements and geochemistry. There was a significant and active prokaryotic community both within and beneath the carbonate mound. Although total cell numbers at certain depths were lower than the global average for other subseafloor sediments and prokaryotic activities were relatively low (iron and sulfate reduction, acetate oxidation, methanogenesis) they were significantly enhanced compared with the Reference site. In addition, there was some stimulation of prokaryotic activity in the deepest sediments (Miocene, > 10 Ma) including potential for anaerobic oxidation of methane activity below the mound base. Both Bacteria and Archaea were present, with neither dominant, and these were related to sequences commonly found in other subseafloor sediments. With an estimate of some 1600 mounds in the Porcupine Basin alone, carbonate mounds may represent a significant prokaryotic subseafloor habitat.

A novel, multi‐layered methanotrophic microbial mat system growing on the sediment of the Black Sea

A novel microbially diverse type of 1- to 5-cm-thick mat performing anaerobic oxidation of methane (AOM) and covering several square metres of the seafloor was discovered in the Black Sea at 180 m water depth. Contrary to other AOM-mat systems of the Black Sea these floating mats are not associated to free gas and are not stabilized by authigenic carbonates. However, supply of methane is ensured by the horizontal orientation of the mats acting as a cover of methane enriched fluids ascending from the underlying sediments. Thorough investigation of their community composition by molecular microbiology and lipid biomarkers, metabolic activities and elemental composition showed that the mats provide a clearly structured system with extracellular polymeric substances (EPS) building the framework of the mats. The top black zone, showing high rates of AOM (15 mumol g(dw) (-1) day(-1)), was dominated by ANME-2, while the following equally active pink layer was dominated by ANME-1 Archaea. The lowest AOM activity (2 mumol g(dw) (-1) day(-1)) and cell numbers were found in the greyish middle part delimited towards the sediment by a second pink, ANME-1-dominated and sometimes a black outer layer (ANME-2). Our work clearly shows that the different microbial populations are established along defined chemical gradients such as methane, sulfate or sulfide.

Assessing sulfate reduction and methane cycling in a high salinity pore water system in the northern Gulf of Mexico

Pore waters extracted from 18 piston cores obtained on and near a salt-cored bathymetric high in Keathley Canyon lease block 151 in the northern Gulf of Mexico contain elevated concentrations of chloride (up to 838 mM) and have pore water chemical concentration profiles that exhibit extensive departures (concavity) from steady-state (linear) diffusive equilibrium with depth. Minimum δ 13C dissolved inorganic carbon (DIC) values of −55.9‰ to −64.8‰ at the sulfate–methane transition (SMT) strongly suggest active anaerobic oxidation of methane (AOM) throughout the study region. However, the nonlinear pore water chemistry-depth profiles make it impossible to determine the vertical extent of active AOM or the potential role of alternate sulfate reduction pathways. Here we utilize the conservative (non-reactive) nature of dissolved chloride to differentiate the effects of biogeochemical activity (e.g., AOM and/or organoclastic sulfate reduction) relative to physical mixing in high salinity Keathley Canyon sediments. In most cases, the DIC and sulfate concentrations in pore waters are consistent with a conservative mixing model that uses chloride concentrations at the seafloor and the SMT as endmembers. Conservative mixing of pore water constituents implies that an undetermined physical process is primarily responsible for the nonlinearity of the pore water-depth profiles. In limited cases where the sulfate and DIC concentrations deviated from conservative mixing between the seafloor and SMT, the δ 13C-DIC mixing diagrams suggest that the excess DIC is produced from a 13C-depleted source that could only be accounted for by microbial methane, the dominant form of methane identified during this study. We conclude that AOM is the most prevalent sink for sulfate and that it occurs primarily at the SMT at this Keathley Canyon site.

On the relationship between methane production and oxidation by anaerobic methanotrophic communities from cold seeps of the Gulf of Mexico

The anaerobic oxidation of methane (AOM) in the marine subsurface is a significant sink for methane in the environment, yet our understanding of its regulation and dynamics is still incomplete. Relatively few groups of microorganisms consume methane in subsurface environments--namely the anaerobic methanotrophic archaea (ANME clades 1, 2 and 3), which are phylogenetically related to methanogenic archaea. Anaerobic oxidation of methane presumably proceeds via a 'reversed' methanogenic pathway. The ANME are generally associated with sulfate-reducing bacteria (SRB) and sulfate is the only documented final electron acceptor for AOM in marine sediments. Our comparative study explored the coupling of AOM with sulfate reduction (SR) and methane generation (MOG) in microbial communities from Gulf of Mexico cold seep sediments that were naturally enriched with methane and other hydrocarbons. These sediments harbour a variety of ANME clades and SRB. Following enrichment under an atmosphere of methane, AOM fuelled 50-100% of SR, even in sediment slurries containing petroleum-associated hydrocarbons and organic matter. In the presence of methane and sulfate, the investigated microbial communities produce methane at a small fraction ( approximately 10%) of the AOM rate. Anaerobic oxidation of methane, MOG and SR rates decreased significantly with decreasing concentration of methane, and in the presence of the SR inhibitor molybdate, but reacted differently to the MOG inhibitor 2-bromoethanesulfonate (BES). The addition of acetate, a possible breakdown product of petroleum in situ and a potential intermediate in AOM/SR syntrophy, did not suppress AOM activity; rather acetate stimulated microbial activity in oily sediment slurries.

Application of lipid biomarkers to detect sources of organic matter in mud volcano deposits and post-eruptional methanotrophic processes in the Gulf of Cadiz, NE Atlantic

The Gulf of Cadiz is known as an active mud volcano (MV) and fluid venting region discovered during the Training Through Research (TTR) expeditions in 1999 (TTR-9). Here we report a study of lipid biomarkers in the mud breccia matrix from three MVs in this region. The lipid biomarker composition revealed strong compositional resemblance as well as similar thermal maturity properties for the studied MVs. This indicates that the primary source of the erupted material for these MVs is located in similar litho-stratigraphic units. In both areas, upward migrated fluid went through the sedimentary series of the allochthonous Olistrostrome and through the Upper Cretaceous horizons. The relatively immature characteristics of organic matter from both mud breccia rock clasts and matrix together with the more mature properties of hydrocarbon gases from the same sampling locations indicate that source-strata for the initial fluid are located deeper in the subsurface than the source-strata for the erupted sedimentary material. The presence of microbial lipid biomarkers derived from archaea and sulfate reducing bacteria in the mud breccias revealed anaerobic oxidation of methane (AOM) as a dominant microbial process in these habitats. Our data suggest that the AOM-derived lipids are, at least in part, fossil in origin (i.e. they do not reflect active AOM communities) and thus reflect AOM activity in these MVs integrated over time. The low concentrations of AOM-related biomarkers in these MVs suggest relatively low intensities of AOM in the studied MV deposits. The distribution of biomarkers suggests an abrupt and brief cold seep activity in the past and the absence of a continuous methane influx in the studied locations.

Microbial methane turnover at mud volcanoes of the Gulf of Cadiz

The Gulf of Cadiz is a tectonically active area of the European continental margin and characterised by a high abundance of mud volcanoes, diapirs, pockmarks and carbonate chimneys. During the R/V SONNE expedition “GAP–Gibraltar Arc Processes (SO-175)” in December 2003, several mud volcanoes were surveyed for gas seepage and associated microbial methane turnover. Pore water analyses and methane oxidation measurements on sediment cores recovered from the centres of the mud volcanoes Captain Arutyunov, Bonjardim, Ginsburg, Gemini and a newly discovered, mud volcano-like structure called “No Name” show that thermogenic methane and associated higher hydrocarbons rising from deeper sediment strata are completely consumed within the seabed. The presence of a distinct sulphate–methane transition zone (SMT) overlapping with high sulphide concentrations suggests that methane oxidation is mediated under anaerobic conditions with sulphate as the electron acceptor. Anaerobic oxidation of methane (AOM) and sulphate reduction (SR) rates show maxima at the SMT, which was found between 20 and 200 cm below seafloor at the different mud volcanoes. In comparison to other methane seeps, AOM activity (<383 mmol m −2 year −1) and diffusive methane fluxes (<321 mmol m −2 year −1) in mud volcano sediments of the Gulf of Cadiz are low to mid range. Corresponding lipid biomarker and 16S rDNA clone library analysis give evidence that AOM is mediated by a mixed community of anaerobic methanotrophic archaea and associated sulphate reducing bacteria (SRB) in the studied mud volcanoes. Little is known about the variability of methane fluxes in this environment. Carbonate crusts littering the seafloor of mud volcanoes in the northern part of the Gulf of Cadiz had strongly 13C-depleted lipid signatures indicative of higher seepage activities in the past. However, actual seafloor video observations showed only scarce traces of methane seepage and associated biological processes at the seafloor. No active fluid or free gas escape to the hydrosphere was observed visually at any of the surveyed mud volcanoes, and biogeochemical measurements indicate a complete methane consumption in the seafloor. Our observations suggest that the emission of methane to the hydrosphere from the mud volcano structures studied here may be insignificant at present.

Methane emission and consumption at a North Sea gas seep (Tommeliten area)

Abstract. The Tommeliten seepage area is part of the Greater Ekofisk area, which is situated above the Tommeliten Delta salt diapir in the central North Sea (56°29.90' N, 2°59.80' E, Norwegian Block 1/9, 75 m water depth). Here, cracks in a buried marl horizon allow methane to migrate into overlying clay-silt and sandy sediments. Hydroacoustic sediment echosounding showed several venting spots coinciding with the apex of marl domes where methane is released into the water column and potentially to the atmosphere. In the vicinity of the gas seeps, sea floor observations showed small mats of giant sulphide-oxidizing bacteria above patches of black sediments as well as carbonate crusts, which are exposed 10 to 50 cm above seafloor forming small reefs. These Methane-Derived Authigenic Carbonates (MDACs) contain 13C-depleted, archaeal lipids indicating previous gas seepage and AOM activity. High amounts of sn2-hydroxyarchaeol relative to archaeol and low abundances of biphytanes in the crusts give evidence that ANaerobic MEthane-oxidising archaea (ANME) of the phylogenetic cluster ANME-2 were the potential mediators of Anaerobic Oxidation of Methane (AOM) at the time of carbonate formation. Small pieces of MDACs were also found subsurface at about 1.7 m sediment depth, associated with the AOM zone. This zone is characterized by elevated AOM and Sulphate Reduction (SR) rates, increased concentrations of 13C-depleted tetraether derived biphytanes, and specific bacterial Fatty Acids (FA). Further biomarker and 16S rDNA based analyses of this horizon give evidence that AOM is mediated by archaea belonging to the ANME-1b group and Sulphate Reducing Bacteria (SRB) most likely belonging to the Seep-SRB1 cluster. The zone of active methane consumption was restricted to a distinct horizon of about 20 cm. Concentrations of 13C-depleted lipid biomarkers (e.g. 500 ng g-dw−1 biphythanes, 140 ng g-dw−1 fatty acid ai-C15:0), cell numbers (1.5×108 cells cm−3), AOM and SR rates (3 nmol cm−3 d−1) in the Tommeliten AOM zone are 2–3 orders of magnitude lower compared to AOM zones of highly active deep water cold seeps such as Hydrate Ridge or the Gulf of Mexico.

Molecular biogeochemistry of sulfate reduction, methanogenesis and the anaerobic oxidation of methane at Gulf of Mexico cold seeps

The anaerobic oxidation of methane in aquatic environments is a globally significant sink for a potent greenhouse gas. Significant gaps remain in our understanding of the anaerobic oxidation of methane because data describing the distribution and abundance of putative anaerobic methanotrophs in relation to rates and patterns of anaerobic oxidation of methane activity are rare. An integrated biogeochemical, molecular ecological and organic geochemical approach was used to elucidate interactions between the anaerobic oxidation of methane, methanogenesis, and sulfate reduction in sediments from two cold seep habitats (one brine site, the other a gas hydrate site) along the continental slope in the Northern Gulf of Mexico. The results indicate decoupling of sulfate reduction from anaerobic oxidation of methane and the contemporaneous occurrence of methane production and consumption at both sites. Phylogenetic and organic geochemical evidence indicate that microbial groups previously suggested to be involved in anaerobic oxidation of methane coupled to sulfate reduction were present and active. The distribution and isotopic composition of lipid biomarkers correlated with microbial distributions, although concrete assignment of microbial function based on biomarker profiles was complicated given the observed overlap of competing microbial processes. Contemporaneous activity of anaerobic oxidation of methane and bicarbonate-based methanogenesis, the distribution of methane-oxidizing microorganisms, and lipid biomarker data suggest that the same microorganisms may be involved in both processes.

Anaerobic oxidation of methane and sulfate reduction along the Chilean continental margin

Anaerobic oxidation of methane (AOM) and sulfate reduction (SR) were investigated in sediments of the Chilean upwelling region at three stations between 800 and 3000 m water depth. Major goals of this study were to quantify and evaluate rates of AOM and SR in a coastal marine upwelling system with high organic input, to analyze the impact of AOM on the methane budget, and to determine the contribution of AOM to SR within the sulfate-methane transition zone (SMT). Furthermore, we investigated the formation of authigenic carbonates correlated with AOM. We determined the vertical distribution of AOM and SR activity, methane, sulfate, sulfide, pH, total chlorins, and a variety of other geochemical parameters. Depth-integrated rates of AOM within the SMT were between 7 and 1124 mmol m −2 a −1, effectively removing methane below the sediment-water interface. Single measurements revealed AOM peaks of 2 to 51 nmol cm −3 d −1, with highest rates at the shallowest station (800 m). The methane turnover was higher than in other diffusive systems of similar ocean depth. This higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis. SR within the SMT was mostly fuelled by methane. AOM led to the formation of isotopically light DIC (δ 13C: −24.6‰ VPDB) and of distinct layers of authigenic carbonates (δ 13C: −14.6‰ VPDB).