Stable carbon isotopic compositions of intact polar lipids reveal complex carbon flow patterns among hydrocarbon degrading microbial communities at the Chapopote asphalt volcano

Seepage of asphalt forms the basis of a cold seep system at 3000 m water depth at the Chapopote Knoll in the southern Gulf of Mexico. Anaerobic microbial communities are stimulated in the oil-impregnated sediments as evidenced by the presence of intact polar membrane lipids (IPLs) derived from archaea and Bacteria at depths up to 7 m below the seafloor. Detailed investigation of stable carbon isotope composition (δ¹³C) of alkyl and acyl moieties derived from a range of IPL precursors with distinct polar head groups resolved the complexity of carbon metabolisms and utilization of diverse carbon sources by uncultured microbial communities. In surface sediments most of the polar lipid-derived fatty acids with phosphatidylethanolamine (PE), phosphatidylglycerol (PG) and diphosphatidylglycerol (DPG) head groups could be tentatively assigned to autotrophic sulfate-reducing bacteria, with a relatively small proportion involved in the anaerobic oxidation of methane. Derivatives of phosphatidyl-(N)-methylethanolamine (PME) were abundant and could be predominantly assigned to heterotrophic oil-degrading bacteria. Archaeal IPLs with phosphate-based hydroxyarchaeols and diglycosidic glyceroldibiphytanylglyceroltetraethers (GDGTs) were assigned to methanotrophic archaea of the ANME-2 and ANME-1 cluster, respectively, whereas δ¹³C values of phosphate-based archaeols and mixed phosphate-based and diglycosidic GDGTs point to methanogenic archaea. At a 7 m deep sulfate-methane transition zone that is linked to the upward movement of gas-laden petroleum, a distinct increase in abundance of archaeal IPLs such as phosphate-based hydroxyarchaeols and diglycosidic archaeol and GDGTs is observed; their δ¹³C values are consistent with their origin from both methanotrophic and methanogenic archaea. This study reveals previously hidden, highly complex patterns in the carbon-flow of versatile microbial communities involved in the degradation of heavy oil including hydrocarbon gases that would not have been evident from classical compound-specific isotope analyses of either bulk IPL or apolar lipid derivatives.     

Factors controlling the distribution of anaerobic methanotrophic communities in marine environments: Evidence from intact polar membrane lipids

Three distinct types of microbial consortia appear to mediate the anaerobic oxidation of methane with sulfate as electron acceptor in marine sediments and are distributed ubiquitously. These consortia consist of ANerobic MEthanotrophic (ANME) archaea of the ANME-1, ANME-2 and ANME-3 clades and their sulfate-reducing bacterial partners either of the Desulfosarcina-Desulfococcus (ANME-1/DSS and ANME-2/DSS) or Desulfobulbus spp. (ANME-3/DBB) branches. Frequently one consortium type dominates the community, but the selective factors are not well constrained. Here we analyzed patterns in the composition of intact polar lipids extracted from bacterial and archaeal communities of different marine seep environments. Further, we investigated if different environmental and geographical factors were responsible for the observed patterns, and hence could be important in the selection of seep communities. Intact polar lipids (IPLs) provide a more robust distinction of the composition of extant communities than their less polar derivatives. In ANME-1/DSS-dominated communities, glycosidic- and phospho-glyceroldialkylglyceroltetraethers were abundant, while ANME-2/DSS and ANME-3/DBB-dominated communities showed abundant archaeol-based IPLs, either with glycosidic and phospho-headgroups or only phospho-headgroups, respectively. The relative proportion of bacterial IPLs varied widely from 0% to 93% and was generally lower in samples of the ANME-1 type, suggesting lower bacterial biomasses in the respective communities. In addition to these lipid signatures, distinctive features were related to the habitat characteristics of these communities: lower amounts of phosphate-based IPLs were generally observed in communities from calcified microbial mats compared to sediments, which may reflect phosphate limitation. Based on statistical analyses of IPLs and environmental data this study constrained for the first time the occurrence of three environmental factors controlling the distribution of different ANME-associated communities in a wide range of hydrocarbon seep systems. Habitats dominated by ANME-1/DSS communities were characterized by high temperature and low oxygen content in bottom waters (or even anoxia), while ANME-2/DSS and ANME-3/DBB-dominated sediments were located in settings with lower temperatures and higher oxygen content in bottom waters. Furthermore, ANME-2/DSS communities were particularly prominent in environments in which a relatively high supply of sulfate was sustained.     

Archaeal lipids in Neogene dolomites (Monterey and Sisquoc Formations, California) - Planktic versus benthic archaeal sources

The distribution of archaeal lipids, including archaeol and glycerol dibiphytanyl glycerol tetraethers (GDGTs), in dolomite concretions and surrounding sediment from the Monterey Formation (Miocene) and the Sisquoc Formation (Miocene-Pliocene) were examined to distinguish planktic from benthic contributions. For this purpose, dolomites with positive δ¹³C values (+7‰ to +13‰) were chosen; such highly positive values point to pronounced methanogenesis of benthic archaea in the sedimentary column. At first glance, distributions and relative abundances of GDGTs in both dolomites and background sediment were similar, resembling patterns of marine planktic crenarchaea. A contribution of benthic euryarchaea to the GDGT pool became evident only from variations in the δ¹³C values of different biphytanes obtained after ether cleavage of GDGTs. Whereas bi- and tricyclic biphytanes had an isotopic signal typical of planktic archaea (δ¹³C −23.6‰ to −20.5‰ and −23.4‰ to −21.2‰, respectively) for both dolomite and background sediment, acyclic and monocyclic biphytanes showed lower values for dolomite samples (−25.1‰ to −22.6‰ and −27.6‰ to −24.7‰, respectively), indicating a contribution of lipids from benthic archaea. The isoprenoid diether archaeol (δ¹³C −23.9‰ to −22.9‰), assigned to euryarchaea, was only detected in dolomite samples, also reflecting additional input from sedimentary archaea, probably autotrophic methanogens. The occurrence of lipids derived from methanogenic archaea agrees with the strong ¹³C-enrichment of dolomites and with mineral formation taking place in the zone of archaeal methanogenesis. This implies that the lipid biomarker inventory of sedimentary strata needs to be interpreted carefully, as it is often not straightforward to discriminate between input from the water column and sedimentary microbial activity.     

Diagnostic lipid biomarker and stable carbon isotope signatures of microbial communities mediating the anaerobic oxidation of methane with sulphate

The anaerobic oxidation of methane (AOM) with sulphate is the most important sink for methane in marine environments. This process is mediated by a consortium of methanotrophic archaea and sulphate reducing bacteria. So far, three groups of anaerobic methane oxidisers (ANME-1, -2 and -3) related to the methanogenic Methanosarcinales and Methanomicrobiales were discovered. The sulphate reducing partner of ANME-1 and -2 are two different eco-types of SRB related to the Desulfosarcina/Desulfococcus cluster (Seep-SRB1), whereas ANME-3 is associated with Desulfobulbus spp. (DBB). In this article, we reviewed literature data to assign statistically significant lipid biomarker signatures for a chemotaxonomic identification of the three known AOM communities. The lipid signatures of ANME-2/Seep-SRB1 and ANME-3/DBB are intriguingly similar, whereas ANME-1/Seep-SRB1 shows substantial differences to these AOM communities. ANME-1 can be distinguished from ANME-2 and -3 by a low ratio of the isoprenoidal dialkyl glycerol diethers sn2-hydroxyarchaeol and archaeol combined with a comparably low stable carbon isotope difference of archaeol relative to the source methane. Furthermore, only ANME-1 contains substantial amounts of isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs), however, with the probable exception of the ANME-2c sub-cluster. In contrast to the ANME-1 archaea, the tail to tail linked hydrocarbon tetramethylhexadecane (crocetane) is unique to ANME-2, whereas pentamethylicosenes (PMIs) with 4 and 5 double bonds without any higher saturated homologues were only found in ANME-3. The sulphate reducing partner of ANME-1 can be discerned from those of ANME-2 and -3 by a low ratio of the fatty acids (FAs) C_16:1ω5 relative to i-C_15:0 and, although to a lesser degree, by a high abundance of ai-C_15:0 relative to i-C_15:0. Furthermore, substantial amounts of ¹³C depleted non-isoprenoidal monoalkyl glycerol ethers (MAGEs) were only found in the sulphate reducing partners of ANME-2 and -3. A differentiation of these SRB is possible based on the characteristic presence of the FAs cy-C_17:0ω5,6 and C_17:1ω6, respectively. Generally, the data analysed here show overlaps between the different AOM communities, which highlights the need to use multiple lipid signatures for a robust identification of the dominating microbes involved.     

南海东沙深海冷泉区973-5重力柱沉积物古菌多样性

中国南海东沙一带冷泉发育,但目前国内外对深海冷泉区微生物研究甚少,特别是缺乏利用高通量测序的记录。对东沙深海冷泉区973-5站位(该站位水深约3 000 m)长约935 cm的重力岩心进行了高通量分析。结果显示:该站位微生物细胞丰度为5.3×10⁸~34.0×10⁸个/g,随深度变深而增加,其变化趋势与甲烷含量变化可对比,与粒度、有机碳的变化也具有相关性。测序结果显示,岩心中主要古菌类群是MBGB(39.9%)、C3(15.8%)以及ANME-1(12.0%),随着深度的变化群落组成有所改变。硫酸盐-甲烷界面(SMI,760 cm)上下出现了大量的MBGB和ANME-1类群,pH也不断增加,暗示了这一区域存在不断增强的甲烷厌氧氧化作用。岩心底部出现了一定量的ANME-1和ANME-2类群,暗示除了在SMI附近甲烷氧化和硫酸盐还原反应强烈,其下部可能还有水合物的分解与甲烷的上涌,为ANME类群生存提供了营养物质。与东沙海区其他站位相比,973-5站位的甲烷通量较高,但没有发现产甲烷菌,推测该区沉积物中高浓度的甲烷来源为周边浅部或深部断裂系统运移供给。     

祁连山水合物分布区不同高寒生态类型冬季表层土壤中古菌群落及变化特征

祁连山冻土区是我国青藏高原重要永久冻土区之一,也是我国陆域天然气水合物分布的重要地区。前期调查表明祁连山木里天然气水合物钻井区一带具 有丰富多样的高寒冻土生态类型。为了解该地区高寒草甸和高寒沼泽草甸表层土壤中古菌群落的多样性及分布特征,对2014年初冬在该区采集的表层土壤样品利用16S rRNA分子生物学技术和 地球化学等方法进行了分析和研究。结果显示,高寒草甸区土壤呈中性,而高寒沼泽草甸区土壤呈弱酸性。钻井区土壤中的TOC和顶空气甲烷含量均显著高于背景区,而在背景区内的两种生态 类型土壤中的TOC和顶空气甲烷含量差别较小。钻井区(除1个点)微生物细胞丰度高于背景区2~5倍。冬季表层土壤中的古菌多样性较低,含泉古菌的3个类群和广古菌的3个类群。不同植被类型 古菌群落的优势种群显著不同,在高寒草甸区为泉古菌门的Group Ⅰ.1b,而高寒沼泽草甸区为广古菌门的甲烷八叠球菌目(Methanosarcinales)。根据研究结果推测,土壤水分可能是导致高 寒草甸区和高寒沼泽草甸区细胞丰度和古菌群落差异的一个主要原因,高寒沼泽草甸区内产甲烷菌占优势可能与土壤高TOC含量有关。高寒沼泽草甸土壤中存在较丰富的产甲烷古菌,它们在厌 氧条件下的甲烷氧化作用也是土壤中甲烷来源之一。     

Archaea mediate anaerobic oxidation of methane in deep euxinic waters of the Black Sea

We evaluate anaerobic oxidation of methane (AOM) in the Black Sea water column by determining distributions of archaea-specific glyceryl dialkyl glyceryl tetraethers (GDGTs) and ¹³C isotopic compositions of their constituent biphytanes in suspended particulate matter (SPM), sinking particulate matter collected in sediment traps, and surface sediments. We also determined isotopic compositions of fatty acids specific to sulfate-reducing bacteria to test for biomarker and isotopic evidence of a syntrophic relationship between archaea and sulfate-reducing bacteria in carrying out AOM. Bicyclic and tricyclic GDGTs and their constituent ¹³C-depleted monocyclic and bicyclic biphytanes (down to −67‰) indicative of archaea involved in AOM were present in SPM in the anoxic zone below 700 m depth. In contrast, GDGT-0 and crenarchaeol derived from planktonic crenarchaeota dominated the GDGT distributions in the oxic surface and shallow anoxic waters. Fatty acids indicative of sulfate-reducing bacteria (i.e., iso- and anteiso-C₁₅) were not strongly isotopically depleted (e.g., −32 to −25‰), although anteiso-C₁₅ was 5‰ more depleted in ¹³C than iso-C₁₅. Our results suggest that either AOM is carried out by archaea independent of sulfate-reducing bacteria or those sulfate-reducing bacteria involved in a syntrophy with methane-oxidizing archaea constitute a small enough fraction of the total sulfate-reducing bacterial community that an isotope depletion in their fatty acids is not readily detected. Sinking particulate material collected in sediment traps and the underlying sediments in the anoxic zone contained the biomarker and isotope signature of upper-water column archaea. AOM-specific GDGTs and ¹³C-depleted biphytanes characteristic of the SPM in the deep anoxic zone are not incorporated into sinking particles and are not efficiently transported to the sediments. This observation suggests that sediments may not always record AOM in overlying euxinic water columns and helps explain the absence of AOM-derived biomarkers in sediments deposited during past periods of elevated levels of methane in the ocean.     

Highly isotopically depleted isoprenoids: molecular markers for ancient methane venting

We propose that organic compounds found in a Miocene limestone from Marmorito (Northern Italy) are source markers for organic matter present in ancient methane vent systems (cold seeps). The limestone contains high concentrations of the tail-to-tail linked, acyclic C₂₀ isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), a C₂₅ homolog 2,6,10,15,19-pentamethylicosane (PME), and a distinctive glycerol ether lipid containing 3,7,11,15-tetramethylhexadecyl (phytanyl-) moieties. The chemical structures of these biomarkers indicate a common origin from archaea. Their extremely ¹³C-depleted isotope compositions (δ¹³C ≈ −108 to −115.6‰ PDB) suggest that the respective archaea have directly or indirectly introduced isotopically depleted, methane-derived carbon into their biomass. We postulate that a second major cluster of biomarkers showing heavier isotope values (δ¹³C ≈ −88‰) is derived from sulfate-reducing bacteria (SRB). The observed biomarkers sustain the idea that methanogenic bacteria, in a syntrophic community with SRB, are responsible for the anaerobic oxidation of methane in marine sediments. Marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined, two-membered bacterial consortium: (1) archaea that perform reversed methanogenesis by oxidizing methane and producing CO₂ and H₂; and (2) SRB that consume the resulting H₂. Furthermore, the respective organic molecules are, unlike other compounds, tightly bound to the crystalline carbonate phase. The Marmorito carbonates can thus be regarded as “cold seep microbialites” rather than mere “authigenic” carbonates.     

Carbon Geochemistry of Gas Hydrate‐associated Sediments in the Southwestern Taiwan Basin

Marine gas hydrates, one of the largest methane reservoirs on Earth, may greatly affect the deep sea sedimentary environment and biogeochemistry; however, the carbon geochemistry in gas hydrate-bearing sediments is poorly understood. In this study, we investigated the carbon variables in sediment core 973-3 from the southwestern Taiwan Basin in the South China Sea to understand the effect of environmental factors and archaeal communities on carbon geochemistry. The carbon profiles suggest the methanogenesis with the incerase of dissolved inorganic carbon(DIC) and high total organic carbon(TOC)(mean = 0.46%) originated from terrigenous organic matter(mean δ~(13)C_(TOC) value of-23.6‰) driven by the abundant methanogen ‘Methanosaeta and Methanomicrobiales'. The active anaerobic oxidation of methane is characterized by the increase of DIC and inorganic carbon(IC), and the depleted δ~(13)C_(IC), coupled with the increase of TOC and the decrease of δ~(13)C_(TOC) values owing to the methanotroph ‘Methanosarcinales/ANME' in 430–840 cm. Environmental factors and archaeal communities in core 973-3 are significantly correlated to carbon variables owing to methane production and oxidation. Our results indicate that the carbon geochemical characteristics are obviously responding to the formation and decomposition of gas hydrates. Furthermore, pH, Eh and grain size, and Methanosaeta greatly affect the carbon geochemistry in gas hydrate-associated sediments.     

Comparison of fluid geochemistry and microbiology of multiple organic-rich reservoirs in the Illinois Basin, USA: Evidence for controls on methanogenesis and microbial transport

Microbial methane in sedimentary basins comprises approximately 20% of global natural gas resources, yet little is known about the environmental requirements and metabolic rates of these subsurface microbial communities. The Illinois Basin, located in the midcontinent of the United States, is an ideal location to investigate hydrogeochemical factors controlling methanogenesis as microbial methane accumulations occur: (1) in three organic-rich reservoirs of different geologic ages and organic matter types - Upper Devonian New Albany Shale (up to 900 m depth), Pennsylvanian coals (up to 600 m depth), and Quaternary glacial sediments (shallow aquifers); (2) across steep salinity gradients; and (3) with variable concentrations of . For all three organic-rich reservoirs aqueous geochemical conditions are favorable for microbial methanogenesis, with near neutral pH, concentrations <2 mM, and Cl⁻ concentrations <3 M. Also, carbon isotopic fractionation of CH₄, CO₂, and DIC is consistent with microbial methanogenesis, and increased carbon isotopic fractionation with average reservoir depth corresponds to a decrease of groundwater flushing rates with average depth of reservoir. Plots of stable isotopes of water and Cl⁻ show mixing between a brine endmember and freshwater, suggesting that meteoric groundwater recharge has affected all microbial methanogenic systems. Additionally, similar methanogenic communities are present in all three reservoirs with comparable cell counts (8.69E3-2.58E6 cells/mL). TRFLP results show low numbers of archaea species with only two dominant groups of base pairs in coals, shale, and limestone aquifers. These results compare favorably with other methanogen-containing deep subsurface environments. Individual hydrogeochemical parameters that have a Spearman correlation coefficient greater than 0.3 to variations in methanogenic species include stable isotopes of water (δ¹⁸O and δD), type of substrate (i.e. coals versus shale), pH, and Cl⁻ concentration. The matching of variations between methanogenic TRFLP data and conservative tracers suggests that deep circulation of meteoric waters influenced archaeal communities in the Illinois Basin. In addition, coalification and burial estimates suggest that in the study area, coals and shale reservoirs were previously sterilized (>80 °C in nutrient poor environments), necessitating the re-introduction of microbes into the subsurface via groundwater transport.     

Molecular characterization of phenanthrene-degrading methanogenic communities in leachate-contaminated aquifer sediment

The existence of polycyclic aromatic hydrocarbons in the various environments has aroused great environmental concerns due to their potential hazards to human health. The presence of polycyclic aromatic hydrocarbons in aquifer is particularly sensitive where groundwater is used as a source of potable water. Anaerobic biodegradation of polycyclic aromatic hydrocarbons is an attractive option for remediation of contaminated aquifer sediment. Bacterial and archaeal community structures of phenanthrene-degrading aquifer sediment under methanogenic condition were investigated using clone library analysis in combination with microcosm study. The bacterial members were all affiliated with ??-Proteobacteria. Phylum Euryarchaeota was the predominant archaeal group, represented by genera Methanosarcina, Methanobacterium and Thermogymnomonas. Both bacteria (genera Citrobacter and Pseudomonas) and archaea (genus Methanosarcina) might have links to phenanthrene degradation process. This work might provide some new insights into developing strategies for the isolation of the putative polycyclic aromatic hydrocarbons degraders under methanogenic condition and bioremediating polycyclic aromatic hydrocarbons in leachate-contaminated aquifer sediment.     

Distribution of glycerol dialkyl glycerol tetraether lipids in the water column of Lake Tanganyika

We studied the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) in suspended particulate matter from the water column of Lake Tanganyika (East Africa), where sediment studies had shown the applicability of the TEX₈₆ proxy for reconstructing surface lake water temperature. GDGTs, in particular crenarchaeol, showed maximum abundance within the suboxic zone (100–180 m), suggesting that this is the preferred niche of ammonia-oxidizing Thaumarchaeota. Despite evidence for anaerobic methane oxidation in deep anoxic water (300–1200 m) no unambiguous evidence for an imprint of methanotrophic archaea on GDGT distribution was found. Comparison of TEX₈₆ and BIT indices with those of surface sediments suggests that the sedimentary GDGTs are derived predominantly from the oxic zone and suboxic zone of the lake.     

Distributions and assemblages of microbial communities along a sediment core retrieved from a potential hydrate-bearing region offshore southwestern Taiwan

Assessing the impacts of methane released from hydrate-bearing environments on global carbon cycling would require detailed insights into the distributions and capacities of microbial communities at different horizons of sediment column. In this study, we conducted geochemical, gene abundance and diversity analyses for a sediment core retrieved from a potential hydrate-bearing region off southwestern Taiwan. Geochemical profiles were characterized by a sulfate-to-methane transition with decreasing total organic carbon and nitrogen in sediments, and increasing dissolved inorganic carbon, ammonium and total sulfur in sediments. Bacterial and archaeal 16S rRNA and amoA gene abundances decreased with depth. In contrast, ANME-1 and -2 16S rRNA gene abundances increased significantly across the sulfate-to-methane transition and peaked at different horizons below this interface. A total of 124,379 bacterial and 130,351 archaeal reads were recovered through tag-pyrosequencing of 16S rRNA genes and categorized into 9014 bacterial and 6394 archaeal operational taxonomic units on the basis of 97% sequence similarity, respectively. Major bacterial phyla/divisions and archaeal groups (>5% of the total reads) detected included Chloroflexi, Planctomycetes, OP9, Deltaproteobacteria, BHI80-139, MBG-B, Halobacteria, MCG, Thermoplasmata, ANME-1 and MG-I. The abundance variations of most major OTUs (>0.5% of the total reads) were statistically correlated with those of geochemical parameters. These lines of evidence suggest that the populations represented by the major OTUs or detected by group-specific primers were compartmentalized into different horizons and involved directly or indirectly in the cycling of methane, sulfate, organic carbon and nitrogen. Overall, this study demonstrates that the deep sequencing coverage combined with the quantification of gene abundance and geochemical characterization would enable to uncover the detailed distributions and potential metabolic capabilities of specific groups from complexly structured microbial communities in methane-rich marine sediments.     

Application of a Simulation Model for the Formation of Methane Hydrate to the Nankai Trough and the Blake Ridge: Natural Examples of Two End-member Cases

Abstract. Simulation experiments with a one-dimensional static model for formation of methane hydrate are used to demonstrate models of hydrate occurrence and its generation mechanism for two end-member cases. The simulation results compare well with experimental data for two natural examples (the Nankai Trough and the Blake Ridge).
At the MITI Nankai Trough wells, the hydrate occurrence is characterized by strongly hydrated sediments developing just above the BGHS. Such occurrence can be reproduced well by simulation in which the end-member case of upward advective fluid flow from below the BGHS is set. The strongly hydrated sediments is formed by oversaturated solution with free gas which directly enters the BGHS by the upward advective fluid flow. The recycling of dissociated methane of preexisting hydrate also contributes to the increase of hydrate saturation.
At the Site 997 in the Blake Ridge area, the hydrate occurrence is characterized by thick zone with poorly hydrated sediments and no hydrate zone developing above the hydrate zone. Such occurrence can be reproduced well by simulation in which the end-member case of in-situ biogenic production of methane in the sediment of methane hydrate zone is set. The distribution pattern of hydrate saturation is basically controlled by that of TOC. However, the hydrate concentration near the bottom of the hydrate zone is increased by the effect of recycling of dissociated methane of pre-existing hydrate. No hydrate zone expresses the geologic time needed until the local concentration of methane exceeds the solubility by gradual accumulation of in-situ biogenic methane with burial.     

Archaeal polar lipids in subseafloor sediments from the Nankai Trough: Implications for the distribution of methanogens in the deep marine subsurface

Although the methane in marine methane hydrates is mainly of microbial origin, information about the distribution of methanogens in subseafloor sediments is limited. To address this issue, we analyzed sediment core samples from two sites in the Nankai Trough, off the Pacific coast of central Japan, including those bearing methane hydrates from depths > 100 m below the seafloor (mbsf), for isopranyl ether-linked polar lipids (i.e. with polar head groups of phosphate, sugar, or both) as biomarkers of archaea, including methanogens. In most samples, including the deepest (381 mbsf), archaeol, and sn-2- and sn-3-hydroxyarchaeols were detected as their hydrolyzed derivatives. Concentrations of these three archaeal lipids correlated strongly with each other, suggesting a common biological source. The δ¹³C values of phytane derived from the phytanyl groups in the archaeal lipids were distinctly higher than those of methane, indicating that methanogens rather than anaerobic methanotrophic archaea were the major biological source. Depth profiles of polar sn-2-hydroxyarchaeol concentration were consistent with those of the potential methane production activity previously estimated from incubation of core sediments from the same sites. This observation, together with results of previous studies showing the presence of sn-2-hydroxyarchaeol mainly in shallow young sediments, strongly suggests that this polar lipid is a valid biomarker for in situ methanogens in sediments. There was a strong correlation between the concentration of polar sn-2-hydroxyarchaeol and that of total organic carbon, suggesting that bulk organic matter concentration is a primary control on the distribution of methanogens in sediments.     

Formation of iron sulfide nodules during anaerobic oxidation of methane

The biomarker compositions of iron sulfide nodules (ISNs; upper Pliocene Valle Ricca section near Rome, Italy) that contain the ferrimagnetic mineral greigite (Fe₃S₄) were examined. In addition to the presence of specific terrestrial and marine biomarkers, consistent with formation in coastal marine sediments, these ISNs contain compounds thought to originate from sulfate reducing bacteria (SRB). These compounds include a variety of low-molecular-weight and branched alkanols and several non-isoprenoidal dialkyl glycerol diethers (DGDs). In addition, archaeal biomarkers, including archaeol, macrocyclic isoprenoidal DGDs and isoprenoidal glycerol dialkyl glycerol tetraethers are also present. Both SRB and archaeal lipid δ¹³C values are depleted in ¹³C (δ¹³C values are typically less than −50‰), which suggests that the SRB and archaea consumed ¹³C depleted methane. These biomarker and isotopic signatures are similar to those found in cold seeps and marine sediments where anaerobic oxidation of methane (AOM) occurs with sulfate serving as the terminal electron acceptor. Association of AOM with formation of greigite-containing ISNs could provide an explanation for documented remagnetization of the Valle Ricca sediments. Upward migration of methane, subsequent AOM and associated authigenic greigite formation are widespread processes in the geological record that have considerable potential to compromise paleomagnetic records.     

天然气水合物分解及其生态环境效应研究进展

了解大陆／大洋边缘地区天然气水合物形成与分解及其在海底沉积物、水体及海底化学自氧生物群落中的一系列物理、化学及生物作用有助于我们在全球和区域尺度上探讨天然气水合物分解对气候变化的影响;天然气水合物在碳循环中的作用和大陆边缘流体活动与物质交换机制等问题。从水合物分解与全球变暖、贫／缺氧甲烷氧化作用、自生矿物沉淀、化学自养生物群落等方面综述了水合物的环境生态效应研究进展。天然气水合物的形成／分解及 其对海洋乃至全球环境生态变化的影响,深刻地反映了地球上岩石圈、水圈、大气圈和生物圈之间相互联系与相互作用,生物,特别是微生物对全球CH 4的平衡和自生矿物沉淀至关重要。     

Carbonate formation by anaerobic oxidation of methane: Evidence from lipid biomarker and fossil 16S rDNA

Carbonate chimneys and other carbonate structures occur widespread in the Gulf of Cadiz and probably reflect the presence of cold seeps and associated release of methane in the geological past, possibly in the Early Pleistocene, but it is unclear under what conditions and by which processes these carbonates were formed. We studied a fossil methane-related carbonate crust collected from the Kidd mud volcano in the gulf. Concentrations of microbial lipids, their stable carbon isotope composition, sequences of fossil 16S rRNA genes of anaerobic methanotrophic archaea in combination with mineralogical and carbon and oxygen isotopic composition of carbonate were obtained for seven different horizons of the crust. This combination of organic and inorganic geochemical techniques with molecular ecological methods gave a consistent view on processes resulting in the formation of the crust and indicated that it took place in two phases and in a downward direction. Archaeal lipid biomarkers and fossil 16S rRNA gene sequence data revealed the dominance of archaeal ANME-2 group and elevated methane partial pressures during the formation of the top part of the crust. The lower part of the carbonate was likely formed in an environment with reduced methane fluxes as revealed by the dominance of fossil remains of ANME-1 archaea. The combination of these methods can be used as an effective tool to reconstruct in unprecedented detail the palaeo-biogeochemical processes resulting in the formation of carbonate fabrics. This interdisciplinary strategy may also be applied for other fossil methane-derived carbonates, generating new concepts and knowledge about past methane-related carbonate systems.     

A comprehensive approach to the study of methane-seep deposits from the Lincoln Creek Formation, western Washington State, USA

ABSTRACT A comprehensive approach using palaeontology, petrography, stable isotope geochemistry and biomarker analyses was applied to the study of seven small methane‐seep carbonate deposits. These deposits are in the Oligocene part of the Lincoln Creek Formation, exposed along the Canyon and Satsop Rivers in western Washington. Each deposit preserves invertebrate fossils, many representing typical seep biota. Authigenic carbonates with δ¹³C values as low as ?51‰ PDB reveal that the carbon is predominately methane derived. Carbonates contain the irregular isoprenoid hydrocarbons 2,6,11,15‐tetramethylhexadecane (crocetane) and 2,6,10,15,19‐pentamethylicosane (PMI), lipid biomarkers diagnostic for archaea. These lipids are strongly depleted in ¹³C (δ¹³C values as low as ?120‰ PDB), indicating that archaea were involved in the anaerobic oxidation of methane. Small filaments preserved in the carbonate may represent methanotrophic archaea. Archaeal methanogenesis induced the formation of a late diagenetic phase, brownish calcite, consisting of dumbbell‐shaped crystal aggregates that exhibit δ¹³C values as high as +7‰ PDB. Clotted microfabrics of primary origin point to microbial mediation of carbonate precipitation. Downward‐directed carbonate aggregation in the seeps produced inverted stromatactoid cavities. Large filaments, interpreted as green algae based on their size, shape, arrangement and biomarkers, imply that deposition occurred, in places, in water no deeper than 210 m.     

Molecular and isotopic partitioning of low-molecular-weight hydrocarbons during migration and gas hydrate precipitation in deposits of a high-flux seepage site

Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C₁–C₄, CO₂]). Molecular ratios of LMWHC (C₁/[C₂ + C₃] > 1000) and stable isotopic compositions of methane (δ¹³C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C₁/C₂₊ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C₁–C₄, CO₂]) relative to the other gas types and the virtual lack of ¹⁴C–CH₄ indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C₁–C₄, CO₂)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of ¹⁴C–CH₄ (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.