Methane-derived Carbon Research Articles

Methane in shallow cold seeps at Mocha Island off central Chile

We studied for the first time the intertidal and subtidal gas seepage system in Mocha Island off Central Chile. Four main seepage sites were investigated (of which one site included about 150 bubbling points) that release from 150 to 240 tonnes CH 4 into the atmosphere per year. The total amount of methane emitted into the atmosphere is estimated in the order of 800 tonnes per year. The gases emanated from the seeps contain 70% methane, and the stable carbon isotopic composition of methane, δ 13C–CH 4 averaged −44.4±1.4‰ which indicates a major contribution of thermogenic gas. Adjacent to one of the subtidal seeps, rocky substrates support a diverse community of microbial filaments, macroalgae, and benthic organisms. While stable carbon isotopic compositions of marine benthic organisms indicate a dominant photosynthesis-based food web, those of some hard-substrate invertebrates were in the range −48.8‰ to −36.8‰, suggesting assimilation of methane-derived carbon by some selected taxa. This work highlights the potential subsidy of the trophic web by CH 4–C, and that its emission to the atmosphere justifies the need of evaluating the use of methane to support the energy requirements of the local community.

Biogenic methane in freshwater food webs

1. It has long been known that substantial amounts of methane are produced in anoxic lake sediments, and the components of the methane cycle in lakes have been well described. At oxic–anoxic interfaces, methane-oxidising bacteria (MOB) convert methane to microbial biomass and can be highly productive. However, only recently has methane been recognised as a potentially important carbon and energy source for lake food webs, and some instances have also been reported of methane contribution to river food webs. Stable isotope analysis (SIA) has provided compelling evidence in this respect and has been supplemented by other lines of evidence. 2. In the benthic food webs of lakes, profundal chironomid larvae appear to be the main conduits for trophic transfer of biogenic methane via grazing on MOB. The mode of feeding of these larvae and the microhabitats they generate both promote larval ability to exploit MOB production. Support to chironomid larvae from methane is rather widespread, but its degree is highly variable; estimates suggest that in some lakes methane-carbon might contribute more than 60% of chironomid carbon biomass. 3. Evidence of crustacean zooplankton in lakes deriving part of their carbon from methane is currently more limited. Reports from some lakes have indicated Daphnia with a substantial (>50%) contribution of methane-carbon in their biomass. However, for this to happen, an oxic–anoxic interface where sufficient MOB production can occur needs to be within the range of vertical migrations by zooplankton, which may only rarely be the case. Hence, a significant methane subsidy of pelagic food webs in lakes is probably much less widespread than for benthic food webs. 4. There is also recent and currently very limited evidence that some stream benthos derives biomass carbon (reported values up to 30%) from methane. This can occur in stagnant backwater pools where conditions can be analogous to those in lake sediments. However, groundwater aquifers can also supply water supersaturated with methane to some rivers, providing a basis for a microbially-mediated transfer of methane-carbon to river benthos. 5. Evidence for significant transfer of methane-derived carbon to higher trophic levels is still very limited. Within some lakes, those fish species that feed extensively on chironomid larvae can derive a substantial part (perhaps up to 20%) of their carbon biomass from methane. It is also likely that methane-carbon produced in lakes or rivers is exported to riparian ecosystems when emerging chironomids or other insects are eaten by invertebrate or avian predators. 6. We argue that conceptual models of freshwater food webs, and especially those for lakes, need to be modified to enable incorporation of biogenic methane as a carbon and energy source. For some types of lakes, carbon and energy budgets certainly need to take account of the production and utilisation of biogenic methane, and the accumulating evidence indicates that this is a more widespread phenomenon that has generally been acknowledged hitherto.

Heterogeneous energetic pathways and carbon sources on deep eastern Mediterranean cold seep communities

Cold seep communities in the Mediterranean Sea have only been discovered two decades ago, and their trophic ecology has been the subject of very few studies. We investigated the benthic food web of two deep chemosynthesis-based ecosystems on the Napoli and Amsterdam mud volcanoes (MVs) in the eastern Mediterranean Sea (~2,000 m depth). Seeping methane has been detected at the surface of both MVs during pioneering cruises and has been hypothesised to be assimilated by benthic fauna as observed in other oceans’ margins. Given the extreme oligotrophic character of the eastern Mediterranean Sea, we a priori expected that chemosynthetic food sources, especially methane-derived carbon (MDC), played a major trophic role in these deep seep communities relative to what has been observed in other seep systems worldwide. We aimed at unravelling the trophic relationships on Napoli and Amsterdam MVs through the analysis of carbon, nitrogen and sulphur isotopes both in the dominant benthic invertebrates including the small endofauna (300 μm < size < 1 cm) and in the sedimented organic matter. In particular, we assessed the fraction of MDC in the tissue of several heterotrophic and symbiotic species. Low mean δ34S and δ13C values (0.4 ± 4.8‰ and −31.6 ± 5.7‰, respectively) obtained for mega- and macrofauna suggested that the investigated benthic food webs are virtually exclusively fuelled by carbon of chemosynthetic origin. A few grazer invertebrates (δ34S up to 11‰) depart from this trend and could complement their diet with sedimented and decayed phytoplanktonic organic matter. Faunal δ13C values indicated that the oxidation of sulphur is likely the predominant energetic pathway for biosynthesis on both MVs. Nevertheless, mytilid bivalves and small capitellid, ampharetid and spionid polychaetes were 13C-depleted (δ13C < −37‰) in a way indicating they assimilated a significant portion of MDC. For these later heterotrophic species, MDC ranged between 21 and 31% (lower estimates) and 97 and 100% (upper estimates). However, our results highlighted that the origin of assimilated carbon may be complex for some symbiotic species. The vestimentiferan tubeworm Lamellibrachia sp., which exclusively depends on its sulphur-oxidising endosymbionts, showed a ~20‰ inter-individual δ13C variability on a very small spatial scale (<1 m) at the summit of Napoli MV. This mostly reflects the variable isotopic composition of pore-water-dissolved inorganic carbon (DIC) and evidenced that tubeworms (and subsequently their endosymbionts) uptake DIC derived from multiple methane oxidation processes in varying proportions. The lower and upper MDC estimates for the vestimentum of Napoli’s individuals were 11–38 and 21–73%, respectively. Finally, data on trophic ecology of Napoli and Amsterdam MVs clearly corroborate previous geophysical results evidencing the spatial heterogeneity of Mediterranean MV environmental conditions.

Evidence for limited microbial transfer of methane in a planktonic food web

Methane-derived carbon may be an important, but overlooked source of energy fueling food webs in a variety of aquatic ecosystems. Although it is commonly assumed that the flow of methane-derived carbon is regulated by aquatic invertebrate consumption of methane-oxidizing bacteria (MOB), few studies have characterized this trophic interaction. We used stable isotope analysis, bioassay experiments, and PCR-based molecular techniques to investigate the interactions between Daphnia and MOB in the pelagic region of a humic lake located in southwestern Michigan, USA. We observed moderate depletion of 13 C in the plankton, but these data alone could not provide evidence for the consumption of MOB by Daphnia. Using quantitative PCR, we determined that MOB attained relatively high densities in the water column (3% of the bacterial community), but we found no evidence that they were grazed upon by Daphnia. Moreover, our results do not support the hypothesis that Daphnia harbored symbiotic MOB. Therefore, the isotopic composition of Daphnia could not be explained by direct trophic interactions with MOB, suggesting the potential importance of indirect trophic interactions (e.g. consumption of MOB-feeding protists) or other processes that alter the isotopic composition of zooplankton resources (e.g. CO2 recycling).

Anaerobic methane oxidation by archaea/sulfate-reducing bacteria aggregates: 2. Isotopic constraints

Recent studies employing novel analytical tools provide detailed, microscopic portraits of archaea/sulfate-reducing bacteria aggregates in sediments from methane seep and vent sites. One of the most striking features of these aggregates is that lipid and cell carbon are highly depleted in ¹³C (δ¹³C &lt; −60‰). Biogenic methane, with δ¹³C values of −50 to −110 permil, is a logical candidate for carbon source of these aggregates. Accordingly, it is widely assumed that the archaea oxidize and assimilate methane, and that methane-derived carbon is transferred to the sulfate-reducing bacteria (SRB) symbionts as CO₂ or as a partially oxidized intermediate. However, methane is not the only possible source of ¹³C-depleted carbon in archaea/SRB aggregates. ΣCO₂ in sediments at seep and vent sites tends to be isotopically “light” due to decomposition of organic matter derived from chemoautotrophic organisms. In addition, CO₂ is depleted in ¹³C by ∼10 permil compared to ΣCO₂ owing to the equilibrium isotope effect. Assimilation of this “light” CO₂ by methanogenic archaea and autotrophic SRB, combined with enzymatic isotope effects, could also yield lipid and biomass that are highly depleted in ¹³C. We derive general equations based on isotope mass-balance and calibrated with laboratory and field data to predict the isotopic composition of archaeal cell carbon and lipids derived from autotrophic methanogenesis and anaerobic methane oxidation. The calculations show that observed δ¹³C values for archaeal biomass and lipids at methane seep and vent sites are readily accounted for by isotope fractionation during methane production from CO₂, and that biomass produced during anaerobic methane oxidation is only slightly depleted in ¹³C relative to methane unless the enzymatic isotope effect associated with the anabolic arm of the assimilation-dissimilation branch point is considerably larger than the isotope effect associated with the catabolic arm. We also apply an isotope diffusion-reaction model to demonstrate that micro-gradients in δ¹³C-CO₂ cannot be maintained within archaea/SRB aggregates. However, ¹³C-depleted carbon in SRB members of the aggregate is readily explained by autotrophic sulfate-reduction with bulk porewater CO₂ as carbon source. These results illustrate that ¹³C-depleted biomass and lipids observed in sediments from methane seep and vent sites may be derived from CO₂-reducing archaea and autotrophic sulfate-reducing bacteria. The inference of anaerobic methanotrophy based on ¹³C depletion in archaeal and sulfate-reducing bacterial cell carbon and/or lipids should be considered tentative unless corroborated by independent, concordant evidence of net methane consumption.

Stable isotope signatures and methane use by New Zealand cold seep benthos

The carbon isotopic composition of seep faunal tissue represents a time-integrated view of the interaction between biology and the biogeochemical gradients within the environment. Here we provide an initial description of carbon and nitrogen stable isotope signatures of dominant symbiont-bearing megafauna and heterotrophic mega- and macrofauna from 10 methane-seep sites on the continental margin of the North Island of New Zealand (662–1201 m water depth). Isotopic signatures suggest that sulfide oxidation supports symbiont-bearing taxa including solemyid and vesicomyid bivalves, and methanotrophic symbionts are present in the seep mussel Bathymodiolus sp. Multiple species of Frenulata (Siboglinidae) are present and have a range of isotopic values that are indicative of both thiotroph- and methanotroph-based nutrition. Isotopic composition of the tubeworm Lamellibrachia sp. varied by 23.3‰ among individuals although there was no consistent difference among sites. Variation in methane use by heterotrophic fauna appears to reflect the availability of hard vs. soft substrate; macrofauna on hard substrates had high δ 13C signatures, reflecting consumption of photosynthetic-derived organic matter. Two unique, biotic assemblages were discovered to be fueled largely by methane: a hard-substrate, multi-phyla sponge-associated community (inhabiting the sponge Pseudosuberites sp.) and a soft-sediment assemblage dominated by ampharetid polychaetes. Isotope signatures yield estimates of 38–100% and 6–100% methane-derived carbon in sponge associates and ampharetid-bed macrofauna, respectively. These estimates are comparable to those made for deeper methane seeps at the Florida Escarpment (3290 m) and Kodiak, Alaska seeps (4445 m). The overall high use of methane as a carbon source by both symbiont-bearing and heterotrophic fauna suggests that New Zealand methane seeps are an ideal model system to study the interaction among metazoans, bacteria, archaea, and their resulting effect on methane cycles.

WIDESPREAD CONTRIBUTION OF METHANE-CYCLE BACTERIA TO THE DIETS OF LAKE PROFUNDAL CHIRONOMID LARVAE

Reports of unexpectedly 13C-depleted chironomid larvae in lakes have led to an hypothesis that significant transfer of detrital organic matter to chironomid larvae may occur via methane-cycle bacteria. However, to date little is known of how such transfer might vary across species and lakes. We gathered data from 87 lakes to determine how widespread this phenomenon might be and to define boundaries for its likely magnitude. Carbon stable isotope values of chironomid larvae varied greatly between taxa. Very marked 13C-depletion was evident only in certain taxa, especially Chironomus plumosus, C. anthracinus, and C. tenuistylus, all characteristic of eutrophic or dystrophic lakes and known to be tolerant of low oxygen conditions. Furthermore, marked 13C-depletion was only found in larvae from lakes in which late-summer hypolimnetic oxygen depletion near the sediment surface was below an apparent threshold concentration of 2-4 mg O2/L. Similarly, application of a two-source mixing model suggested that methanotrophic bacteria made the greatest contribution to profundal chironomid growth (0-70% of larval carbon) when the late-summer oxygen concentration dropped below approximately 2 mg O2/L. Our study demonstrates that methane-derived carbon is an important, but often neglected, contribution to the flux of carbon through the food webs of many productive or dystrophic lakes.

Temperature-programmed Studies of Coke Resistant Ni Catalyst for Carbon Dioxide Reforming of Methane

Argon glow discharge plasma was applied for drying the impregnated Ni/SiO2 catalyst instead of the drying thermally. Such plasma treated catalyst significantly inhibits the coke formation from methane decomposition. The methane-derived carbon shows an improved reactivity against CO2. This can result in a better balance of coke formation and gasification by CO2 when the plasma-treated Ni/SiO2 catalyst is used for the CO2 reforming of methane.

Authigenic carbon entombed in methane-soaked sediments from the northeastern transform margin of the Guaymas Basin, Gulf of California

Extensive ROV-based sampling and exploration of the seafloor was conducted along an eroded transform-parallel fault scarp on the northeastern side of the Guaymas Basin in the Gulf of California to observe the nature of fluids venting from the seafloor, measure the record left by methane-venting on the carbonates from this area, and determine the association with gas hydrate. One gas vent vigorous enough to generate a water-column gas plume traceable for over 800 m above the seafloor was found to emanate from a ∼10-cm-wide orifice on the eroded scarp face. Sediment temperature measurements and topography on a sub-bottom reflector recorded in a transform-parallel seismic reflection profile identified a subsurface thermal anomaly beneath the gas vent. Active chemosynthetic biological communities (CBCs) and extensive authigenic carbonates that coalesce into distinct chemoherm structures were encountered elsewhere along the eroded transform-parallel scarp. The carbon isotopic composition of methane bubbles flowing vigorously from the gas vent (−53.6±0.8‰ PDB) is comparable to methane found in sediment cores taken within the CBCs distributed along the scarp (−51.9±8.1‰ PDB). However, the δ13C value of the CO2 in the vent gas (+12.4±1.1‰ PDB) is very distinct from those for dissolved inorganic carbon (DIC) (−35.8‰ to −2.9‰ PDB) found elsewhere along the scarp, including underneath CBCs. The δ13C values of the carbonate-rich sediments and rocks exposed on the seafloor today also span an unusually large range (−40.9‰ to +12.9‰ PDB) and suggest two distinct populations of authigenic carbonate materials were sampled. Unconsolidated sediments and some carbonate rocks, which have lithologic evidence for near-seafloor formation, have negative δ13C values, while carbonate rocks that clearly formed in the subsurface have positive δ13C values (up to +23.0‰) close to that measured for CO2 in the vent gas. There appears to be two carbon sources for the authigenic carbonates: (1) deeply-sourced, isotopically heavy CO2 (∼+12‰); and (2) isotopically light DIC derived from local anaerobic oxidation of methane at the sulfate–methane interface in the shallow subsurface. Addition of isotopically light methane-derived carbon at the seafloor may completely mask the isotopically heavy CO2 signature (+12.4‰) in the underlying sediments. Thus, the authigenic carbonates may have formed from the same methane- and carbon dioxide-bearing fluid, but under different migration and alteration conditions, depending on how it migrated through the sediment column.

Modeling CO 2 chemistry, δ 13C, and oxidation of organic carbon and methane in sediment porewater: Implications for paleo-proxies in benthic foraminifera

I present a numerical diffusion–advection-reaction model to simulate CO 2 chemistry, δ 13C, and oxidation of organic carbon and methane in sediment porewater. The model takes into account detailed reaction kinetics of dissolved CO 2 compounds, H 2O, H +, OH −, boron and sulfide compounds. These reactions are usually assumed to be in local equilibrium, which is shown to be a good approximation in most cases. The model also includes a diffusive boundary layer across which chemical species are transported between bottom water and the sediment–water interface. While chemical concentrations and δ 13 C TCO 2 at these locations are frequently assumed equal, I demonstrate that they can be quite different. In this case, shells of benthic foraminifera do not reflect the desired properties of bottom water, even for species living at the sediment–water interface ( z = 0 cm). Environmental conditions recorded in their shells are strongly influenced by processes occurring within the sediment. The model is then applied to settings in the Santa Barbara Basin and at Hydrate Ridge (Cascadia Margin), locations of strong organic carbon and methane oxidation. In contrast to earlier studies, I show that a limited contribution of methane-derived carbon to porewater TCO 2 in the Santa Barbara Basin cannot be ruled out. Simulation of methane venting shows that at oxidation rates greater than F CH 4 ∼ 50 μ mol cm - 2 y - 1 , the δ 13C of porewater TCO 2 at z > 1 cm is depleted by more than 15‰ relative to bottom water. Depletions of this magnitude have not been observed in living benthic foraminifera, even at methane vents with much higher oxidation rates. This suggests that foraminifera at these sites either calcify at very shallow sediment depth or during times when oxidation rates are much lower than ∼50 μmol cm −2 y −1.

Linking larval chironomids to methane: seasonal variation of the microbial methane cycle and chironomid δ13C

Methane-derived carbon has been shown to be an important carbon source for macroinvertebrates in several studies of lake ecosystems using stable isotopes. Furthermore, season and lake morphology appear to influence the importance of methane as a carbon source. However, rarely have the dynamics of the methane cycle been measured concurrently with the isotope signatures of chironomid larvae. We examined the methane dynamics in 2 lakes with contrasting mixing regimes (polymictic and dimictic), while monitoring corresponding changes in chironomid larval δ13C throughout an annual cycle. Both methane turnover rates and abundance of methane-oxidising bacteria were higher in the dimictic lake, where correspondingly lower mean larval δ13C values of –44.2 to –61.7‰ were recorded. In contrast, potential methane production and oxidation rates, as well as cell numbers of methane-oxidising bacteria were always lower in the polymictic lake; corresponding larval δ13C values ranged from –32.3 to –29.6‰. Furthermore, seasonal variation in larval δ13C was more pronounced in the dimictic lake (–50.1 ± 5.9‰) compared to the polymictic lake (–31.1 ± 1.2‰), reflecting the amplitude of turnover rates. This suggests strongly that lake characteristics have an influence on methane turnover rates and, in conjunction with season, affect the subsequent incorporation of methane-derived carbon into freshwater food webs via macroinvertebrates.

Possible sources of methane-derived carbon for chironomid larvae

In freshwater food webs, a substantial contribution of methane-derived carbon for the nutrition of larvae of the macroinvertebrate species Chironomus plumosus has been proposed from stable carbon isotope analysis. We investigated if the larvae feed on methanogenic Archaea or methane-oxidising bacteria (MOB), and whether the carbon transfer is based on digestion of the microbial biomass or on a symbiotic relationship. Fluorescence in situ hybridisation (FISH) with lar- val tissue thin sections showed that the larvae are not inhabited by methanogens or MOB as sym- bionts or part of their tissue and gut microflora. Furthermore, clone libraries for the bacterial 16S rRNA gene from larval tissue and comparative denaturing gradient gel electrophoresis (DGGE) of sediment and larval DNA samples showed that the eubacterial community present in the larval gut is dominated by anaerobic and facultatively anaerobic bacteria, whereas MOB were only rarely found. Additionally, comparative quantitative PCR assays for Eubacteria and Archaea from sediment samples, larval tissue and gut content revealed a strong enrichment of Eubacteria in the larval gut. Consequently, MOB and methanogens were not detected in amounts high enough to explain the larval δ 13 C signatures by direct feeding on their biomass. Thus, the microorganisms might be degraded too fast in the larval gut to be detected by the methods used, or the larvae feed indirectly on MOB- or methanogen-derived carbon, e.g. by feeding on meiofauna which itself is grazing on these microorganisms, or by feeding on organisms growing on MOB-derived carbon metabolites like methanol or carbon dioxide.

Methane‐derived carbon flows through methane‐oxidizing bacteria to higher trophic levels in aquatic systems

Recent investigations have shown that biogenic methane can be a carbon source for macro invertebrates in freshwater food webs. Stable carbon isotopic signatures, used to infer an organism's food source, indicated that methane can play a major role in the nutrition of chironomid larvae. However, the pathway of methane-derived carbon into invertebrate biomass is still not confirmed. It has been proposed that chironomid larvae ingest methane-oxidizing bacteria (MOB), but this has not been experimentally demonstrated to date. Using (13)C-labelled methane we could show for the first time that chironomid larvae assimilate methane-derived carbon through MOB. Chironomid larval biomass was significantly (13)C-enriched after dwelling for 10 days in lake sediment enriched with labelled methane. Moreover, phospholipid fatty acids diagnostic for MOB were detected in larval tissue and were significantly (13)C-enriched, which encompasses the (13)C-uptake predicted for a methane-based nutrition. Additionally, chironomid larvae fed on sediment and water-column derived MOB biomass.

Carbon stable isotopic composition of benthic foraminifera from Pliocene cold methane seeps, Cascadia accretionary margin

Cold methane seep carbonates preserve the distinctive carbon stable isotope signatures of their ambient porewaters, thus facilitating the identification of methane sources and fluid migration pathways. This study employs an isotopic indicator from microfossils in a diffuse fossil seep for the recognition and characterization of methane seepage in the rock record. Benthic foraminifera from the Pliocene Quinault Formation, western Washington State, are plentiful and provide a temporal link between modern and more ancient seeps. Stable carbon isotope analyses of individual foraminiferal tests revealed a clear distinction between seep and non-seep foraminifera. Infaunal species Globobulimina pacifica and Nonion basispinatum collected from seep samples exhibited δ 13C values that were highly variable and often strongly depleted: values ranged from − 33.1‰ to − 55.3‰ PDB in G. pacifica and from + 2.1‰ to − 42.9‰ PDB in N. basispinatum. The epifaunal species Cibicidoides mckannai, though less depleted, also showed considerable variability with δ 13C values ranging from + 1.7‰ to − 3.1‰ PDB. In contrast, non-seep specimens were much more enriched and less variable: non-seep specimens of N. basispinatum displayed δ 13C values clustering between − 0.5‰ and − 1.5‰ PDB, while non-seep specimens of C. mckannai yielded a range of + 1.1‰ to + 1.8‰ PDB. SEM analyses and oxygen isotope values of foraminiferal tests indicated minimal diagenetic alteration. δ 13C values from authigenic blebs (small nodules with wispy, indistinct boundaries) ranged from + 8.7‰ to − 38.4‰ PDB. By combining these inorganic carbonate data with the foraminiferal values, the association of seep-specific bivalves, and the present-day leakage of thermogenic methane from the nearby Garfield gas mound, we conclude that these foraminifera precipitated their tests in the presence of methane-derived carbon. Thus they record pore-water 13C depletion and syn-depositional fluid-flow pathways near the sediment/water interface in a seep system active 3.4 mya and continuing onshore and offshore today. The range and degree of 13C depletion in the Pliocene microfossils is due to the heterogeneity of seepage caused by different methane sources (i.e. thermogenic and biogenic), the tortuosity of fluid migration pathways reaching the seafloor sediments, and mixing of seep fluids with dissolved inorganic carbon (DIC) and marine organic carbon.

Experimental d13C evidence for a contribution of methane to pelagic food webs in lakes

We tested the hypothesis that low stable carbon isotope (δ13C) values commonly observed for zooplankton in humic lakes are due to their feeding on isotopically light methane‐oxidizing microbes, and thus that methane‐derived carbon is important in the food webs of these lakes. In replicate laboratory cultures, Daphnia longispina, a common crustacean zooplankter in humic lakes, were fed microbial suspensions with or without enrichment by biogenic methane. The δ13C values of Daphnia indicated consumption of 13C‐depleted methanotrophic bacteria, while growth rates, survival, and reproduction of Daphnia in cultures enriched with methane were equal to or greater than those in nonenriched cultures. Results from lake enclosures during the autumn overturn period revealed a decrease in δ13C of adult Daphnia from ‐40.5% to ‐50.3%, reflecting extensive consumption of 13C‐depleted methanotrophic bacteria. Methane‐derived carbon is a more important contribution to carbon flux through lake pelagic food webs than has previously been suspected.

Meiobenthos at the Arctic Håkon Mosby Mud Volcano, with a parental-caring nematode thriving in sulphide-rich sediments

Hakon Mosby Mud Volcano (HMMV, SW Barents Sea slope, 1280 m) is one of the numerous cold methane-venting seeps existing along the continental margins. Analyses of video- guided core samples revealed extreme differences in the diversity and density of the metazoan meiobenthic communities associated with the different sub-habitats (centre, microbial mats, Pogonophora field, outer rim) of this mud volcano. Diversity was lowest in the sulphidic, microbial mat sediments that supported the highest standing stock, with unusually high densities (11 000 ind. 10 cm -2 ) of 1 nematode species related to Geomonhystera disjuncta. Stable carbon isotope analyses revealed that this nematode species was thriving on chemosynthetically derived food sources in these sediments. Ovoviviparous reproduction has been identified as an important adaptation of parents securing the survival and development of their brood in this toxic environment. The proliferation of this single species in exclusive association with free-living, sulphide-oxidising bacteria (Beggiatoa) indicates that its dominance is strongly related to trophic specialisation, evidently uncommon among the meiofauna. This chemoautotrophic association was replaced by copepods in the bare, sulphide- free sediments of the volcano's centre, dominated by aerobic methane oxidation as the chemosyn- thetic process. Copepods and nauplii reached maximum densities and dominance in the volcano's centre (500 ind. 10 cm -2 ). Their strongly depleted carbon isotope signatures indicated a trophic link with methane-derived carbon. This proliferation of only selected meiobenthic species supported by chemosynthetically derived carbon suggests that, in addition to the sediment geochemistry, the asso- ciated reduced meiobenthic diversity may equally be related to the trophic resource specificity in HMMV sub-habitats.

Archaeal and bacterial lipids in authigenic carbonate crusts from eastern Mediterranean mud volcanoes

We investigated the distributions and δ 13C values of lipid biomarkers in authigenic methane-related carbonate crusts formed on eastern Mediterranean mud volcanoes. A highly diverse suite of microbial membrane lipids was detected. Strongly 13C-depleted lipids diagnostic for archaea include isoprenoid glycerol diethers, isoprenoid glycerol dialkyl glycerol tetraethers and several C 20–C 35 isoprenoid hydrocarbons. Bacterial lipids mainly consist of dialkyl glycerol diethers, branched fatty acids and hopanoids, showing important 13C-depletions, albeit less than archaeal lipids. The data provide further evidence for the involvement of archaea and bacteria in the anaerobic oxidation of methane (AOM) and in the sequestration of methane-derived carbon in authigenic carbonate crusts. Varying distributions and isotopic compositions of 13C-depleted lipids argue for heterogeneous AOM-related microbial consortia, expanding on existing work at cold seeps. Distinctive biomarker features – and inferred microbial assemblages – are observed in these crusts, in agreement with available phylogenetic data for the samples. Our results support the potential of lipids for distinguishing methanotrophic communities. Despite the observed variances, conservative features in the abundances and isotopic offsets of microbial biomarkers occur, likely reflecting the overall relationships between archaea and bacteria and the nature of carbon flow between them.

In Vitro Study of Lipid Biosynthesis in an Anaerobically Methane-Oxidizing Microbial Mat

The anaerobic oxidation of methane (AOM) is a key process in the global methane cycle, and the majority of methane formed in marine sediments is oxidized in this way. Here we present results of an in vitro 13CH4 labeling study (delta13CH4, approximately 5,400 per thousand) in which microorganisms that perform AOM in a microbial mat from the Black Sea were used. During 316 days of incubation, the 13C uptake into the mat biomass increased steadily, and there were remarkable differences for individual bacterial and archaeal lipid compounds. The greatest shifts were observed for bacterial fatty acids (e.g., hexadec-11-enoic acid [16:1Delta11]; difference between the delta13C at the start and the end of the experiment [Deltadelta13C(start-end)], approximately 160 per thousand). In contrast, bacterial glycerol diethers exhibited only slight changes in delta13C (Deltadelta13C(start-end), approximately 10 per thousand). Differences were also found for individual archaeal lipids. Relatively high uptake of methane-derived carbon was observed for archaeol (Deltadelta13C(start-end), approximately 25 per thousand), a monounsaturated archaeol, and biphytanes, whereas for sn-2-hydroxyarchaeol there was considerably less change in the delta13C (Deltadelta13C(start-end), approximately 2 per thousand). Moreover, an increase in the uptake of 13C for compounds with a higher number of double bonds within a suite of polyunsaturated 2,6,10,15,19-pentamethyleicosenes indicated that in methanotrophic archaea there is a biosynthetic pathway similar to that proposed for methanogenic archaea. The presence of group-specific biomarkers (for ANME-1 and ANME-2 associations) and the observation that there were differences in 13C uptake into specific lipid compounds confirmed that multiple phylogenetically distinct microorganisms participate to various extents in biomass formation linked to AOM. However, the greater 13C uptake into the lipids of the sulfate-reducing bacteria (SRB) than into the lipids of archaea supports the hypothesis that there is autotrophic growth of SRB on small methane-derived carbon compounds supplied by the methane oxidizers.

Depleted methane-derived carbon in waters of Lake Baikal, Siberia

Results of hydrochemical and stable isotope measurements during the ice-breaking period on Lake Baikal indicate an apparent lack of relationship between measured δ13C of dissolved inorganic carbon (DIC) and phytoplankton below the trophogenic layer. While planktonic values of −31.7 to −33.5‰ are within a typical lacustrine range, the δ13C values of DIC turned out to be very negative, from −28.9 to −35.6‰. These isotopic values of DIC appear to be associated with oxidation of methane that accumulated during winter ice cover period. At the time of sampling, however, the observed depletion did not affect the phytoplankton/DIC fractionation relationship, because the difference between phytoplankton and DIC (−20 to −22‰ in surface waters) lies within the expected range of the fractionation coefficient. By analogy with small lakes, we explain this lack of relationship by the time lag between peak productivity and peak methane oxidation. Our interpretation of the Baikal DIC isotopic signature is consistent with methanogenesis in bottom sediments and with the known presence of widespread unstable gas hydrates and active methane seeps on the lake floor. Our findings suggest that methane is an important component of the Baikal carbon cycle, that late winter concentrations of methane in Baikal under ice may be 3–4 orders of magnitude higher than previously reported values for summer, and that the lake may be emitting a significant amount of methane to the atmosphere.

Lipid biomarkers preserved in hydrate-associated authigenic carbonate rocks of the Gulf of Mexico

Anaerobic oxidation of methane (AOM) is common in ocean-margin sediments, where it is mediated by consortia of Archaea and Bacteria and can result in the formation of authigenic carbonate, including extensive carbonate crusts. Previous work indicates that AOM is associated with Gulf of Mexico hydrocarbon seeps and is mediated by similar organisms as identified in other settings; however, biological investigations have not been done on the associated 13C-depleted carbonates. Here, we show that 13C-depleted archaeal and bacterial biomarkers are abundant in Gulf of Mexico authigenic carbonate rocks, revealing that AOM-mediating organisms are closely associated with carbonate authigenesis. Moreover, the rocks share general characteristics of the background (soft) sedimentary archaeal and bacterial community inferred from biomarker analysis, suggesting that the organisms associated with carbonate authigenesis are the same as those that live elsewhere in the hydrocarbon seep environment. This provides further evidence that AOM by Archaea and sulfate-reducing bacteria can result in the sequestration of significant quantities of methane-derived carbon in carbonate rocks.