Biomarker Evidence for Widespread Anaerobic Methane Oxidation in Mediterranean Sediments by a Consortium of Methanogenic Archaea and Bacteria

Abstract

Although abundant geochemical data indicate that anaerobic methane oxidation occurs in marine sediments, the linkage to specific microorganisms remains unclear. In order to examine processes of methane consumption and oxidation, sediment samples from mud volcanoes at two distinct sites on the Mediterranean Ridge were collected via the submersible Nautile. Geochemical data strongly indicate that methane is oxidized under anaerobic conditions, and compound-specific carbon isotope analyses indicate that this reaction is facilitated by a consortium of archaea and bacteria. Specifically, these methane-rich sediments contain high abundances of methanogen-specific biomarkers that are significantly depleted in 13 C (␦ 13 C values are as low as ؊95‰). Biomarkers inferred to derive from sulfate-reducing bacteria and other heterotrophic bacteria are similarly depleted. Consistent with previous work, such depletion can be explained by consumption of 13 Cdepleted methane by methanogens operating in reverse and as part a consortium of organisms in which sulfate serves as the terminal electron acceptor. Moreover, our results indicate that this process is widespread in Mediterranean mud volcanoes and in some localized settings is the predominant microbiological process.Methane can have a stronger greenhouse effect than CO 2 , and recent work has highlighted its potential climatic impact on glacial timescales (24) and in relation to major geologic events (8,12). Consequently, controls on methane production and consumption are important concerns in the evaluation of past and future climate change. In marine sediments, anaerobic methane oxidation could be the dominant pathway for methane consumption (2,4,6,15,25,26), but the organisms involved have not been isolated and the mechanism remains controversial. Methanogenic archaea operating in reverse (9,11,14) or novel, previously uncharacterized archaea (13) have been proposed to play a vital role, but current evidence remains ambiguous.Methane tends to be highly depleted in 13 C, and organisms that consume methane either directly or indirectly via heterotrophic consumption of methanotroph biomass will be similarly depleted in 13 C. For this study, we determined distributions and carbon isotope abundances of organic components in methane-rich mud volcano sediments of the Eastern Mediterranean Ridge. In particular, we examined the ␦ 13 C values of compounds derived from specific organisms (i.e., biomarkers) presumed to play a major role in methane oxidation, including methanogens, aerobic methanotrophs, sulfate-reducing bacteria, and anaerobic heterotrophic bacteria. From these results, we evaluate the controls on and microorganisms responsible for methane oxidation in these sediments and the conditions under which methane oxidation occurs. MATERIALS AND METHODSSamples. Using a bordeaux core, samples were collected from surface sediments (upper 30 cm) of several mud volcano flows (mud breccias) in the Olimpi (Milano and Napoli mud volcanoes) and Anaximander (Amsterdam mud volcano) fi...

Archaeal lipids in Mediterranean cold seeps: molecular proxies for anaerobic methane oxidation

“…Hoehler et al (1994) argued on the basis of inhibition studies that the Archaea are methanogens that operate in reverse under certain conditions. In general, Elvert et al (1999) agreed with this idea, and Pancost et al (2000) and Thiel et al (1999) provided further support for the Hoehler et al (1994) proposal by showing that methane-consuming Archaea co-existed with sulfate reducers and other bacteria in cold seep sediments. In contrast, phylogenetic analyses revealed that Archaea present in California Margin sediments consist of an unidentified Methanosarcina species and apparently two new orders of Archaea (Hinrichs et al, 1999).…”

“…For example, Hinrichs et al (1999) and Pancost et al (2000) observed abundant hydroxyarchaeol in some samples, but Elvert et al (1999) and Thiel et al (1999) did not. Based on comparison of their data with that of Elvert et al (1999), Hinrichs et al (1999) suggested that at least two archaeal assemblages could be responsible for anaerobic methane oxidation in different settings.…”

“…This is particularly useful when coupled to compound-specific carbon isotope analyses. Because methane tends to be highly depleted in 13 C, the carbon isotopic composition of archaeal lipids can be used to examine carbon flow in such communities and further evaluate the ecology of the Archaea from which such lipids are derived (Hinrichs et al, 1999;Elvert et al, 1999;Pancost et al, 2000). Archaea are distinguished from the Bacteria and Eukarya partly because they contain ether-bound membrane lipids with isoprenoidal carbon skeletons rather than ester-linked alkyl lipids (Woese et al, 1990).…”

“…Archaea are distinguished from the Bacteria and Eukarya partly because they contain ether-bound membrane lipids with isoprenoidal carbon skeletons rather than ester-linked alkyl lipids (Woese et al, 1990). Such isoprenoid ether lipids or their degradation products are present in diverse environments (Teixidor et al, 1993;Pauly and van Fleet, 1986;Hoefs et al, 1997;DeLong et al, 1998;Schouten et al, 1998) and been shown to be useful in studying archaeal communities in methane seeps (Hinrichs et al, 1999;Pancost et al, 2000). Additional diagnostic compounds for the Archaea include pentamethylicosane (PMI; Brassell et al, 1981;Schouten et al, 1997) and probably crocetane (Elvert et al, 1999) and the corresponding unsaturated components (Sinninghe Damste et al, 1998).…”

Hydrogen production by methanogens under low-hydrogen conditions

“…Initial conditions were as follows: 37°C, pH 7, inoculum 205 ml, gas flow 20 ml min -1 (at 0°C), gas composition 5.2×10 4 Pa CO 2 , 5.2×10 4 Pa CH 4 . Polysulfides (0.5 ml, 1 M) were added as indicated tium of archaea and sulfate-reducing bacteria (Boetius et al 2000;Elvert et al 1999;Hinrichs et al 1999;Hinrichs et al 2000;Hoehler and Alperin 1996;Hoehler et al 1994;Pancost et al 2000;Thiel et al 1999;. Hoehler et al (1994) outlined a consortium hypothesis to explain anaerobic methane oxidation, in which they postulated that methanogens operate in reverse to consume methane and produce H 2 when ambient H 2 is held low by sulfate-reducing bacteria.…”