Anaerobic methane oxidation associated with marine ... - Springer Link

Naturwissenschaften 86, 295–300 (1999)

Springer-Verlag 1999

Anaerobic methane oxidation associated with marine gas hydrates: superlight C-isotopes from saturated and unsaturated C20 and C25 irregular isoprenoids Marcus Elvert, Erwin Suess “Dynamics of Global Cycles within the System Earth,” University of Kiel, and GEOMAR, Research Center for Marine Geosciences, Wischhofstrasse 1–3, D-24148 Kiel, Germany Michael J. Whiticar School of Earth and Ocean Sciences, University of Victoria, B.C. V8 W 2Y2, Canada Received: 12 October 1998 / Accepted in revised form: 27 January 1999

The activity of anaerobic methaneoxidizing microbes is revealed by extremely light C-isotope values of saturated and unsaturated C20 and C25 isoprenoids (e.g., crocetane, 2,6,10,15,19-pentamethyleicosane) in reducing sediments of the Hydrate Ridge of the Cascadia continental margin. The 13C values were up to –123.8% vs. Pee Dee Belemnite; they represent the isotopically lightest carbon so far isolated from the marine environment. At Hydrate Ridge destabilization of gas hydrate generates an enormous methane plume in the lower water column and saturates the sediments with methane. Based on isotope evidence of the biomarkers we conclude that methanogenic archaebacteria in a syntrophic consortium with sulfate reducers may be able to switch their metabolism from one of methane formation to another, favoring methane consumption. Methane hydrate destabilization at Hydrate Ridge and globally at cold seeps at all convergent margins supplies sufficiently high amounts of free methane to favor net methane oxidation under anaerobic conditions. Correspondence to: M. Elvert (e-mail: melvert6geomar.de, Tel.: c49-431-6001407, Fax: c49-431-6001400) All experiments presented here complied with the current laws of Germany and Canada.

Methanotrophic processes are widespread in nature and play a significant role in the global biogeochemical cycle of methane (King 1992; Hovland et al. 1993; Reeburgh et al. 1993). Methane-oxidizing bacteria typically occur at the aerobic/anaerobic interface in terrestrial environments such as lake sediments, rice paddies, and water-saturated peat lands (Sundh et al. 1994; Gilbert and Frenzel 1995). In contrast, there is strong evidence in marine environments for anaerobic methane oxidation, evidence is indirect and based largely on geochemical data (Iversen and Jørgensen 1985; Suess and Whiticar 1989). Neither has the mechanism been elucidated in detail, nor has the organism isolated. Several authors propose “reverse methanogenesis”, a process by which a syntrophic consortium of methanogenic bacteria and sulfate reducers is responsible for the net oxidation of methane (Hoehler et al. 1994; Hansen et al. 1998). Here we report new data which support this hypothesis: significant 12C enrichment in irregular C20 and C25 isoprenoid lipids of anaerobic sediments from a methane-sulfide gas hydrate environment of the Cascadia continental margin. To our knowledge, these are the most extreme Cisotope values so far observed in the marine environment. Gas hydrate deposits are quite common and well-known from accreted sediments at plate collision zones

Naturwissenschaften 86 (1999) Q Springer-Verlag 1999

such as the Cascadia margin and from sedimentary sequences at passive margins throughout the ocean basins (Hyndman et al. 1992; Kvenvolden et al. 1993). Hydrates typically occur tens to hundreds of meters below the sea floor, depending on methane availability, pressure, and temperature imposed by water depth, and the sub-sea floor geothermal regime. Only a few areas have been reported associated with gas vents or oil seeps near the sediment surface (e.g., Brooks et al. 1991; Ginsburg et al. 1993; MacDonald et al. 1994). At the northern summit of Hydrate Ridge (585 m water depth) rising bubbles from the decomposition of gas hydrates generate an enormous methane plume in the lower water column with methane concentrations of up to 50,000 nl/l (Suess and Bohrmann 1997). Video surveys show prolific vent communities and authigenic carbonates associated with faults that act as fluid conduits, with warm fluids flowing upwards (Westbrook et al. 1994). Carbon isotope values of these carbonates are between –35‰ and –55‰ relative to the Pee Dee Belemnite standard (PDB) and clearly identify methane as the dominant carbon source (Greinert 1998). Microbial incorporation of methanederived carbon into cellular biomass enriches 13C in the residual methane and 12C in the synthesized biomass. Specific biomarkers in sediments and petroleum contain 12C-enriched compounds of up to –90‰ vs. PDB (Hayes et al. 1987; Freeman et al. 1990; Collister et al. 1992). This depletion in d 13C implicates a source of these compounds from organisms which utilize methane, since it is the only natural carbon substrate commonly found with such light C-isotope values. At Hydrate Ridge there are abundant irregular saturated and unsaturated C20 and C25 isoprenoid biomarkers which are highly depleted in 13C of up to –123.8‰ PDB (Table 1, Fig. 1). In contrast, biomarkers derived from algal sources such as the isoprenoid pristane show isotope values of –27.5‰. The isotopic composition of individual compounds was determined by continuous flow–isotope ratio mass spectrometry (CF-IRMS, Finnigan MAT 252), as described pre295

Fig. 1. Structures of identified regular and irregular isoprenoids isolated from Hydrate Ridge sediments

viously (Hayes et al. 1990; Merritt and Hayes 1994). The mass spectrometer was connected to a Varian 3300 gas chromatograph equipped with 30 m DB-1 column with 0.25 mm internal diameter and a stationary cross linked methyl silicone phase of 0.25 mm. The carrier gas was He at a flow rate of 1.5 ml/min. The samples (dissolved in hexane) were on-column injected at 607 C, and after 1 min the oven was programmed to 3007 C at 2.57 C/min and then kept at 3007 C for 20 min. All values reported were determined by duplicate analysis and the results averaged. Identification of irregular isoprenoids was carried out by gas chromatography–mass spectrometry (GC-MS). Compounds such as crocetane (2,6,11,15-tetramethylhexadecane), an isomer of phytane (2,6,10,14-tetramethylhexadecane), 2,6,10,15,19-pentamethyleicosane (PME), or unsaturated C25 isoprenoids were identified (Figs. 2, 3). Other isoprenoids such as squalene or archaeal ether-linked C40 isoprenoids were absent in all of our samples. For crocetane, only minor coelution with phytane was observed. The difference in the mass spectrum of the regular (head-to-tail) isoprenoid phytane and irregular (tail-totail) isoprenoid crocetane is the much higher abundance of the m/z 169 fragment in the mass spectrum of crocetane while phytane has a characteristic fragment of m/z 183. Although crocetane has been known for over 60 years, it has only recently been recognized in recent marine sediments because coelution with phytane tended to obscure its presence (Bian 1994). The isotope data suggest that crocetane is natural and possibly a biomarker for methane-oxidizing bacteria (see Table 1). On the other hand, PME (characteristic fragment 296

m/z 239) is a well-known C25 isoprenoid believed to be synthesized by methanogenic archaebacteria (Holzer et al. 1979; Tornabene et al. 1979; Risatti et al. 1984). Also, although PME has been reported to originate at least partially from algal sources based on its isotope signature (Kohnen et al. 1992; Freeman et al. 1994), our C-isotope data clearly suggest that under certain conditions this compound may partake in methanotrophic bacterial activity. The cluster of unsaturated C25 isoprenoids in the hydrocarbon fraction of hydrated sediments from the sampling site contains compounds with one to five double bonds. Such unsaturated isoprenoids were previously isolated from various strains of methanogenic archaea such as Methanosarcina mazei and Methanolobus bombayensis and determined by mass spectral analysis (Schouten et al. 1997). Additionally, 1H and 13C magnetic resonance spectroscopy analysis has identified the most present unsaturated C25 isoprenoid in M. mazei as 2,6,10,15,19-pentamethyleicosa2,6,14,18-tetraene (Sinninghe Damsté et al. 1997). This compound has mass

spectral data identical to one of our unsaturated isoprenoids (PME:4) extracted from hydrated sediments which shows a very light C-isotope value of –107.3‰. Higher amounts of unsaturated C25 isoprenoids have been recognized only at TVG site 431. All other samples yielded minor or no detectable amounts of these molecules which made them unsuitable for isotopic investigations (see Fig. 2). The similarity of stable isotopic compositions of irregular C20 and C25 isoprenoids from anaerobic sediments at all locations indicate a common source for these compounds (Table 1). These biomarkers are possible end members of methanotrophic activity. They reflect not only the isotopic composition of the abundant methane pool from gas hydrates but also the isotope effect accompanying methane consumption with a depletion in 13C of approximately 50‰ relative to free gaseous methane (d 13CCH4 ranged from –62 to –72‰). While for crocetane there is no known source in the marine environment other than methanotrophy, for PME and its unsaturated counterparts until now mostly methanogenic archaebacteria were thought to be the source. Our isotope values in 13C for PME and PME:4 of up to –123.8‰ prove that these biomarkers participate in methanotrophic processes as well. From these evidences we conclude that methanogenic archaebacteria may be able to switch from a metabolism of methane formation to one favoring methane consumption. This possibility was recently confirmed by 16 S rRNA analysis of

Table 1. Stable carbon isotope values of irregular isoprenoid biomarkers from anaerobic sediments of Hydrate Ridge associated with methane oxidation at coring sites TVG 41-1, TVG 43-1, and TVG 43-2 Sample

Pr

Cr

PME

PME:4

SO109-1 TVG 41-1 SO109-1 TVG 41-2 SO109-1 TVG 41-3

P27.5 P27.7 P27.6

P107.6 P117.9 P113.8

P101.7 P123.8 P112.6

n.d. P107.3 a n.d.

The isoprenoid pristane is shown as reference for biomarkers of algal origin. Isotope values are reported in the notation vs. PDB. An internal standard (C36) with a known d 13C value was used to determine the isotopic composition of individual hydrocarbons. Additionally 3-methylnonadecane and 5b(H)-cholane were used as internal standards for monitoring reproducibility and precision; standard deviation and isotope values of ten replicated runs were P27.0B0.2‰ and P18.9B0.3‰, respectively. n.d., Not detected a13 C-enriched isotope value because of coelution with other polyunsaturated isoprenoids Naturwissenschaften 86 (1999) Q Springer-Verlag 1999

Fig. 2. GC-MS traces of hydrocarbon fractions obtained from anaerobic sediments at coring sites TVG 43-1, TVG 43-2, and TVG 41-1. Hydrocarbon lipids were extracted from the wet sediment ultrasonically using dichloromethane-methanol 2 : 1 (v/v, 3!) and separated by medium pressure liquid chromatography on silica gel with hexane as eluent. Elemental sulfur was removed by passing the fractions over activated copper. The GCMS was operated at 70 eV with a mass range of m/z 40–600 and a cycle time of 0.9 s (resolution 1000) using He as carrier gas; samples were injected in splitless mode (injector temperature: 2857 C) and separation of hydrocarbons was achieved on a 30 m apolar DB-1 fused silica capillary column (0.25 mm i.d., film thickness; J&W Scientific); after 2 min hold time at 507 C the oven was programmed at 57 C/min to 3007 C, at which it was held for 15 min. *Strong even over odd predominance for n-alkanes in this part of the chromatogram at coring site TVG 43-1 is probably due to contamination of the sample


297

Fig. 3. Mass spectra of irregular isoprenoid biomarkers such as crocetane, PME and PME:4 (mass spectra identical to PME-2,6,14,18-te-

anaerobic sediments of the Eel River Basin on methanogenic archaea with similar depleted isoprenoid compounds (K.-U. Hinrichs, personal communication). The anaerobic oxidation of methane may be accomplished via a reversal pathway of CO2 reduction (i.e., “reverse methanogenesis”) or an up to now unrecognized nonenzymatic route. Thermodynamically a reversal pathway of CO2 reduction is quite feasible using water as electron acceptor at very low hydrogen and CO2, and at very high methane concentrations (Zehnder and Brock 1979; Harder J (1997). These conditions are met in Hydrate Ridge sediments because of CO2 removal by carbonate precipitation and low hydrogen concentrations caused by sulfate reducers. It appears very likely that hydrate destabilization, such as at Hydrate Ridge of the Cascadia convergent margin, supplies free methane sufficiently to exceed a threshold beyond which net methane oxidation is favored. The special gas hydrate environment which provides the necessary conditions for anaerobic methane oxidation also contains abundant authigenic carbonates with extreme 13C depletion. Therefore in the absence of gas 298

traene; Schouten et al. 1997; Sinninghe Damsté et al. 1997) associated with methanotrophic processes at coring site TVG 43-1

Fig. 4. Gas chromatogram of the hydrocarbon fraction isolated from extracted residue of cemented Mg-calcite sediment from SO97 TVG 97. The quantity of 25 g of previously cleaned Mg-calcite (washed with acetone) was dissolved by adding stepwise 500 ml of 1 N HCl and stirring for 6 h. After centrifugation, the residue was washed twice with preextracted water and the hydrocarbons were extracted and isolated as previously described. GC analysis was performed on a 30 m apolar DB-5 fused silica capillary column (0.25 mm i.d., film thickness; J&W Scientific) using a Carlo Erba 5160 gas chromatograph equipped with an on-column injector and a flame ionization detector. The samples were dissolved in hexane and injected at 607 C. After a 1-min hold time at 607 C the oven was programmed to 1407 C at 107 C/min, then to 3107 C at 57 C/min, and finally kept at 3107 C for 22 min. The carrier gas was H2 with a flow rate of 2.5 ml/min. IS, Internal standards [3-methylnonadecane, 2-methyleicosane, 5b(H)-cholane]


hydrates or if hydrate destabilization and CH4 emission have ceased, the carbonates may retain evidence of organomineralization and methanotrophic biomarkers. First results from a Mg calcite retrieved from a vent site at the Aleutian trench (water depth 4860 m) show that the gas chromatogram is dominated by crocetane and other compounds which might be unsaturated C20 isoprenoids (Fig. 4). Neither GC-MS nor carbon isotopic investigations have been undertaken yet. Small amounts of PME and PME:4 were detected in the carbonate sample. All our evidence points to anaerobic methane oxidation carried out by methanogenic archaebacteria in a syntrophic consortium with sulfate reducers at convergent tectonic settings with special deep sea ecosystems. However, there is still a more exciting possibility for the oxidation of methane that might be accomplished by a single as yet undetected bacterium. We thank Magnus Eek and Paul Eby for their invaluable help concerning the operation of GC-MS and CFIRMS, and other technical assistance at the School of Earth and Ocean Sciences, Victoria, Canada. We thank J. Greinert for providing one of his Mg calcites from SO97, and D. Schulz-Bull and J. Maaßen for providing laboratory space and time at the Sonderforschungsbereich 313 in Kiel. Financial support was provided by the Federal Ministry of Education, Research, and Technology (Bonn, Germany, grant 03G0109B).

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European Energy Policies in a Changing Environment. Hrsg. von Francis McGowan, Energy Economics and Policy. Bd. 1, Heidelberg; PhysicaVerlag, 1996, 28 Tabellen, 183 S., DM 75, ISBN 3–7908–0951–9. Long-Term Integration of Renewable Energy Sources into the European Energy System. Hrsg. von der LTIResearch Group. Heidelberg; Physica-Verlag, 1998, 66 Abb., 268 S., DM 90,-, ISBN 3–7908–1104–1. Nachhaltige Entwicklung im Energiesektor? Erste deutsche Branchenanalyse zum Leitbild von Rio. Hrsg. von der Prognos AG. Heidelberg; Physica-Verlag, 1998, 28 Abb., 10 Tabellen, 357 S., DM, ISBN 3–7908–1104–1. Diese drei Bücher thematisieren die Entwicklung und Erklärung der Energiepolitiken der EU und ihrer vier großen Mitgliedsstaaten Frankreich, Deutschland, Italien und Großbritannien in der Nachkriegszeit; sie bieten einen Ausblick auf die langfristige Integration erneuerbarer Energiequellen bis zum Jahr 2050 und untersuchen in einer Branchenanalyse die Rückwirkungen des Konzepts der Nachhaltigkeit auf den deutschen Energiesektor. Die ersten beiden Auftragsstudien wurden von der EUKommission und der dritte Band vom Bundesministerium für Wirtschaft gefördert. Während der erste Band von einer Gruppe von Energiepolitikexperten des European Network for Energy Research (ENER) an den Universitä-

ten von Sussex, Grenoble, Mailand und vom FhG-ISI verfaßt wurde, waren in der LTI-Research Group neun Institutionen beteiligt: das ZEW, Mannheim, das CIRED, Paris, das Polytechnikum Mons, die Univ. Roskilde und das Wuppertal-Institut sowie das FhG-ISI, die EUREC, Novator und die Univ. Aalborg. Autoren des dritten Bandes sind Peter Hofer, Janina Scheelhaase und Heimfrid Wolff vom Europäischen Zentrum für Wirtschaftsforschung und Strategieberatung der Prognos AG in Basel. Francis McGowan arbeitet in ihrem einleitenden Kapitel zur Energiepolitik in der EU die unterschiedlichen energiewirtschaftlichen Rahmenbedingungen heraus, die sich zwischen 1960 und 1993 in allen Staaten unterschiedlich veränderten. Die Energieselbstversorgungsrate stieg in Großbritannien überdurchschnittlich von 72% auf 101%, in Dänemark von 11% auf 69%, in den Niederlanden von 50% auf 97%, in Griechenland von 15% auf 38%, in Schweden von 36% auf 42%, während sie in Österreich von 71% auf 35%, in Belgien von 60% auf 23%, in Deutschland von 88% auf 44%, in Italien von 45% auf 18%, in Portugal von 50% auf 11% und in Spanien von 66% auf 33% fiel. Diese wachsende Energieimportabhängigkeit wurde damit zugleich zu einer zentralen Determinante der nationalen Energiepolitiken. In der Kernenergie gibt es sowohl hin-

sichtlich der Relevanz und Akzeptanz wichtige nationale Unterschiede innerhalb der EU. Bei der Privatisierung und Liberalisierung übernahm Großbritannien die Führung, während andere Staaten hinterher hinkten. In zwei der Gründungsverträge der EU, der EGKS und EURATOM, stand die Energiepolitik zwar im Zentrum, aber weder im Vertrag von Maastricht noch von Amsterdam ist diesem Politikbereich ein eigenes Kapitel gewidmet. Dennoch hat die Europäische Kommission seit Mitte der 1980er Jahre eine wachsende Rolle bei energiepolitischen Entscheidungen u.a. durch die Vorlage von Grünund Weißbüchern und den Erlaß von Richtlinien und Verordnungen erlangt: z.B. bei der Marktliberalisierung (Netze für Elektrizität und Gas), beim Umweltschutz und bei der Forschung für erneuerbare Energien. Nicht zuletzt aber wegen der zahlreichen Unterschiede bleibt die Energiepolitik – so die These der Autorin – eine nationale Angelegenheit, wenngleich die EU den nationalen Handlungsspielraum im Energiesektor zunehmend beschneidet. Die folgenden vier nationalen Fallstudien von Dominique Finon zu Frankreich, von Eberhard Jochem, Edelgard Gruber und Wilhelm Mannsbart zu Deutschland, von Luigi de Paoli zu Italien und von Francis McGowan zu Großbritannien arbeiten die unterschiedlichen Ausgangspunkte, Reak-

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