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| Titel |
Evidence for microbial methane oxidation at cold seeps along the main active fault in the Marmara Sea |
| VerfasserIn |
Nicolas Chevalier, Daniel Birgel, Purificacion Lopez-Garcia, Marie-Helene Taphanel, Ioanna Bouloubassi |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040614
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| Zusammenfassung |
| The North Anatolian Fault in the Marmara Sea is a spectacular example of a seismically
active fault where, in recent years, numerous sites of active fluid venting have been
discovered and explored. During the MARNAUT cruise (2007), multidisciplinary sampling
was carried out with the Nautile submersible in order to investigate biogeochemical and
hydrogeological processes taking place at these newly discovered cold seeps.
We have studied short sediment cores (< 20 cm) and authigenic carbonate crusts
retrieved with the Nautile submersible from sub-basins of the Marmara Sea, aiming at
gaining insight into microbial processes and assemblages in this recently discovered
methane-rich setting and at comparing it with previously studied cold seeps. To do so, we
investigated diagnostic microbial lipids and their carbon isotope composition, and, in
selected sediment samples, we carried out cloning and sequencing of 16S rRNA
genes.
The sediment core retrieved from the southern slope of the Çinarcik Basin, in an area of
black patches, bacterial mats and polychaetes, contained abundant and strongly
13C-depleted archaeal and bacterial lipids. Archaeal lipids consisted mainly of archaeol,
sn-2-hydroxyarchaeol, crocetane, and unsaturated PMIs, and showed δ13C values as low as
–125 per mille. Concurrently, bacterial lipids (e.g. cyclopropyl-C17:0, C16:1Ï5, i-/ai-C15:0,
and non-isoprenoidal glycerol monoethers), previously assigned to sulphate reducing bacteria
(SRB), were identified with low δ13C values (-55 to –115 per mille). The structural and
isotopic features of microbial lipids provided compelling evidence for anaerobic oxidation of
methane (AOM) taking place within the upper 17 cm of the sediment core, mediated by
methanotrophic archaea (ANME) and sulphate reducing bacteria. No biomarker evidence for
aerobic oxidation of methane was found. Depth profiles of microbial lipids revealed
the vertical zonation of AOM and associated microbial biomass, and implied that
AOM is highest at the 10-12 cm sediment horizon, in agreement with pore water
profiles.
Lipid fingerprints suggested the occurrence of significantly diverse archaeal/bacterial
assemblages. Among them, ANME-2 (likely ANME-2c) methanotrophic archaea and
sulphate reducing bacteria from the Desulfosarcina/Desulfococcus (DSS) cluster appeared to
dominate the ANME/SRB communities. Complementary evidence was provided by analyses
of the archaeal and bacterial diversity based on the amplification, cloning and sequencing
of 16S rRNA genes from the core surface and the 10-12 cm-deep layer. A large
variety of archaeal and bacterial phylotypes was identified. In accordance with lipid
profiles, sequences belonging to the groups ANME-2a and ANME-2c within the
archaea and to δ-proteobacterial SRB within the bacteria were dominant in our gene
libraries.
In contrast with the core from the south of Çinarcik Basin, cores retrieved from the north
of Çinarcik Basin and the Central Basin did not contain lipids diagnostic for AOM. Evidence
for aerobic methanotrophy was also absent. However, pore water profiles have implied
that, in these cores, the methane/sulphate transition zone occurs in deeper sediment
layers.
Diagnostic microbial lipids with very low δ13C values were also very abundant in
carbonate crust samples from the Marmara Sea pointing to their formation through AOM.
Alike in sediments, lipid fingerprints indicated the prevalence of ANME-2/DSS
assemblages. Yet, significant differences in the composition of bacterial lipids were
observed.
Overall, our data provide first evidence for AOM taking place in the newly explored
active fluid venting sites along the main active fault in the Marmara Sea, expand on existing
observations and knowledge of AOM at cold seeps, and highlight similarities/differences with
cold seeps in the Mediterranean/Black Sea realm. |
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