The archaeal lipid composition of partially lithified cold seep mats

doi:10.1016/j.orggeochem.2008.03.020

Organic Geochemistry

Volume 39, Issue 8, August 2008, Pages 1000-1006

https://doi.org/10.1016/j.orggeochem.2008.03.020 Get rights and content

Abstract

The anaerobic oxidation of methane (AOM), by converting methane to bicarbonate which is then precipitated as extensive carbonate crusts, is an important methane sink in the Earth’s ocean systems. Here we employ a multidisciplinary approach to investigate the role of microorganisms in carbonate precipitation using biomarker analysis, scanning electron microscopy (SEM) and X-ray diffraction. We examined two microbial mats from the Black Sea and found that one comprised carbonate in both aragonite and Mg calcite forms and most likely ANME-1 archaea, whereas the other contained only Mg calcite and most likely ANME-2 archaea. We conclude, as have others, that the different microbial communities could impart different influences on carbonate mineralogy and morphology. Although further research is needed, this is a contribution to our understanding of those relationships, which could prove critical in the interpretation of ancient sedimentary deposits.

Introduction

The anaerobic oxidation of methane (AOM) plays a major role in the sequestration of methane generated in marine sediments. The process is microbially mediated (Boetius et al., 2000) and occurs under anoxic conditions where methane oxidation is coupled to sulfate reduction, resulting in overall production of bicarbonate that is ultimately precipitated as calcium or magnesium carbonate. The microorganisms responsible for AOM are generally thought to comprise consortia of methane-consuming archaea and sulfate-reducing bacteria (Boetius et al., 2000, Hinrichs et al., 2000, Pancost et al., 2000). The two main groups of methane-consuming archaea are both euryarchaeota and have been termed ANME-1 and ANME-2 (Hinrichs et al., 1999, Orphan et al., 2001). A third group, ANME-3, has recently been described and is also apparently associated with AOM (Niemann et al., 2006).

Due to the difficulty in growing AOM-related microorganisms in the lab, inferences about the role of AOM in forming carbonate crusts derive from analysis of naturally occurring deposits. This includes both modern crusts from the Black Sea (Michaelis et al., 2002, Thiel et al., 2001), Mediterranean mud volcanoes (Aloisi et al., 2002) and the Gulf of Mexico (Pancost et al., 2005b), as well as fossil crusts from ancient seep sites (Peckmann et al., 2001, Peckmann et al., 2004, Reitner et al., 2005). The microbial community associated with such crusts has been examined by analysing solvent extractable archaeal (e.g. archaeol, hydroxyarchaeol, pentamethylicosane, crocetane) and bacterial (e.g. branched fatty acids, non-isoprenoidal diethers) lipid biomarkers. However, questions remain about the structure of both the microbial community and the lithified material and it is not clear how reliable biomarker data are over time.

To evaluate further the role of the AOM microbial community in the formation of cold seep crusts, biomarker lipids were extracted from various sections of two different Black Sea mats. Heterogeneities in biomarker distributions are attributed to either differences in microbial assemblages or the preservation state of the mat. These observations are then compared to the carbonate mineralogy and morphology of the mats evaluated using scanning electron microscopy (SEM).

Section snippets

Methods

We analysed two partially lithified microbial mats collected by the submersible Jago from active cold seeps of the Black Sea during the 2004 sailing of the RV Poseidon as part of the EU METROL programme. Both mats were collected from below the oxic/anoxic chemocline; one was collected from the Dniepr shelf region (44°46.54′N, 031°58.97′E, 191 m) and one from the Danube canyon (44°57.48′N, 030°17.26′E, 295 m). The Dniepr mat is from a carbonate chimney and is similar to those described by

XRD and SEM

The inorganic sections of the mats were analysed using powder X-ray diffraction (XRD). The inorganic carbon content of the Danube mat ranges between 4.6% and 11.9% (mean 8.8%) and the mineral is predominantly magnesian calcite. To further evaluate the mineral morphology, the mats were examined using SEM after removal of the organic matter (Fig. 1A and B) revealing morphology consistent with polycrystalline calcite. Some variation in structure was observed. While the majority of the mat

Summary of morphological and biomarker investigations

Elemental, SEM, XRD and biomarker analyses of two partly lithified mats from the Black Sea reveal differences in both mineralogy and organic matter assemblage. Specifically, the Danube mat does not contain aragonite, has generally higher % CaCO₃ and lower % TOC, is apparently dominated by ANME-2, as reflected by high Ar–OH:Ar values and the presence of crocetane, and contains a significant proportion of matter as necromass with respect to the putatively active microbial community, as

Acknowledgements

We are grateful to R. Berstan and I. Bull and the Bristol Node of the NERC Life Sciences Mass Spectrometry Facility for help with GC-MS, S. Davis, V. Swinerd and J. Jones for help with SEM and J. Charmant and L. Hall for help with XRD. Also to D. Jones for photographs of the samples. We also thank the METROL Scientists and the Captain, Crew and Scientific Party of the Poseidon Black Sea Cruise and the EU for funding the METROL project for samples. We also thank the NERC for funding Z.R and W.

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The build-up consists of Mg-calcite (MgCO₃ = 9–13 mol%) with minor aragonite. The carbonate matrix encloses organic matter, and Emiliania huxleyi coccoliths, as well as minor framboidal pyrite, gypsum, barite, and diatomite fragments. The contribution of detrital silicate material is negligible. Micritic Mg-calcite in the inner zone of the chimney forms obtuse rhombohedrons clustered in hemispherical aggregates (clots) and hosts isolated prismatic aragonite (≪ 1 vol.%). In the outer zone, Mg-calcite exists as foliated crystals and spherulites, while aragonite spherulites are restricted to the top surface. The carbon isotope compositions of carbonates (δ¹³С = − 46.5 to − 33.0‰ VPDB) and remnant bacterial mats (δ¹³С = − 76.9 to − 81.6‰ VPDB) provide evidence of a biogenic methane source for the build-up formation. Oxygen came mainly from sea water, judging by a narrow range of δ¹⁸O values in carbonates (+ 0.2 to 1.3‰ VPDB). The prevalence of Mg-calcite throughout the build-up indicates an ANME-2/SBR consortium being the main component of the bacterial mats living at the seep. Aragonite grew mostly late in the seep history, which hints to environmental changes and the ensuing predominance of the ANME-1/SBR consortium. The observed relationship between the prokaryotic communities and the main carbonate phases is common to the microbial mats of the Black Sea, but is actually opposed to the pattern of AOM-community and AOM-related carbonates found in sediments. Different morphologies of carbonates in the outer and inner zones of the build-up point to variations in the methane flux gradient, which controlled the biomass density of the AOM consortia. The REE + Y patterns of Mg-calcite from the inner zone are typical of carbonates crystallised in a freshened sea-water environment, while both Mg-calcite and aragonite from the outer zone show distinct sea-water signatures. Inasmuch as the time span of build-up growth (radiocarbon ages from 9000 ± 180 to 7500 ± 180 year BP) overlaps the period when the Black Sea ‘Lake’ reconnected to the Mediterranean Sea, the progressive arrival of sulphate-rich waters can be inferred to be another possible control of REE + Y patterns in seep carbonates besides the methane flux gradient and related activity of AOM consortia.
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The archaeal lipid composition of partially lithified cold seep mats

Abstract

Introduction

Section snippets

Methods

XRD and SEM

Summary of morphological and biomarker investigations

Acknowledgements

Earth and Planetary Science Letters

Organic Geochemistry

Organic Geochemistry

Organic Geochemistry

Journal of Crystal Growth

Geochimica et Cosmochimica Acta

Palaeogeography, Palaeoclimatology, Palaeoecology

Marine Geology

Journal of Crystal Growth

Palaeogeography, Palaeoclimatology, Palaeoecology

Marine Geology

Marine Chemistry

Algal and archaeal polyisoprenoids in a recent marine sediment: molecular isotopic evidence for anaerobic oxidation of methane

Geochemistry Geophysics Geosystems