25th International Meeting on Organic Geochemistry IMOG 2011

P-161
Biosignatures of microbial methane turnover in the water
column and sediments of the central Baltic Sea (Gotland Deep)
Martin Blumenberg 1 , Christine Berndmeyer 1 , Oliver Schmale 2 , Volker Thiel 1
1 Geoscience Center, Geobiology Group, Georg-August-University Göttingen, Göttingen, Germany, 2 Leibniz
Institute for Baltic Sea Research Warnemünde, Rostock, Germany (corresponding
author:martin.blumenberg@geo.uni-goettingen.de)
Methane (CH4) is an important atmospheric trace gas,
influencing the climate with a global warming potential
25 times higher than that of CO2. Marine and limnic
sediments are considerable sources of methane
originating from thermogenic deep sources or from
microbial methanogenesis [1]. However, in sediments
and overlying water bodies, most methane is oxidized
by phylogenetically and metabolically versatile
prokaryotes [2]. In marine environments, the sulfatedependent
anaerobic oxidation of methane (AOM) is
of particularly high relevance because it eliminates
the vast majority of methane before reaching
overlying oxygenated waters or the atmosphere.
However, other anaerobic pathways of methane
turnover, potentially prevailing in freshwater,
terrestrial and brackish environments were recently
described [3,4].
In the Baltic Sea geochemical and seismoacoustic
studies of the seabed reveal large areas with high
concentrations of dissolved methane and free gas
occurring within an organic-rich postglacial sediment
layer of the Littorina facies [5]. Moreover, surface
waters of the Baltic Sea are considerably methaneoversaturated
with respect to the atmospheric
equilibrium [5]. In this regard of particular interest are
the deep basins in the central Baltic (Gotland-Deep
and Landsort-Deep). Here the strong permanent
density stratification leads to large vertical redox
gradients in the water column, which are perturbed by
saline inflows from the North Sea. Although dissolved
methane is highly abundant in the deep anoxic water
body of these basins (up to 1090nM) [5], little is
known about the methane turnover in this
environment.
Here we present preliminary biomarker data on the
distribution of lipids specific for methanotrophic
prokaryotes in the water column of the central Baltic
Sea (Gotland Deep). These molecules include
specific bacteriohopanepolyols (BHPs; i.e., 35aminobacteriohopanetetrol)
that show a maximum
abundance in the particulate organic matter sampled
at the oxic/anoxic transition zone of the water column
at about 120 m water depth. Consequently, the
occurrence of aerobic methane consumption is
indicated, a process which is also recorded in the
underlying sediments. In anoxic deeper waters
(sample from 135 m water depth), these BHPs were
absent, but specific biomarkers for sulphate reducing
bacteria (ai15/ai15-dialkyl glycerol diether; hexadec-
11-enoic acid) and archaea (archaeol) were found.
The majority of the source organisms, however,
appears to be not involved in the consumption of
methane since compound specific δ 13 C-analyses
revealed no significant 13 C-depletions (-28 to -33‰
vs. VPDB), as it would be expected for AOM
performing organisms. Assessing the importance of
sulphate-dependent and -independent AOM in the
Baltic Sea clearly requires further studies. However,
our data indicate the existence of a ‗microbial filter‘ of
most likely aerobic methanotrophs at the oxic/anoxic
boundary of the central Baltic Sea water column that
considerably reduces the amounts of sedimentary
methane before entering the atmosphere.
[1] Reeburgh, W.S. (1976) Earth and Planetary
Science Letters 28, 337-344.
[2] Knittel, K., Boetius A. (2009) Annual Review of
Microbiology 63, 311-334.
[3] Ettwig, K.F., et al. (2010) Nature 464, 543-548.
[4] Raghoebarsing, A.A., et al. (2006) Nature 440,
918-921
[5] Schmale, O., et al. (2010) Geophysical Research
Letters 37, L12604.
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