25th International Meeting on Organic Geochemistry IMOG 2011

O-60
Assessing subsurface microbial carbon assimilation by lipid
13CDIC-DH2O stable isotope probing
Gunter Wegener 1,2 , Marlene Bausch 1,2 , Nguyen Manh Thang 1 , Matthias Kellermann 2 ,
Xavier Pietro 2 , Kai-Uwe Hinrichs 2 , Antje Boetius 1,3
1 Max Planck Institute for Marine Microbiology, Bremen, Germany, 2 MARUM, University Bremen, Bremen,
Germany, 3 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
(corresponding author:gwegener@mpi-bremen.de)
Microbial metabolism predominates
remineralization of organic matter in marine
sediments. The catabolic reactions during carbon
mineralization in sediments are fairly well understood
and quantified, whereas in vivo anabolic activity of the
microbiota, including the extent of (lipid) biomass
formation and the biomass carbon sources of the
different organisms, are poorly constrained.
To characterize microbial membrane lipid
production we developed a combined 13 CDIC-DH2O
stable isotope probing (SIP) approach and tested it on
cultures of Desulfosarcina variables, grown on
heterotrophic and autotrophic substrates. At both
conditions hydrogen isotopic compositions of formed
lipids were almost in equilibrium with the deuterium
labelled water, indicating that D2O assimilation can be
used to integrate total lipid production. The dominant
pathway (hetero- or autotrophy) is displayed in
different 13 CDIC / DH2O assimilation ratios.
We applied our method to natural sediments from
a Swedish fjord (Himmersfjarden, Baltic Sea).
Sediments were incubated with 13 CDIC and DH2O
labelled brackish medium for 21 days. Total lipids
were extracted using Bligh and Dyer protocol. TLEs
were saponified or cleaved with boron tribromide to
yield total bacterial fatty acids and archaeal
isoprenoids, respectively. Deuterium and carbon
isotopic compositions of bacterial lipids were heavily
altered compared to the start conditions, especially in
terminally branched and unsaturated fatty acids,
whereas isotopic changes in archaeal biphytanes
were low. In control experiments (incubation of killed
sediments with 13 CDIC and DH2O amended medium),
no significant changes were detected in carbon or
hydrogen isotopic compositions.
Lipid formation rates, determined from changes
of deuterium isotopic compositions, labelling strength
and lipid concentrations, show that more than ¾ of
anaerobic microbial membrane lipid formation in a
72 cm core appears in its uppermost 5 cm (Fig 1). By
comparing 13 CDIC and DH2O derived lipid assimilation
rates we conclude that most lipids are produced by
heterotrophs (Fig 1). Only for some lipids (e.g. C16:1ω5)
autotrophic origin is dominant. Comparison of
sediment age and DH2O-SIP derived lipid turnover
times indicates that bacterial fatty acids were most
likely formed within the sediment. Turnover times of
archaeal biphytanes are several times higher than the
sediment ages. Hence we assume mainly
allochthonous origin (from the water column) of these
compounds.
Due to the high sensitivity of deuterium labelling
and the absence of artificial energy source additions,
we propose our method as tool to assess microbial
assimilatory activity in deep subsurface habitats.
Fig. 1. Left: Concentrations of bacterial lipids in
different horizons (bars) and their averaged weight
balanced carbon isotopic composition (black dots)
before incubation with 13 C-DIC and D2O-amended
medium. Right: Bacterial lipid formation in the
different horizons based on D2O-labeling (black bars)
and 13 C-DIC labelling (grey bars) and the ratio of the
lipid formation proxies (white circles).
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