25th International Meeting on Organic Geochemistry IMOG 2011

P-451
Methane fluxes modulating the molecular and carbon isotopic
composition of microbial lipids in gas hydrate bearing
sediments from the northern Cascadia margin
Marcos Yukio Yoshinaga 1 , John W. Pohlman 2 , Tobias Goldhammer 1 , Nadine Broda 1 ,
Michael Riedel 3 , Marcus Elvert 1 , Kai-Uwe Hinrichs 1
1 MARUM, University of Bremen, Bremen, Germany, 2 U.S. Geological Survey, Woods Hole Science Center,
Woods Hole, MA, United States of America, 3 Geological Survey of Canada, Sydney, BC, Canada
(corresponding author:marcosyukio@gmail.com)
A consortium of methane oxidizing archaea and
sulfate reducing bacteria critically regulates the flux of
methane between marine sediments and the oceans.
The process, known as the anaerobic oxidation of
methane (AOM), occurs within the sulfate-methane
transition (SMT)[1]. Under methane flux conditions
typical of most continental margins, AOM consumes
virtually all methane migrating to the seafloor.
However, at gas hydrate-bearing seeps and other
seep types (e.g., mud volcanoes[2]), the methane flux
often overwhelms the oxidative capacity of the AOM
consortium and passes into the water column. Seeps
presently emitting methane are analogs for largescale
methane releases that could be stimulated by
global warming. Understanding how the structure and
function of the AOM consortium responds to different
flux regimes at methane emitting seeps is crucial for
predicting how the marine methane cycle might be
affected by extreme environmental changes.
The objective of this study is to understand how
methane flux intensities and the sedimentary
microbial communities are spatially oriented around a
gas hydrate-bearing seep on the northern Cascadia
margin. We conducted detailed pore water
geochemical measurements in combination with
molecular and carbon isotopic analysis of intact polar
lipids (IPLs) in shallow subsurface sediments (ca. 5 m
depth).
The highest concentration and diversity of IPLs (a
proxy for the overall microbial diversity) occurred at
the SMT, with higher concentrations found in the
higher methane-flux core with the shallower SMT. In
general, the archaeal IPL concentration and diversity
was higher than the bacterial IPLs (Fig. 1). Bacterial
diether IPLs, likely derived from sulphate-reducing
bacteria, were found in high concentrations at the
SMT. The archaeal IPLs above and below the SMT
have mainly glycosyl (Gly) headgroups, while the
archaeal IPLs with phosphatidyl (Phos) headgroups
were concentrated near the SMT.
� 13 C values of archaeal and bacterial lipids were
more negative at the SMT than at the other horizons,
suggesting methane and/or CO2 assimilation [3].
Interestingly, in the SMT the � 13 C values of the
Phos-glyceroldialkylglyceroltetraethers (GDGTs) are
more negative than the Gly-GDGTs. This novel
intermolecular isotopic variation in archaeal lipids
suggests archaea with phosphatidyl headgroups are
more closely affiliated with AOM than those with
glycosyl headgroups, a conclusion supported by pore
water profiles that suggest uptake of phosphate at the
SMT.
Our results confirm that the methane flux into the
SMT is correlated with microbial biomass
accumulation. We also demonstrate that the flux
intensity influences the magnitude of isotopic lipid
fractionation, and that different microbial metabolisms
and communities may be differentiated by careful
analysis of the IPL structure.
Figure 1. Porewater profiles of methane, sulfate and
phosphate and selected IPLs groups concentrations at
Amnesiac Vent from the Northern Cascadia margin.
References
[1] Iversen, N. and Jorgensen,B.B. (1985) Limnol Oceanogr
30: 944-955.
[2] Niemann, H. et al. (2006) Nature 443: 854-858.
[3] Wegener, G. et al. (2008) Environ Microbiol 10: 2287-
2298.
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