25th International Meeting on Organic Geochemistry IMOG 2011

O-26
A radiotracer experiment for investigating the degradation
kinetics of intact polar lipids in sediments
Sitan Xie 1 , Julius Sebastian Lipp 1 , Gunter Wegener 2 , Timothy G. Ferdelman 2 , Kai-Uwe
Hinrichs 1
1 Marum Center for Marine Environmental Sciences, University of Bremen, Organic Geochemistry Group,
Bremen, Germany, 2 Max Planck Institute for Marine Microbiology, Bremen, Germany (corresponding
author:sitan.xie@uni-bremen.de)
Intact polar lipids (IPLs) constitute the cellular
membrane of every organism. As a consequence,
their analysis may provide an unselective view on
microorganisms present in an environmental sample.
Due to the nature of the bond between the headgroup
and the glycerol backbone, IPLs are assumed to be
unstable after cellular decay and are therefore used
as biomarkers for living microbial cells [1,2,3]. To act
as a proxy for live cells, the half life of extracellular
IPLs needs to be lower than the presumed cell
population turnover which is estimated to be on the
order of thousands of years [4]. Conversely, if the half
life is much higher, the fossil component could
potentially mask in situ IPL production [5,6]. A few
previous studies have investigated the degradation
kinetics of IPLs in shallow sediments. These studies
suggested that headgroups of bacterial phospholipids
degrade rapidly after cell lysis [1,2], whereas
glycosidic ether lipids, which are dominantly produced
by archaea, may be preserved longer [2]. However,
stability of IPLs has not been studied systematically
under conditions relevant to low-activity subsurface
sediments and the degradation kinetics of IPLs still
remains unclear. Therefore a better understanding of
IPL degradation is essential for the interpretation of
lipid signals in natural environments.
In this study, the degradation kinetics of IPLs has
been evaluated by a radiotracer incubation
experiment. Phosphatidylethanolamine diacylglycerol
(PE-DAG) and 1Gly-archaeol (1Gly-AR) were used as
model compounds representative for typical bacterial
and archaeal membrane lipids, respectively. The
carbons in the headgroups of IPLs were labeled by
14 C. Radio labeled IPLs were added to anaerobic
slurries of sediment from the Wadden Sea surface
and Cascadia Margin subsurface (138.21 mbsf)
sediments. The slurries were incubated at 4°C and
20°C to study the effect of temperature on the
degradation of IPLs. Independent of the chemical fate
of the polar headgroup after its hydrolysis from the
glycerol backbone, the 14 C enters the aqueous or gas
phase while the intact lipid is insoluble and remains in
the solid phase. Reaction progress is monitored by
quantifying the increase of radioactivity in the
aqueous solution and in the gas phase with a
scintillation counting technique.
The initial results of
14 C labeled PE-DAG
experiment (Fig.1) showed that about 48% of PE-
DAG was degraded after fourteen days of incubation
at 20°C. The degradation rate of PE-DAG and 1Gly-
AR is currently being examined by long-term
incubation. We will discuss the results in the context
of current applications of IPLs as proxies for live cells.
Fig.1. Initial degradation of 14 C labeled PE-DAG in Cascadia
Margin sediment slurry incubated at 20°C. A: negative killed
control experiment showing that no abiotic degradation takes
place. B: strong lipid degradation in the alive sediment.
References
[1] White, D.C., Davis, W.M., Nickels, J.S., King, J.D., Bobbie, R.J. (1979)
Oecologia 40, 51-62.
[2] Harvey, H.R., Fallon, R.D., Patton, S. (1986) Geochim. Cosmochim. Acta
50, 795-804.
[3] Lipp, J.S., Morono, Y., Inagaki, F., Hinrichs, K.U. (2008) Nature 454, 991-
994.
[4] Biddle, J.F., Lipp, J.S., Lever, M.A., Lloyd, K.G., Sørensen, K.B.,
Anderson, R., Fredricks, H.F., Elvert, M., Kelly, T.J., Schrag, D.P., Sogin,
M.L., Brechley, J.E., Teske, A., House, C.H., Hinrichs, K.U. (2006) Proc.
Natl. Acad. Sci. USA 103, 3846-3851.
[5] Lipp, J.S. and Hinrichs, K.U. (2009) Geochim. Cosmochim. Acta 73,
6816-6833.
[6] Schouten, S., Middelburg, J.J., Hopmans, E.C., Damsté J.S.S. (2010)
Geochim Cosmochim. Acta 74, 3806-3814.
85