25th International Meeting on Organic Geochemistry IMOG 2011

P-492
Investigating the microbial populations controlling the
production and oxidation of methane in water-saturated mineral
soils
Katie Lim 1 , Peter Maxfield 1 , Edward Hornibrook 2 , Richard Pancost 1 , Richard Evershed 1
1 Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol, United Kingdom,
2 Department of Earth Sciences, University of Bristol, Bristol, United Kingdom (corresponding
author:k.lim@bristol.ac.uk)
There is significant concern regarding the projected
emissions of the prominent greenhouse gas methane
(CH4) from known sources, e.g. wetlands, and the
potential impact on future climate change. In contrast,
mineral soils are generally regarded as CH4 sinks,
due to the presence of high abundances of
methanotrophic populations able to consume ambient
levels of CH4. However, the flux of CH4 from certain
mineral soils is a delicate balance between the
simultaneous production and oxidation carried out by
methanogenic archaea and methanotrophic bacteria
they contain. For example, mineral soils experiencing
regular water saturation, although not always fully
anoxic, may host methanogenic communities and act
as CH4 sources (Teh et al., 2005). It has become a
concern that in such mineral soils, termed ‗transitional
soils‘, internal CH4 production may be significantly
underestimated, and that a ‗tipping-point‘ may occur
where marginal increases in water-content may
increase their capacity to act as a net CH4 source.
The aim of this research was to investigate CH4
cycling in transitional soils susceptible to frequent
seasonal water-logging using a combination of CH4
flux measurements, 13 C-stable isotope probing and
biomarker analyses. A major aim was to develop new
methods for monitoring C flow between methanogenic
and methanotrophic microbes. Methanotrophic activity
is based upon phospholipid fatty acid (PLFA) profiling
in combination with 13 C-stable isotopic probing (SIP;
Maxfield et al., 2006), confirming a dominance of
18:1�7 producing high affinity bacteria. In order to
assess methanogenic archaeal communities we
developed the use of archaeol, determined using gas
chromatography/mass spectrometry (GC/MS), as a
proxy for in situ methanogenesis. This holds particular
potential, as evidenced by the vertical distribution of
archaeol in both free, phospholipid- and glycolipidbound
forms, in mineral soils (see Fig. 1). These
results show for the first time in UK mineral soils an
increased population of methanogenic archaea at
depth due to the increase in anoxia induced by
increased water content. Significantly, >90% of the
total archaeol was present in ‗bound‘ glycolipid and
phospholipid forms, indicating an origin from the living
archaeal biomass. The link to methanogenesis was
confirmed through increased CH4 production rates.
We are now using these techniques to assess the
true ‗sink‘/‗source‘ capacity of mineral soils subjected
to frequent water saturation as basis for further
predicting trends in emissions related to greenhouse
gas driven climate change.
Depth / cm
0
2
4
6
8
10
12
14
Concentration / µg g -1 dry wt.
0.00 0.05 0.10 0.15
Free archaeol
Phospholipid bound archaeol
Glycolipid bound archaeol
Figure 1. Vertical distribution of free and bound
archaeol detected using GC/MS in a mineral soil.
References
[1] Maxfield, P. J., Hornibrook, E. R. C. and
Evershed, R. P., 2006. Applied and
Environmental Microbiology 72 (6), 3901-
3907.
[2] Teh, Y. A., Silver, W. L. and Conrad, M. E.,
2005. Global Change Biology 11 (8), 1283-
1297.
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