25th International Meeting on Organic Geochemistry IMOG 2011

P-176
Microbial consortium mediating carbon cycle in a subsurface
and oliogotrophic karst cave in China
Yang Huan, Xie Shucheng
Key laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of
Geosciences, Wuhan, Wuhan, China (corresponding author:yanghuansailing@163.com)
The karst topography is widely distributed in
southwestern China and exerts significant influence
on people‘s lives in this region. The stone
desertification in karstified areas has become a
serious problem in agriculture because crops are not
able to grow on the barren lands. In addition,
stalagmites in karst caves are extraordinarily good
archives of paleoenvironmental reconstruction due to
their precise and high-resolution chronology, and little
diagenesis influence.
The inorganic carbon kinetics in soil-dripwaterstalagmite
system has been well documented to
delineate the paleoenvironmental information of δ 13 C
values for stalagmites [1]. However, most carbon
kenetic models in karst environment are restricted to
thermodynamic processes, whether the biological,
especially microbial activities may influence carbon
cycle is poorly understood. The karst cave can be
considered as an oligotrophic and aphotic ‘extreme’
environment as nutrients can not be readily
transported to caves through subterranean water
drainage system and light are usually unavailable.
The phototrophs, e.g. cyanobacteria can not survive
and thus provide no dissolved organic matter to the
heterotrophic bacteria in the cave. Although cavederived
stalagmites have attracted numerous
attentions, little is known about the microbial
community structure and their function in
biogeochemical cycles in this subsurface habitat.
We collected stalagmites, dripwater, cave
sediments, the surface sediments of growing
stalagmites and weathered surface of collapsed
parent rock inside the Heshang cave in central China
to explore the microbial community structure and their
putative function in carbon cycle in karst cave. The
microbial lipids were extracted repeatedly with
dichloromethane/methanol (9:1, v/v) in an ultrasonic
apparatus. The archaeal and bacterial tetraether
membrane lipids, glycerol dialkyl glycerol tetraethers
(GDGTs) were analyzed by high performance liquid
chromatogram and mass spectrometer while fatty
acid, alcohols, sterols and hopanoids were analyzed
by gas chromatography and mass spectrometer.
Previous work has demonstrated that the majority of
archaeal isoprenoid and bacterial branched GDGTs in
stalagmites were probably both in situ produced on
the surface of stalagmites [2]. A newly-recovered
surface sample of a growing stalagmite shows also
significant predominance of crenarchaeotal GDGTs
over bacterial GDGTs and very high diversity of
bacterial lipids including branched fatty acids,
branched fatty alcohols, monoalkyl glycerol ethers
(MAGEs), diethers, hopanols and hopanoic acids
with little contribution from plant lipids in overlying
soils. Most crenarchaeota and MAGEs-producing
bacteria are believed to be chemoautotrophic
microbes and they may uptake bicarbonate as the
sole carbon source for biosynthesis. Therefore the
chemoautotrophs living on the surface of stalagmites
may influence the carbon dynamic fractionation during
the stalagmite formation.
Phylogenetic analysis reveals that marine group
1.1a and 1.1b crenarchaeota, two groups of
ammonia-oxidizers, dominate in the Heshang cave
sediments. The high abundance of crenarchaeol and
low abundance of branched GDGTs were present in
most cave sediments and all stalagmites samples,
which were flushed continuously or discontinuously by
bicarbonate-saturated dripwater. In contrast,
sediments distant from dripwater site show much
higher branched GDGTs than archaeal GDGTs,
indicating that crenarchaeota may uptake bicarbonate
and play a vital important part in carbon and nitrogen
cycle in karst cave. The microbial lipids from the
sediments in the drainage system show apparently
higher abundance than that away from drainage
system, implying that dripwater carry bicarbonate and
other nutrients, thus may fuel the microbial growth in
sediments. Therefore the dripwater may function as
the key energy source for microbes living in the karst
cave environment.
References
[1] Dreybrodt, W. and Scholz, D. (2011) Geochim.
Cosmochim. Acta 75, 734-752.
[2] Yang, H., Ding, W., Zhang, C.L., et al., (2011) Org.
Geochem. 42:108–115.
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