25th International Meeting on Organic Geochemistry IMOG 2011

O-38
Microbial communities associated to deep subsurface coal
layers: A review of the DEBITS project
Kai Mangelsdorf 1 , Clemens Glombitza 1,2 , Andrea Vieth 1 , Tiem Vu Thi Anh 1 , Jens
Kallmeyer 2 , Klaus Zink 3 , Richard Sykes 3 , R. John Parkes 4 , John Fry 4 , Brian Horsfield 1
1 Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany,
2 University of Potsdam, 14476 Golm, Germany, 3 GNS Science, Lower Hutt 5040, New Zealand, 4 School of
Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, United Kingdom
(corresponding author:K.Mangelsdorf@gfz-potsdam.de)
The DEBITS (Deep Biosphere in Terrestrial System)
project was a joint study between geologists,
biogeochemists and microbiologists from New Zealand
(NZ), Germany and the UK and was dedicated to the
investigation of deep microbial communities associated
to lignite and coal layers in terrestrial environments.
Within the project a 148 m deep well was drilled in the
Waikato coal area on the North Island of NZ, penetrating
several lignite/coal layers intercalated between sand, silt
and clay lithologies. Additionally, a coal series covering a
maturity range from 0.28 to 0.8% vitrinite reflectance (R0,
diagenesis to beginning of main catagenesis) was
collected from several mines and outcrops on the North
and South Island of NZ.
Microbiological investigations reveal a high molecular
prokaryotic diversity, with some groups of bacteria and
archaea similar to the marine deep biosphere. While
there are partly similar bacteria above and below the
unconformity, archaeal populations are different.
Microbial communities and activity changed with
lithology. Substantial numbers of viable heterotrophs and
lignite-degrading bacteria were present throughout the
whole DEBITS core.
Phospholipids, being molecular indicators for living
bacteria, were detected in sedimentary succession
associated to lignite and coal layers indicating that
substrates are released from the organic carbon rich
lignite and coal layers to sustain a microbial ecosystem in
adjacent clay and mainly silt and sand layers. In selected
transects petrological investigations show an enhanced
permeability in the sand layers.
Water extraction of the lignites and coals shows that the
organic carbon rich lithologies contain substantial
amounts of small organic acids such as formiate, acetate
and oxalate being important substrates for microbial
metabolism. Thus, pore water of the coal layers provide
sufficient substrates to feed associated microbial
populations.
To investigate the potential of these coals to provide
appropriate substrate for deep microbial life, the amount
of kerogen bound low molecular weight organic acids
(LMWOA) were determined in a series of NZ coals of
different thermal maturity. Kerogen-bound formate,
acetate and oxalate were detected in all samples.
However, their abundance decreases with increasing
maturity of the coals suggesting that the main release of
these compounds is related to the diagenetic and early
catagenetic phases, a range where temperature
conditions are still compatible with microbial life. The
release of these substrates might be controlled by abiotic
(equilibrium reactions with the pore water) and biotic
processes. First assessment of the feedstock pool
indicates that the NZ coals exhibit the potential to sustain
associated deep terrestrial microbial life over geological
time spans.
At greater depth, when geothermally driven processes
become more and more important, an additional
substrate pool becomes available for deep microbial
ecosystems. For NZ coals the onset of the main phase of
geothermal hydrocarbon generation was determined at
0.55 to 0.6% R0 (main catagenesis) at temperatures
being presumably above the limit of deep subsurface life.
However, it was shown that already at lower maturity (0.4
% R0) the thermal generation of hydrocarbons slowly
begins. Thus, during early catagenesis geothermally
released hydrocarbons form another possible feedstock
for deep microbial life. Indications for this are provided by
NZ coals from the early catagenetic stage. The C16 and
C18 fatty acids, being also main constituents of bacterial
cell membranes, initially decrease with ongoing
maturation, but during the early catagenesis they
significantly increase before decreasing again at higher
maturity.
The interdisciplinary approach in the DEBITS project
allowed a deep insight into the type, abundance and
distribution of terrestrial microbial ecosystems associated
to lignite and coal layers. Furthermore, it broadened our
understanding on feedstock supply and optimal life
habitats for deeply buried microorganisms.
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