25th International Meeting on Organic Geochemistry IMOG 2011

P-030
Trace analysis of methylated substrates in marine sediment
Guangchao Zhuang, Yu-Shih Lin, Eoghan Reeves, Kai-Uwe Hinrichs
MARUM, Center for Marine Environmental Sciences, University of Bremen, Organic Geochemistry Group,
D-28334 Bremen, Germany (corresponding author:gzhuang@marum.de)
Methane in marine sediment has received great
attention because of its role as greenhouse gas and
as major constituent in gas hydrate and its
significance in driving the benthic carbon cycle in
many seep environments. Among all the biological
pathways leading to methane production in marine
sediment, methyltrophic methanogenesis via noncompetitive
substrates, such as methylated sulfides,
methylamines, and methanol, is the least understood
one. The existing knowledge of methylated substrates
and their precursors (summarized in Fig.1) mostly
came from studies of hypersaline environments and
freshwater sediments [1]. Activity studies
demonstrated that degradation of volatile methylated
compounds could account for the substantial fraction
of methanogenesis in some anoxic sulfate-rich
sediments [2].
Fig.1. Summary of source and sink of methylated substrates
for methanogenesis in marine sediments. Solid lines indicate
direct process, and dashed lines represent a series of
sedimentary, chemical, biological processes. Involved
conversion pathways have been proposed through
incubation experiments, but further evidence is needed for
in-situ sediments.
There has been sporadic evidence suggesting that
methylotrophic methanogenesis could contribute to
biogenic methane in subseafloor sediment.
Microbiological studies showed that the potential
methanogenic rates from methylated substrates were
high enough to be detected in subseafloor sediment
samples, and known methylotrophs, members of
Methanosarcinales, have been detected in hydratebearing
sediments drilled by IODP [3]. Yet, the
significance of this process under in situ conditions
remains unclear. A main reason for the uncertainty
comes from the lack of concentration data of these
compounds in subseafloor sediment. The only
published data, to our knowledge, came from the
carbonate sediments of the Great Australian Bight [4]
where the biogenic carbonates are suggested to
contain a protein matrix that is embedded in the
mineral structure and releases amino acids during
diagenesis. The relatively high concentrations of
glycine (up to 1 mmol/kg) and hence elevated levels
mol/kg) could
explain the substantial amounts of methane in the
sulfate-reducing zone. Nevertheless, this case may
not apply to the majority of clastic sediments at
continental margins. Constraining the sources and
fluxes of methylated low-molecular-weight organic
compounds in subseafloor sediments is of great
importance for our understanding carbon flow and
methane biogeochemistry in subseafloor sediments.
We are currently establishing analytical protocols
suitable for the quantitative and isotopic analysis of
methlyated sulfides, methylamines, and methanol. We
are adapting and improving two existing preconcentration
methods, i.e., solid-phase microextraction
and purge-and-trap techniques, for
quantification of these compounds in diverse
sediment samples. Once the analytical barrier is
overcome, we envision extensive application of the
method, such as down-core survey of marine
sediment and further adaptation of the method for
isotope ratio analysis. Our paper will discuss the
success and challenges of the molecular-isotopic
analysis of these trace constituents.
Reference:
1. Lomans, B. P., et al. (2002). Cell. Mol. Life Sci.,
59, 575-588.
2. Oremland, R. S. and Polcin, S. (1982). Appl.
Environ. Microbiol., 44, 1270-1276.
3. Yoshioka, H., et al. (2010). Geobiology, 8, 223-
233.
4. Mitterer, R. M., et al. (2001). Geophys. Res. Lett.,
28(20), 3931-3934.
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