25th International Meeting on Organic Geochemistry IMOG 2011

0 - 69.99
0 - 69.99
P-052
Incorporation of archaeal and bacterial lipids into
geomacromolecules: implications for organic matter
preservation
Lidia Chaves Torres, Katie L. H. Lim, Paul S. Monaghan, Richard P. Evershed, Richard
D. Pancost
Organic Geochemistry Unit, University of Bristol, Bristol, United Kingdom (corresponding
author:chylc@bristol.ac.uk)
The incorporation of organic matter into
geomacromolecules during diagenesis contributes to
their formation and composition. These
geomacromolecules are generally resistant to
degradation, and thus, their formation is a crucial
aspect of understanding the bioavailability of organic
carbon in the geosphere, the carbon cycle and its role
in climate regulation, and the formation of source
rocks. However, the mechanisms by which
geomacromolecules are formed, including the
incorporation of lipids, remains poorly understood.
Sulfurisation processes have been shown to be
important in S-rich marine contexts, but in other
settings, oxidative cross linking appears to be
extensive (e.g. Versteegh et al., 2004 [1]).
In order to better understand geomacromolecule
formation, we are examining the nature and timing of
the incorporation pathways of archaeal and bacterial
lipids, as they have diagnostic structures that act as
robust tracers among the different organic matter
fractions. Furthermore, the wide range of different
functionalities of these compounds allow us to
examine which chemical moieties might be most
prone to cross-linking reactions. Thus, the purpose of
the research is analyzing the fate of structurally
diagnostic archaeal and bacterial membrane lipids
through the comparison of free biomarkers (free;
those that are extractable with organic solvents,
including core lipids and intact polar lipids) with their
structural equivalents released from
geomacromolecular fractions via sequential chemical
degradation (incorporated).
In the first instance we have focused on a core with
significant methanogenic activity, whose redox
gradient across the water table enables the
exploration of both aerobic and anaerobic conditions.
In such a terrestrial setting sulfur does not play a
predominant role, and instead oxidative
polymerization may be the key pathway responsible
for the geomacromolecule formation. We have started
by characterizing the free archaeal and bacterial
membrane lipids, archaeol and branched fatty acids,
respectively, and their occurrence as either core
lipids, phospholipids or glycolipids. Selected sections
Relative Abundance
of the core were analyzed and the results show that
archaeol concentrations increased with depth,
especially below the water table, whereas bacterial
phospholipid fatty acid concentrations decreased.
These observations collectively show a shift in the
population towards a relatively smaller bacterial
community and larger archaeal community.
Ongoing work is now focussed on the incorporation
of these different compounds into the non-extractable
fraction. Initially, we have hydrolysed the peat residue
under basic and then acidic conditions releasing
bacterial and archaeal biomarkers. Given the age of
the core (