Lipid biomarker geochemistry
An important component of every organism is the cytoplasmatic membrane. This lipid bilayer functions as a permeability barrier, and prevents leakage of solutes in or out of the cell. The molecular structure and distribution of certain membrane lipids is specific for archaea, bacteria and higher organisms (eucaryotes). The presence and abundance of characteristic lipids in environmental samples therefore provides information on the community structure and (indirectly) on biogeochemical processes within an ecosystem. For example, typical biomarkers of AOM communities are isoprenoidal glycerol ethers archaeol and sn2-hydroxyarchaeol, and the irregular, tail-to-tail linked isoprenoidal hydrocarbons pentamethylicosane (PMI) and its unsaturated analogues. Aerobic methanotrophs, on the other hand, can be detected by characteristic fatty acids, hopanoids and 4α-methylstyroids. Particularly in combination with their stable carbon isotope signatures, lipid biomarkers can provide valuable information on the community structure and methabolic pathways, as well as their importance in the environment. We routinely use biomarker-based approaches (often in combination with biogeochemical and DNA-based constraints) in both marine and lacustrine studies.
Similarly, the chemotaxonomic specificity of lipids in geologic/sedimentary archives can be used to gain information about environmental conditions at the time when the lipids were biosynthesized and buried. For instance, certain anoxygenic phototrophs inhabit H2S-rich environments, and lipids that are intrinsic to these organisms (specific carotenoids) provide evidence for euxinic conditions at the time of archive formation. A second application of lipid analyses for reconstructing paleo-environmental conditions is based on the ability of microbial organisms to modify the lipid composition of their cellular membranes in order to adapt to environmental changes. The relative abundance of soil-bacterial methyl-branched and cyclic dialkyl glycerol tetraethers (GDGTs) can, for example, be used for paleo-T and pH reconstruction. Most recently, (in collaboration with the Royal Netherlands Institute for Sea Research - NIOZ) we started to investigate the applicability of the GDGT-based paleothermometer to lacustrine sediment records.