Glycerol dialkyl glycerol
tetraethers are large membrane lipids with up to 86 carbon atoms produced
by Archaea and some Bacteria. These fossil lipids are found in sedimentary
organic matter of a variety of environments such as oceans. They are used as a
biomarker due to the relation of its relative distribution with temperature and
good preservation.


biomarker-based paleothermometers are built upon the biochemical principle that
communities adjust the chemical structure of their cell membranes in response
to environmental temperature (ref). The Archaea and Bacteria response
in this case is to adjust the rigidity of their cell membrane structures by
altering the number of rings or branches. In addition, the composition of the lipid
membrane is variable in order to adjust the membrane fluidity to different environments
(De Jonge et al.,2014).

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GDGTs can be divided
in two main types by their chain architecture: isoprenoidal GDGTs (isoGDGTs) and branched
GDGTs (brGDGTs). The isoGDGTs are characteristic
lipids of Archaea, and are produced by methanogenic, hyperthermophilic, and
mesophilic species. The dominant producers of isoGDGTs are members of the phylum
Thaumarchaeota, which are a one of the dominant prokaryotes in today’s ocean, representing
up to 20% of the ocean’s picoplankton (Karner et al., 2001).  The isoGDGTs structure consist of two
head-to-head isoprenoid
chains with a varying number of cyclopentane and cyclohexane rings, connected
by ether bonds to two terminal glycerol groups (Figure 1).


brGDGTs are most likely biosynthesized by heterotrophic bacteria, however the
organism responsible for these compounds have not been identified.  These lipids have branched  alkyl chains containing: 4 to 6 methyl branches
(depend primarily on mean annual air temperature) and up to two cyclopentane moieties
(depend primarily on soil pH), connected by ether bonds to two terminal
glycerol groups (Figure 1). Although some features are typically seen only in
Archaea, the chain structure and the stereochemistry of the glycerol groups on
the brGDGTs are diagnostic of Bacteria (Weijers et al., 2006a). BrGDGTs are
commonly found in soils, peats, lakes, and marginal/deltaic environments, and
are not present in pelagic marine environments (Hopmans et al., 2004).

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