Glycerol dialkyl glycerol tetraether
Their presence, structure, and relative abundances in natural materials can be useful as proxies for temperature, terrestrial organic matter input, and soil pH for past periods in Earth history.
[5][6] The chemical nature of GDGTs is succinctly described by its name: they consist of two glycerol molecules connected via two alkyl chains, being held together at four ether linkages.
[1] They were first recognized as being associated with extremophilic archaea,[2] but research in recent decades has discovered the compounds in a wide range of mesophilic environments as well, including soils, lake sediment, and marine deposits.
[1] Archaeal phylogenetic classes Nitrososphaerota (formerly Thaumarchaeota), Thermoproteota (formerly Crenarchaeota), "Euryarchaeota", and "Korarchaeota" produce GDGTs.
[1] The evolution of the cyclohexane ring was likely to adjust the density of the membrane packing to more optimally function at the cooler ocean temperatures to which Nitrososphaerota adapted.
The larger number of cyclopentane moieties facilitates a more densely packed membrane lipid structure, which better inhibits trans-membrane passage of protons and ions.
[13] The building blocks of brGDGT, specifically the long alkyl groups (iso-diabolic acid), are detected in acidobacteria subdivisions 1, 3, 4, and 6.
[14] Because the number of cyclopentane moieties in a GDGT compound is related to the temperature of the growth environment, with increasing numbers of cyclopentane rings resulting in increased thermal stability and allowing for survival at higher temperatures, GDGT distribution and abundance can be employed as paleoclimate proxies.
[1] Seasonal variability in archaeal productivity and depth in the water column at which the archaea grow should be considered prior to employing this proxy.
It is calculated by ratioing a sum of bacterially-produced brGDGT abundances over a sum of archaeal isoGDGT abundances and is based on the fundamental idea that brGDGTs are produced most commonly in terrestrial environments (most ubiquitous in soils and peats) while archaeal isoGDGTs (particularly crenarchaeol) are produced in marine environments.
[1] Further research is needed to assess seasonal bias, appropriate calibration protocols, and whether the brGDGT distributions record air or soil temperature.
High-precision liquid chromatography mass spectrometry (HPLC-MS) is the primary means by which GDGTs are analyzed due to this method's tolerance for high temperatures.
