Glycerol dialkyl glycerol tetraether

GDGT-0 has zero cyclopentane moieties and is the most ubiquitous isoGDGT synthesized by archaea. Halophilic archaea are the only group of archaea not known to produce GDGT-0.[1] Carbon isotope analyses and association with sites of anaerobic methane oxidation suggest that GDGT-0 is produced via methanotrophs.[1] In older microbiology literature GDGT-0 is sometimes referred to as caldarchaeol.[1]

GDGT-1, GDGT-2, and GDGT-3 have one, two, and three cyclopentane rings respectively within their isoprenoid biphytane carbon structures, respectively. Nitrososphaerota are the largest producers of these groups in marine and lacustrine environments.[1] Methanogens are not thought to be large synthesizers of these molecules, with the exception of Methanopyrus kandleri, which does produce them.[1] These classes are lower in abundance than GDGT-0 and GDGT-4 and are used in the TEX86 paleothermometer.[1]

Crenarchaeol is mainly attributed to ammonium-oxidizing Nitrososphaerota and has four cyclopentane rings plus one cyclohexane ring, which distinguishes it from GDGT-4 and is unique to the Nitrososphaerota phylum.[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.[6] Due to their structural similarities, crenarchaeol and GDGT-4 have similar GC/MS elution times.[1] They are similar in prevalence to GDGT-0 and therefore are not included in the TEX₈₆ paleothermometer because their abundance overwhelms the less abundant GDGT groups. A crenarchaeol isomer, however, is a part of the TEX₈₆ paleothermometer.[7]

GDGTs -5 through -8 are nearly exclusive to extreme high-temperature environments such as hot springs. The larger number of cyclopentane moieties facilitates a more densely packed membrane lipid structure, which better inhibits trans-membrane passage of protons and ions. Doing so increases the molecules' thermal stability, which is necessary to survive at extreme temperatures.[8][6]

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] TEX86 is one such paleothermometer which relates distribution and relative abundance of GDGT-1, GDGT-2, GDGT-3, and crenarchaeol isomer to past sea surface temperature (SST) (see TEX86).[1] GDGT-0 and GDGT-4 (crenarchaeol) are excluded from consideration for this proxy due to their very high abundances relative to isoGDGTs 1–3.[1] The relationship between isoGDGT distribution and temperature is not linear, and some studies have demonstrated its distinctive bias towards unrealistically cold temperatures in the lower latitudes.[9] Current research suggests TEX86 works best in the temperature range 15-34 degrees Celsius.[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.[1]

The branched:isoprenoid tetraether (BIT) index relates the relative abundances of brGDGTs in a natural sample to the relative abundance of soil organic matter in that sample. 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] While caveats and analytical uncertainties remain an issue, the BIT index is a potentially useful proxy for assessing the amount of fluvially transported soil organic matter compared to marine organic matter.[1]

The methylation of branched tetraethers (MBT) and cyclization of branched tetraethers (CBT) indices relate abundances and distributions of bacterially-produced brGDGTs to relative changes in soil pH and mean annual air temperature.[1] Further research is needed to assess seasonal bias, appropriate calibration protocols, and whether the brGDGT distributions record air or soil temperature.[1]