Glycerol dialkyl glycerol tetraether lipids (GDGTs) are a class of

membrane lipid Membrane lipids are a group of compounds (structurally similar to fats and oils) which form the double-layered surface of all cells (lipid bilayer). The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol. Lipid ...

s synthesized by

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

and some

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

, making them useful biomarkers for these organisms in the geological record. 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. Some structural forms of GDGT form the basis for the

TEX86 TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Nitrososphaerota (formerly Marine Group 1 Crenarchaeota). Basics The membrane lipids of Nitrososphaerota are composed of glycerol dialkyl glycerol tetraeth ...

paleothermometer A paleothermometer is a methodology that provides an estimate of the ambient temperature at the time of formation of a natural material. Most paleothermometers are based on empirically-calibrated proxy relationships, such as the tree ring or TEX ...

. Isoprenoid GDGTs, now known to be synthesized by many archaeal classes, were first discovered in

extremophilic An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme tempe ...

archaea cultures. Branched GDGTs, likely synthesized by

acidobacteriota Acidobacteriota is a phylum of Gram-negative bacteria. Its members are physiologically diverse and ubiquitous, especially in soils, but are under-represented in culture. Description Members of this phylum are physiologically diverse, and can be ...

, were first discovered in a natural Dutch peat sample in 2000.

Chemical structure

The two primary structural classes of GDGTs are

isoprenoid The terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives called terpenes, diterpenes, etc. While sometimes used interchangeably with "terpenes", ...

(isoGDGT) and branched (brGDGT), which refer to differences in the carbon skeleton structures. Isoprenoid compounds are numbered -0 through -8, with the numeral representing the number of cyclopentane rings present within the carbon skeleton structure. The exception is

crenarchaeol Crenarchaeol is a glycerol biphytanes glycerol tetraether (GDGT) biological membrane lipid. Together with archaeol, crenarcheol comprises a major component of archaeal membranes. Archaeal membranes are distinct from those of bacteria and eukaryote ...

, a

Nitrososphaerota The Nitrososphaerota (syn. Thaumarchaeota) are a phylum of the Archaea proposed in 2008 after the genome of ''Cenarchaeum symbiosum'' was sequenced and found to differ significantly from other members of the hyperthermophilic phylum Thermoprote ...

product with one cyclohexane ring moiety in addition to four cyclopentane rings. Branched GDGTs have zero, one, or two cyclopentane moieties and are further classified based the positioning of their branches. They are numbered with roman numerals and letters, with -I indicating structures with four modifications (i.e. either a branch or a cyclopentane moiety), -II indicating structures with five modifications, and -III indicating structures with six modifications. The suffix a after the roman numeral means one of its modifications is a cyclopentane moiety; b means two modifications are cyclopentane moieties. For example, GDGT-IIb is a compound with three branches and two cyclopentane moieties (a total of five modifications). GDGTs form as

monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials. ...

s and with ether bonds to glycerol, as opposed to as bilayers and with

ester bond In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

s as is the case in eukaryotes and most bacteria.

Biological origin

GDGTs originate as archaeal

s, whose

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

s are converted to

glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

via

esterification In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

. They were first recognized as being associated with

archaea, but research in recent decades has discovered the compounds in a wide range of

mesophilic A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, with an optimum growth range from . The optimum growth temperature for these organisms is 37°C. The term is mainly applied to microorganisms. Organi ...

environments as well, including soils, lake sediment, and marine deposits. Archaeal phylogenetic classes

(formerly Thaumarchaeota),

Thermoproteota The Thermoproteota (also known as crenarchaea) are archaea that have been classified as a phylum of the Archaea domain. Initially, the Thermoproteota were thought to be sulfur-dependent extremophiles but recent studies have identified characteris ...

(formerly Crenarchaeota), "

Euryarchaeota Euryarchaeota (from Ancient Greek ''εὐρύς'' eurús, "broad, wide") is a phylum of archaea. Euryarchaeota are highly diverse and include methanogens, which produce methane and are often found in intestines, halobacteria, which survive extre ...

", and "

Korarchaeota In taxonomy, the Korarchaeota are a phylum of the Archaea. The name is derived from the Greek noun koros or kore, meaning ''young man'' or ''young woman,'' and the Greek adjective archaios which means ''ancient.'' They are also known as Xenarchae ...

" produce GDGTs. Branched GDGTs are most commonly detected in peats and soils and are most associated with terrestrial settings. To date, no direct evidence for an unequivocal source organism has been reported, but the structural similarity of acidobacterial lipid to brGDGT alkyl chains strongly suggests that

synthesize brGDGT.

GDGT-0

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.

Carbon isotope Carbon (6C) has 15 known isotopes, from to , of which and are stable. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by t ...

analyses and association with sites of

anaerobic methane oxidation Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

suggest that GDGT-0 is produced via

methanotroph Methanotrophs (sometimes called methanophiles) are prokaryotes that metabolize methane as their source of carbon and chemical energy. They are bacteria or archaea, can grow aerobically or anaerobically, and require single-carbon compounds to s ...

s. In older microbiology literature GDGT-0 is sometimes referred to as

caldarchaeol Caldarchaeol is a membrane-spanning lipid found in hyperthermophilic archaea. Membranes made up of caldarchaeol are more stable since the hydrophobic chains are linked together, allowing the microorganisms to withstand high temperatures. It is a ...

GDGT-1 – GDGT-3

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. Methanogens are not thought to be large synthesizers of these molecules, with the exception of ''

Methanopyrus kandleri In taxonomy, ''Methanopyrus'' is a genus of the Methanopyraceae. ''Methanopyrus'' is a genus of methanogen, with a single described species, ''M. kandleri''. It is a rod-shaped hyperthermophile, discovered on the wall of a black smoker from the ...

'', which does produce them. These classes are lower in abundance than GDGT-0 and GDGT-4 and are used in the TEX86 paleothermometer.

Crenarchaeol

Crenarchaeol is mainly attributed to ammonium-oxidizing

and has four cyclopentane rings plus one cyclohexane ring, which distinguishes it from GDGT-4 and is unique to the Nitrososphaerota phylum. 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. Due to their structural similarities, crenarchaeol and GDGT-4 have similar GC/MS elution times. 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 In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

, however, is a part of the TEX₈₆ paleothermometer.

GDGT-5 – GDGT-8

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.

GDGT-based proxies

TEX86

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.

is one such

which relates distribution and relative abundance of GDGT-1, GDGT-2, GDGT-3, and crenarchaeol isomer to past

sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air mass ...

(SST) (see

). GDGT-0 and GDGT-4 (crenarchaeol) are excluded from consideration for this proxy due to their very high abundances relative to isoGDGTs 1–3. 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. Current research suggests TEX86 works best in the temperature range 15-34 degrees Celsius. Seasonal variability in archaeal productivity and depth in the water column at which the archaea grow should be considered prior to employing this proxy.

BIT Index

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. 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.

MBT/CBT index

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. Further research is needed to assess seasonal bias, appropriate calibration protocols, and whether the brGDGT distributions record air or soil temperature.

Measurement techniques

GDGTs are identified via organic geochemical analysis as the polar head groups of the membrane lipids. High-precision

liquid chromatography mass spectrometry A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

(HPLC-MS) is the primary means by which GDGTs are analyzed due to this method's tolerance for high temperatures.

References

{{Reflist Biomarkers Lipids Ethers