Glycerol dialkyl glycerol tetraether (GDGT) lipids are membrane-spanning lipids that over the last two decades have become an important research focus for organic geochemists because of their application in temperature and pH proxies. Currently two different types of GDGTs are known to exist in nature; isoprenoidal and branched GDGTs (Fig. 1). Isoprenoidal GDGTs (isoGDGTs) are synthesized by a wide range of Archaea while branched GDGTs (brGDGTs) are synthesized by specific Bacteria.
The uniqueness of the structure of isoGDGTs and GDGTs in general is that it consists of ether bonds that form a monolayer. Most organisms use ester-bonds and/or bilayers to form their membrane lipids. isoGDGTs were first discovered in the 1970s in cultures of extremophilic Archaea (1). Archaea are characterized by an extremely high ecotypic diversity and can be found in settings ranging from the deep biosphere characterized by high pressure and low temperature (2) to thermal hot springs characterized by temperatures as high as 90 °C (3, 4).The ability to synthesize tetraether membrane lipids, which are nearly impermeable to ions and protons, is key for their survival in these extreme environments (5).
Till the 1990s it was thought that isoGDGTs were only found in extreme settings and were unique to extremophiles. However, during the last two decades it has become apparent that Archaea in general and isoGDGTs specifically are ubiquitous in mesophillic settings such as the oceans, sediments, and soils (6-10). The majority of Archaea synthesize isoGDGT-0, which lacks cyclopentane rings, but many Archaea can synthesize isoGDGTs with up to 8 (!) cyclopentane rings (Fig. 2). In addition, thaumarchaeota, a phylum of Archaea, are unique because they synthesize crenarchaeol, an isoGDGT that contains a cyclohexane ring in addition to four cyclopentane rings (11).
a) Interest to paleoceanographers/paleoclimatologists
Paleoceanographers became interested in isoGDGTs at the start of the 21st century when the NIOZ-group led by Dr Stefan Schouten, building on early culture studies (13), demonstrated that the distribution of isoGDGTs in marine sediments is correlated to the overlying sea surface temperature (Fig. 3) and used this to developed the TEX86 temperature proxy (14). TEX86 reflects the relative distribution of the minor isoGDGTs: isoGDGT-1, isoGDGT-2, isoGDGT-3 and the crenarchaeol regioisomer. As in the modern ocean TEX86 is correlated to SSTs, it allows paleoceanograhpers to use the distribution of isoGDGTs in ancient sediments to estimate SSTs in the geological past. TEX86 is widely used in the paleoceanographic community and has successfully been applied to sediments as old as the Jurassic (140 million years ago).
b) Why temperature dependence
The general hypothesis is that the cyclopentane rings provide increased (thermal) stability due to denser packing of isoGDGTs, decreasing the permeability of the cell membrane (15). After decades of research it is now well established that Archaea synthesize isoGDGTs with different numbers of cyclopentane rings depending on environmental stressors, mainly pH and temperature, with a greater number of cyclopentane rings when stress is high (13, 14, 16-18). This has been shown in both natural samples and culture studies.
Different from isoGDGTs that were first discovered in culture experiments, brGDGTs were first discovered in natural samples, a Dutch peat to be precise (19). Based on their stereochemistry, which is different in Archaea and Bacteria, Bacteria have to be the source organism of brGDGTs (20). The presence of ether-based monolipids in Bacteria is surprising as most Bacteria use ester-bonds and bilayers. Although brGDGTs are ubiquitous in mesophillic settings, especially in soils and peats (10, 12, 21), the exact source organism(s) of brGDGTs has not yet been identified. However several lines of evidence suggest that it is likely that members of the Bacterial phylum Acidobacteria are the source of brGDGTs (22). As Archaea can do with isoGDGTs, the bacteria that produce brGDGTs can synthesize a range of GDGTs. They can add up to two cyclopentane rings and/or add up to two methyl groups (Fig. 4).
a) Interest to paleoceanographers/paleoclimatologists
brGDGTs are often the dominant GDGT in terrestrial setting such as soils and peats, being much more abundant then isoGDGTs produced by Archaea. As such the ratio of the three main bacterial brGDGTs over isoGDGT crenarchaeol, known as the BIT index (23), is often used in marine sediments to infer the relative contribution of terrestrial organic matter to aquatic organic matter (Fig. 5).
In addition, and analogous to isoGDGTs, the distribution of brGDGTs appears to be controlled by environmental stress. In soils the methylation and cyclisation of brGDGTs appears to be correlated to soil temperature and pH (21, 24). As such, brGDGTs are increasingly used to reconstruct past soil temperature and pH, constraining terrestrial temperatures as far back as 50 million years ago (25). However it is important to remember that because the source organism of brGDGTs is unknown, independent culture data to verify an environmental control on the brGDGTs distribution is lacking. In addition, several studies have shown that application of the soil calibration(s) to other environments such as peat, in which brGDGTs are very abundant, results in unrealistic temperatures (26), arguing for setting-specific calibrations.
- Analytical methods
In order to measure the distribution of GDGTs in natural samples (e.g. marine sediment or rocks), organic geochemists extract the organic matter from bulk sediment/rock using organic solvents and elevated temperatures (~ 70 oC). Typically a mixture of dichloromethane and methanol is used to extract the organic matter. The resulting total lipid extract (TLE), containing GDGTs as well as a range of other biomarkers, is normally further separated into different compound classes using open column flash chromatography. GDGTs typically end up in the polar fraction, which after the open column chromatography is filtered to remove fine particles. The distribution of GDGTs is then determined using high-pressure liquid chromatography mass spectrometery (HPLC-MS).
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