Caldarchaeol


Caldarchaeol is a membrane-spanning lipid of the isoprenoid glycerol dialkyl glycerol tetraether class, produced and used by archaea. Membranes made up of caldarchaeol are more stable since the hydrophobic chains are linked together, allowing archaea to withstand extreme conditions.

Chemical Structure

Caldarchaeol is also known as dibiphytanyl diglycerol tetraether, or GDGT-0. Two glycerol units are linked together by two biphytanes, each of which consist of two phytanes linked together to form a linear chain of 32 carbon atoms.
The configuration of the macrocyclic tetraether has been determined by total synthesis of the C40-diol and comparison with a sample obtained by degradation of natural tetraether. A synthesis of tetraether has also been carried out.
Caldarchaeol is not currently described as having any hazards. Due to its high molecular weight, it is neither volatile nor flammable. Caldarchaeol and other GDGTs are present across environments at low concentrations, and no adverse affects or evidence of toxicity are known.

Nomenclature

for archaeal lipids is widely varied across history and fields, and caldarchaeol is no exception. It had originally been defined as dibiphytanyl diglycerol tetraether, a large lipid molecule with two biphytane chains, with or without cyclopentane rings, connected by ethers to glycerols on either end. It is used to describe the entire class of isoprenoid GDGTs in many papers, both historical and recent. However, as GDGTs began to be incorporated into paleoclimate investigations, many began defining caldarchaeol specifically as GDGT-0, the isoprenoid GDGT with no cyclopentane moieties, especially when this specific structure is used in the analysis.

Biosynthesis

The biosynthesis of caldarchaeol and other iGDGTs has been the subject of investigation for decades, due both to the complexity of the pathway and the difficulty of culturing archaea in laboratory settings. Isoprenoid-based molecules are synthesized by all three domains of life using isopentyl pyrophosphate and dimethylallyl pyrophosphate, 5-carbon structural isomers. Archaea make archaeol from these building blocks, which is then condensed into tetraether structures using a radical S-adenosylmethionine protein called tetraether synthase.

Biomarker Applications

Caldarchaeol is a widely distributed lipid across archaea, making it a relatively poor biomarker for specific taxa within the domain. However, comparisons between caldarchaeol concentrations and other biomarkers are frequently used to reveal community composition and/or paleoclimate proxies.

Caldarchaeol to Crenarchaeol Ratio

Formula: Caldarchaeol/Crenarchaeol
Formula: TEX86 = /
  • The number of rings in iGDGT structures has been correlated to mean annual sea surface temperature
  • Used frequently since originally described
  • Rather than directly being determined by temperature, the iGDGT ring number has since been found to actually be a function of NH3 oxidation rates from microbial growth, which is influenced by temperature
  • This metric does not include a term for caldarchaeol due its synthesis by methanogens as well as the crenarchaeota that are meant to be impacting this proxy; however, methanogen growth are still affected by temperature
  • * To clarify this consideration and quantify the contribution of caldarchaeol to the iGDGT pool, some studies have compared TEX86 to the Ring Index
  • ** RI = 0* + 1* + 2* + 3* + 4* + 4*
  • ** While the caldarchaeol term is multiplied by 0, it impacts the RI value through its impact on the total iGDGT pool and calculations of relative concentrations for other structures

    Archaeol and Caldarchaeol Ecometric (ACE)

Formula: ACE = 100 ×
Category:Lipids
Category:Macrocycles