TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Nitrososphaerota (formerly Marine Group 1 Crenarchaeota). [2] [3]
TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Nitrososphaerota (formerly Marine Group 1 Crenarchaeota). [2] [3]
The membrane lipids of Nitrososphaerota are composed of glycerol dialkyl glycerol tetraethers (GDGTs) which contain 0-3 cyclopentane moieties (commonly annotated as GDGT-n where n = numbers of cyclopentane moieties). Nitrososphaerota also synthesise crenarchaeol (cren) which contains four cyclopentane moieties and a single cyclohexane moiety and a regio-isomer (cren'). The cyclohexane and cyclopentane rings, formed by internal cyclisation of one of the biphytane chains, [4] have a pronounced effect on the thermal transition points of the Nitrososphaerota cell membrane. Mesocosm studies demonstrate that the degree of cyclisation is generally governed by growth temperature. [5]
Based upon the relative distribution of isoprenoidal GDGTs, Schouten et al. (2002) [6] proposed the tetraether index of 86 carbon atoms (TEX86) as a proxy for sea surface temperature (SST). GDGT-0 is excluded from the calibration as it can have multiple sources [7] while cren is omitted as it exhibits no correlation with SST and is often an order of magnitude more abundant than its isomer and the other GDGTs. The most recent TEX86 calibration invokes two separate indices and calibrations: [8] TEX86H uses the same combination of GDGTs as in the original TEX86 relationship:
GDGT ratio-2 is correlated to SST using the calibration equation:
TEX86H has a calibration error of ±2.5 °C and is based upon 255 core-top sediments.
TEX86L employs a combination of GDGTs that is different from TEX86H, removing GDGT-3 from the numerator and excluding cren’ entirely:
GDGT ratio-1 is correlated to SST using the calibration equation:
TEX86Lhas a calibration error of ±4 °C and is based upon 396 core-top sediment samples.
Other calibrations exist (including 1/TEX86, [9] TEX86' [10] and pTEX86 [11] ) and should be considered when reconstructing temperature.
There are several caveats to this proxy and this list is by no means exhaustive. For more information, consult [12]
The branched vs isoprenoidal tetratether (BIT) index can used to measure the relative fluvial input of terrestrial organic matter (TOM) into the marine realm. [13] The BIT index is based upon the premise that crenarchaeol is derived from marine-dwelling Nitrososphaerota and branched GDGTs are derived from terrestrial soil bacteria. When BIT values exceed 0.4, a deviation of >2 °C is incorporated into TEX86-based SST estimates. However, isoprenoidal GDGTs can be synthesised in the terrestrial environment and can render BIT values unreliable (Weijers et al., 2006; [14] Sluijs et al., 2007; Xie et al., 2012). A strong co-variation between GDGT-4 and branched GDGTs in modern marine and freshwater environments also suggests a common or mixed source for isoprenoidal and branched GDGTs (Fietz et al., 2012).
The Methane Index (MI) was proposed to help distinguish the relative input of methanotrophic Euryarchaeota in settings characterised by diffuse methane flux and anaerobic oxidation of methane (AOM). [15] These sites are characterised by a distinct GDGT distribution, namely the predominance of GDGT-1. -2 and -3. High MI values (>0.5) reflect high rates of gas-hydrate-related AOM.
Thermal maturity is only thought to affect GDGTs when temperature exceed 240 °C. This can be tested using a ratio of specific hopane isomers. Oxic degradation, which is a selective process and degrades compounds at different rates, has been shown to affect TEX86 values and can bias SST values by up to 6 °C.
The oldest TEX86 record is from the middle Jurassic (~160Ma) and indicates relatively warm sea surface temperatures. [16] TEX86 has been used to reconstruct temperature throughout the Cenozoic era (65–0 Ma) [17] [18] and is useful when other SST proxies are diagenetically altered (e.g. planktonic foraminifera [19] ) or absent (e.g. alkenones [20] ).
TEX86 has been extensively used to reconstruct Eocene (55-34Ma) SST. During the early Eocene, TEX86 values indicate warm high southern hemisphere latitude SSTs (20-25 °C) in agreement with other, independently derived proxies (e.g. alkenones, CLAMP, Mg/Ca). During the middle and late Eocene, high southern latitude sites cooled while the tropics remained stable and warm. Possible reasons for this cooling include long-term changes in carbon dioxide and/or changes in gateway reorganisation (e.g. Tasman Gateway, Drake Passage).
The Eocene Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name Eocene comes from the Ancient Greek ἠώς and καινός and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.
The Paleocene–Eocene thermal maximum (PETM), alternatively "Eocene thermal maximum 1" (ETM1), and formerly known as the "Initial Eocene" or "Late Paleocene thermal maximum", was a time period with a more than 5–8 °C global average temperature rise across the event. This climate event occurred at the time boundary of the Paleocene and Eocene geological epochs. The exact age and duration of the event is uncertain but it is estimated to have occurred around 55.5 million years ago (Ma).
In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between 56 and47.8 Ma, is preceded by the Thanetian Age and is followed by the Eocene Lutetian Age. The Ypresian is consistent with the lower Eocene.
Phytane is the isoprenoid alkane formed when phytol, a constituent of chlorophyll, loses its hydroxyl group. When phytol loses one carbon atom, it yields pristane. Other sources of phytane and pristane have also been proposed than phytol.
Membrane lipids are a group of compounds which form the lipid bilayer of the cell membrane. The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol. Lipids are amphiphilic: they have one end that is soluble in water ('polar') and an ending that is soluble in fat ('nonpolar'). By forming a double layer with the polar ends pointing outwards and the nonpolar ends pointing inwards membrane lipids can form a 'lipid bilayer' which keeps the watery interior of the cell separate from the watery exterior. The arrangements of lipids and various proteins, acting as receptors and channel pores in the membrane, control the entry and exit of other molecules and ions as part of the cell's metabolism. In order to perform physiological functions, membrane proteins are facilitated to rotate and diffuse laterally in two dimensional expanse of lipid bilayer by the presence of a shell of lipids closely attached to protein surface, called annular lipid shell.
Nitrosopumilus maritimus is an extremely common archaeon living in seawater. It is the first member of the Group 1a Nitrososphaerota to be isolated in pure culture. Gene sequences suggest that the Group 1a Nitrososphaerota are ubiquitous with the oligotrophic surface ocean and can be found in most non-coastal marine waters around the planet. It is one of the smallest living organisms at 0.2 micrometers in diameter. Cells in the species N. maritimus are shaped like peanuts and can be found both as individuals and in loose aggregates. They oxidize ammonia to nitrite and members of N. maritimus can oxidize ammonia at levels as low as 10 nanomolar, near the limit to sustain its life. Archaea in the species N. maritimus live in oxygen-depleted habitats. Oxygen needed for ammonia oxidation might be produced by novel pathway which generates oxygen and dinitrogen. N. maritimus is thus among organisms which are able to produce oxygen in dark.
The Azolla event is a scenario hypothesized to have occurred in the middle Eocene epoch, around 49 million years ago, when blooms of the freshwater fern Azolla are thought to have happened in the Arctic Ocean. As they sank to the stagnant sea floor, over a period of about 800,000 years, they were incorporated into the sediment; the resulting draw-down of carbon dioxide has been speculated to have helped transform the planet from a "greenhouse Earth" state, hot enough for turtles and palm trees to prosper at the poles, to the current icehouse Earth known as the Late Cenozoic Ice Age.
Caldarchaeol is a membrane-spanning lipid of the glycerol dialkyl glycerol tetraether class. It is 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 also known as dibiphytanyldiglycerol tetraether. Two glycerol units are linked together by two strains which consist of two phytanes linked together to form a linear chain of 32 carbon atoms.
Fuselloviridae is a family of viruses. Sulfolobus species, specifically shibatae, solfataricus, and islandicus, serve as natural hosts. There are two genera and nine species in the family. The Fuselloviridae are ubiquitous in high-temperature (≥70 °C), acidic hot springs around the world.
Archaeol is composed of two phytanyl chains linked to the sn-2 and sn-3 positions of glycerol. As its phosphate ester, it is a common component of the membranes of archaea.
The Nitrososphaerota 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 Thermoproteota. Three described species in addition to C. symbiosum are Nitrosopumilus maritimus, Nitrososphaera viennensis, and Nitrososphaera gargensis. The phylum was proposed in 2008 based on phylogenetic data, such as the sequences of these organisms' ribosomal RNA genes, and the presence of a form of type I topoisomerase that was previously thought to be unique to the eukaryotes. This assignment was confirmed by further analysis published in 2010 that examined the genomes of the ammonia-oxidizing archaea Nitrosopumilus maritimus and Nitrososphaera gargensis, concluding that these species form a distinct lineage that includes Cenarchaeum symbiosum. The lipid crenarchaeol has been found only in Nitrososphaerota, making it a potential biomarker for the phylum. Most organisms of this lineage thus far identified are chemolithoautotrophic ammonia-oxidizers and may play important roles in biogeochemical cycles, such as the nitrogen cycle and the carbon cycle. Metagenomic sequencing indicates that they constitute ~1% of the sea surface metagenome across many sites.
Eocene Thermal Maximum 2 (ETM-2), also called H-1 or the Elmo event, was a transient period of global warming that occurred around either 54.09 Ma or 53.69 Ma. It appears to be the second major hyperthermal that punctuated the long-term warming trend from the Late Paleocene through the early Eocene.
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This article discusses the Unique properties of hyperthermophilic archaea. Hyperthermophiles are organisms that can live at temperatures ranging between 70 and 125 °C. They have been the subject of intense study since their discovery in 1977 in the Galapagos Rift. It was thought impossible for life to exist at temperatures as great as 100 °C until Pyrolobus fumarii was discovered in 1997. P. fumarii is a unicellular organism from the domain Archaea living in the hydrothermal vents in black smokers along the Mid-Atlantic Ridge. These organisms can live at 106 °C at a pH of 5.5. To get energy from their environment these organisms are facultatively aerobic obligate chemolithoautotrophs, meaning these organisms build biomolecules by harvesting carbon dioxide (CO2) from their environment by using hydrogen (H2) as the primary electron donor and nitrate (NO3−) as the primary electron acceptor. These organisms can even survive the autoclave, which is a machine designed to kill organisms through high temperature and pressure. Because hyperthermophiles live in such hot environments, they must have DNA, membrane, and enzyme modifications that help them withstand intense thermal energy. Such modifications are currently being studied to better understand what allows an organism or protein to survive such harsh conditions. By learning what lets these organisms survive such harsh conditions, researchers can better synthesize molecules for industry that are harder to denature.
Nitrososphaera gargensis is a non-pathogenic, small coccus measuring 0.9 ± 0.3 μm in diameter. N. gargensis is observed in small abnormal cocci groupings and uses its archaella to move via chemotaxis. Being an Archaeon, Nitrososphaera gargensis has a cell membrane composed of crenarchaeol, its isomer, and a distinct glycerol dialkyl glycerol tetraether (GDGT), which is significant in identifying ammonia-oxidizing archaea (AOA). The organism plays a role in influencing ocean communities and food production.
Jessica E. Tierney (born 1982) is an American paleoclimatologist who has worked with geochemical proxies such as marine sediments, mud, and TEX86, to study past climate in East Africa. Her papers have been cited more than 2,500 times; her most cited work is Northern Hemisphere Controls on Tropical Southeast African Climate During the Past 60,000 Years. Tierney is currently an associate professor of geosciences and the Thomas R. Brown Distinguished Chair in Integrative Science at the University of Arizona and faculty affiliate in the University of Arizona School of Geography, Development and Environment Tierney is the first climatologist to win NSF's Alan T Waterman Award (2022) since its inception in 1975.
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 eukaryotes because they contain isoprenoid GDGTs instead of diacyl lipids, which are found in the other domains. It has been proposed that GDGT membrane lipids are an adaptation to the high temperatures present in the environments that are home to extremophile archaea
Paula Veronica Welander is a microbiologist and professor at Stanford University who is known for her research using lipid biomarkers to investigate how life evolved on Earth.
Glycerol dialkyl glycerol tetraether lipids (GDGTs) are a class of membrane lipids synthesized by archaea and some bacteria, 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 paleothermometer. Isoprenoid GDGTs, now known to be synthesized by many archaeal classes, were first discovered in extremophilic archaea cultures. Branched GDGTs, likely synthesized by acidobacteriota, were first discovered in a natural Dutch peat sample in 2000.
Biphytane (or bisphytane) is a C40 isoprenoid produced from glycerol dialkyl glycerol tetraether (GDGT) degradation. As a common lipid membrane component, biphytane is widely used as a biomarker for archaea. In particular, given its association with sites of active anaerobic oxidation of methane (AOM), it is considered a biomarker of methanotrophic archaea. It has been found in both marine and terrestrial environments.
