Highly branched isoprenoid

Highly branched isoprenoids (HBIs) are long-chain alkenes produced by a small number of marine diatoms. There are a variety of highly branched isoprenoid structures, but C25 Highly branched isoprenoids containing one to three double bonds are the most common in the sedimentary record. Highly branched isoprenoids have been used as environmental proxies for sea ice conditions in the Arctic and Antarctic throughout the Holocene, and more recently, are being used to reconstruct more ancient ice records.

Background Highly branched isoprenoids are a type of lipid produced by marine diatoms. Highly branched isoprenoids are biomarkers, and their presence or absence in sedimentary and ice records can be used as a direct proxy for the presence of sea ice. Generally, the highly branched isoprenoids that are used as sea ice proxies are 25-carbon molecules containing one to three double bonds. The longest carbon chains in the C25 highly branched isoprenoids used for sea ice reconstructions are 15 carbons, but these molecules are highly branched and have shorter carbon chains attached to the primary carbon chain. There are 3 C25 highly branched isoprenoids used as ice proxies: a C25 monoene (HBI I), a C25 diene (HBI II), and a C25 triene (HBI III). Highly branched isoprenoid I and II are unique in that they are primarily produced by sympagic diatoms. Sympagic diatoms live in channels at the base of sea ice, making them a highly accurate proxy for sea ice. During the spring, highly branched isoprenoids are produced by diatoms in the sea ice. In the summer, the ice melts, releasing the highly branched isoprenoids into the water column, where they sink and are then deposited in the sediments. Highly branched isoprenoids I and II are generally absent from regions which experience no sea ice cover, supporting their use as a proxy for seasonal sea ice. Highly branched isoprenoid III is produced by pelagic algae, or algae that thrives in the open ocean. Highly branched isoprenoid III can be used as a biomarker for seasonal sea ice in the open ocean. Highly branched isoprenoids were first discovered in 1976 by Patrick Gearing in sediments in the Gulf of Mexico off the coast of Florida in a survey of hydrocarbons in shelf sediments. Following this initial identification, highly branched isoprenoids were identified in a variety of marine environments, such as in the Puget Sound, Antarctica, Spain, and Peru. C25 Highly branched isoprenoids were first identified in marine diatoms by John Volkman in 1994, when he isolated seven different C25 highly branched isoprenoids from the marine diatom Haslea ostrearia. This provided initial evidence that highly branched isoprenoids are produced by marine diatoms. Currently, the precise biological functions of highly branched isoprenoids are not well-understood. Highly branched isoprenoids are a type of isoprenoid lipid, which have a variety of vital biological functions. Isoprenoids are important in regulating gene expression, making up cell membranes, and are important in electron transport and photosynthesis.

Highly branched isoprenoid I

Highly branched isoprenoid (HBI I), a C25 monoene, is also known as IP25 (ice proxy with 25 carbon atoms). IP25 serves as a biomarker for ice conditions in the Arctic. This highly branched isoprenoid is characterized by a single double bond and was first identified in marine diatoms by Thomas Brown in 2014. A variety of diatoms have been shown to produce IP25, with the majority of highly branched isoprenoids produced by the Arctic diatoms Haslea crucigeroides, Haslea spicula, Haslea kjellmanii, and Pleurosigma stuxbergii var. rhomboids. Despite the fact that these species do not comprise much of the sympagic diatom communities globally, they are common in the Arctic. To date, IP25 has been identified in over 500 Arctic samples.

Highly branched isoprenoid II

Highly branched isoprenoid II (HBI II), a C25 diene, is also known as IPSO25 (ice proxy for the Southern Ocean with 25 carbon atoms). IPSO25 is a biomarker proxy for paleo ice in the Southern Ocean. IPSO25 has also been found to co-occur with IP25 in the Arctic. HBI II contains two double bonds and was first identified in marine diatoms by Simon Belt in 2016. The primary source of IPSO25 the sympagic diatom Berkeleya adeliensis, which lives within platelet ice. The IPSO25 proxy is a less-developed biomarker than IP25, and its Arctic sources are unclear. IPSO25 has also been identified in the diatom Haslea ostrearia and in sediments in non-polar locations, indicating that more work is needed to fully understand and develop IPSO25 as a paleo ice proxy.

Highly branched isoprenoid III

Highly branched isoprenoid III (HBI III), a C25 triene, is a biomarker useful for the analysis of the marginal ice zone (MIZ), a zone between the open ocean and sea ice. Highly branched isoprenoid III is primarily produced by pelagic algae of the genus Rhizosolenia, particularly Rhizsolenia setigera, Rhizosolenia herbetata f. semispina, and Rhizosolenia polydactyla. Its source was determined by Simon Belt in 2017, who isolated highly branched isoprenoid III from phytoplankton samples from western Svalbard and the South Atlantic. The production of highly branched isoprenoid III appears to be enhanced in the MIZ, however why this occurs is currently not well-understood. The absence of highly branched isoprenoid III in sediments is typically attributed to sea ice cover in the region, given that ice cover would not allow for pelagic algae production.

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