About this Research Topic
Diether- and tetraether lipids are widespread in Archaea and Bacteria. In particular, archaeal and bacterial glycerol dialkyl glycerol tetraethers (GDGTs) are different in structure, with the former having isoprenoidal hydrocarbon chains at the sn 2,3 positions between the glycerol backbones whereas the latter having alkyl chains at sn 1,2 positions. While archaeal diethers and GDGTs occur in diverse environmental settings, to date bacterial GDGTs have been identified predominately in the soil environment.
Both types of lipids have been used to develop temperature or environmental proxies that are increasingly popular in studies of paleo-climates or paleo-environments. For example, a sea surface temperature proxy called TEX86 (TetraEther Index of tetraethers having 86 carbons) has been developed based on the relative proportions of archaeal GDGTs that respond to changes in modern sea water temperature. TEX86 has also been used to estimate surface lake water temperatures in large lakes that receive minimal soil runoff. A branched and isoprenoidal tetraether index (BIT) has been developed based on the relative proportions of archaeal and bacterial GDGTs, which estimates the contribution of soil organic carbon to the ocean. The ratio of diether archaeol and tetraether caldarchaeol (a GDGT that does not contain any cyclopentyl ring) is known to indicate changes in salinity and may be a valuable proxy for research in paleohydrology and climate change. Based solely on bacterial GDGTs, the degree of cyclization of branched tetraethers (CBT) and the extent of methylation of branched tetraethers (MBT) have been used to estimate soil pH and continental air temperature, respectively.TEX86 has been applied primarily to marine environments and CBT/MBT proxies primarily to soil or lakes, with application to other systems, such as terrestrial hot springs, being far more limited. These proxies may also respond to changes in environmental variables other than pH or temperature, which, however, have barely been examined. Furthermore, the links between the production of archaeal and bacterial GDGTs and their biological sources are seldom explored. Lastly and most excitingly the development of new approaches is allowing us to identify and quantify the intact polar ether lipids, which will significantly enhance our capability to address the questions mentioned above.
The proposed Research Topic on archaeal and bacterial ether lipids from the terrestrial critical zone (soils, lakes, rivers, peat bogs, terrestrial hot springs, etc.) seeks to expand studies on the possible application of diether and tetraether proxies in diverse terrestrial environments, to enhance understanding of the mechanisms of environmental control on the distribution and abundance of diethers and tetraethers, and to identify the underlying biological sources of these compounds in natural systems.
Manuscripts to be included in this Research Topic will contain reviews, development of methods for improving archaeal or bacterial ether lipids quantification or identification, and research papers addressing the distribution, environmental control, or biological sources of archaeal and bacterial ethers in diverse terrestrial settings.
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