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Archaea produce unique membrane-spanning lipids (MSLs), termed glycerol dialkyl glycerol tetraethers (GDGTs), which aid in adaptive responses to various environmental challenges. GDGTs can be modified through cyclization, cross-linking, methylation, hydroxylation, and desaturation, resulting in structurally distinct GDGT lipids. Here, we report the identification of radical SAM proteins responsible for two of these modifications—a glycerol monoalkyl glycerol tetraether (GMGT) synthase (Gms), responsible for covalently cross-linking the two hydrocarbon tails of a GDGT to produce GMGTs, and a GMGT methylase (Gmm), capable of methylating the core hydrocarbon tail. Heterologous expression of Gms proteins from various archaea inThermococcus kodakarensisresults in the production of GMGTs in two isomeric forms. Further, coexpression of Gms and Gmm produces mono- and dimethylated GMGTs and minor amounts of trimethylated GMGTs with only trace GDGT methylation. Phylogenetic analyses reveal the presence of Gms homologs in diverse archaeal genomes spanning all four archaeal superphyla and in multiple bacterial phyla with the genetic potential to synthesize fatty acid–based MSLs, demonstrating that GMGT production may be more widespread than previously appreciated. We demonstrate GMGT production in three Gms-encoding archaea, identifying an increase in GMGTs in response to elevated temperature in twoArchaeoglobusspecies and the production of GMGTs with up to six rings inVulcanisaeta distributa.The occurrence of such highly cyclized GMGTs has been limited to environmental samples and their detection in culture demonstrates the utility of combining genetic, bioinformatic, and lipid analyses to identify producers of distinct archaeal membrane lipids. 

Studies reveal that the sea-surface temperature (SST) of the Northern Hemisphere decreased at a smaller amplitude than that of the Southern Hemisphere during the Eocene−Oligocene transition (EOT). This interhemispheric temperature asymmetry has been associated with intensified Atlantic Meridional Overturning Circulation (AMOC) that may have driven enhanced precipitation and weathering in low latitudes and the subsequent drawdown of atmospheric carbon dioxide. However, no quantitative constraints on paleo-precipitation have been reported in low latitudes to characterize the AMOC effect across the EOT. Here, we present the results of high-resolution (ca. 6 k.y. per sample) isotopic and biomarker records from the Gulf of Mexico. Reconstructed precipitation using leaf wax carbon isotopes shows an increase of 44% across the EOT (34.1−33.6 Ma), which is accompanied by a secular increase in SST of ∼2 °C during the latest Eocene. We attribute the enhanced precipitation in the Gulf of Mexico to the northward shift of the Intertropical Convergence Zone that was driven by an enlarged polar-tropic temperature gradient in the Southern Hemisphere and an invigorated AMOC. Our findings link changes in meridional temperature gradient and large-scale oceanic circulation to the low-latitude terrestrial hydroclimate and provide paleohydrological evidence that supports CO2-weathering feedback during the EOT “greenhouse” to “icehouse” transition. 

Glycerol dibiphytanyl glycerol tetraethers (GDGTs) are distinctive archaeal membrane-spanning lipids with up to eight cyclopentane rings and/or one cyclohexane ring. The number of rings added to the GDGT core structure can vary as a function of environmental conditions, such as changes in growth temperature. This physiological response enables cyclic GDGTs preserved in sediments to be employed as proxies for reconstructing past global and regional temperatures and to provide fundamental insights into ancient climate variability. Yet, confidence in GDGT-based paleotemperature proxies is hindered by uncertainty concerning the archaeal communities contributing to GDGT pools in modern environments and ambiguity in the environmental and physiological factors that affect GDGT cyclization in extant archaea. To properly constrain these uncertainties, a comprehensive understanding of GDGT biosynthesis is required. Here, we identify 2 GDGT ring synthases, GrsA and GrsB, essential for GDGT ring formation in Sulfolobus acidocaldarius . Both proteins are radical S-adenosylmethionine proteins, indicating that GDGT cyclization occurs through a free radical mechanism. In addition, we demonstrate that GrsA introduces rings specifically at the C-7 position of the core GDGT lipid, while GrsB cyclizes at the C-3 position, suggesting that cyclization patterns are differentially controlled by 2 separate enzymes and potentially influenced by distinct environmental factors. Finally, phylogenetic analyses of the Grs proteins reveal that marine Thaumarchaeota, and not Euryarchaeota, are the dominant source of cyclized GDGTs in open ocean settings, addressing a major source of uncertainty in GDGT-based paleotemperature proxy applications.