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1. Marine Cold Seep ANME ‐2/ SRB Consortia Produce Their Lipid Biomass From Inorganic Carbon

   https://doi.org/10.1111/1462-2920.70213  

   Stock, Lennart; Wegener, Gunter; Wang, Yueqing; Zander, Yannick; Elvert, Marcus (December 2025, Environmental Microbiology)

   ABSTRACT In cold seeps, anaerobic methanotrophic archaea (ANME) and sulphate‐reducing bacteria (SRB) oxidise methane to inorganic carbon (IC) coupled to sulphate reduction. While catabolic pathways are well resolved, carbon flow into biomass as well as the functional roles of lipid biomarkers remain unclear. We conducted lipid stable isotope probing (lipid‐SIP) experiments with Astoria Canyon sediments dominated by ANME‐2/SRB consortia and incubated samples with either¹³C‐labelled methane (¹³CH₄) or dissolved IC (DI¹³C). Lipid‐specificδ¹³C analysis showed higher¹³C incorporation from DI¹³C than from¹³CH₄. After 30 days,δ¹³C values were up to +417‰ in SRB‐specific fatty acids (e.g., C_16:1ω5c, cyC_17:0ω5,6) and +126‰ in ANME‐2‐specific isoprenoid lipids (e.g., archaeol, crocetane). Based on these values, we calculated carbon assimilation rates and found that both partners primarily assimilate IC. Remarkably, IC assimilation in SRB lipids was eight times higher than in ANME lipids, suggesting that ANME may use additional yet‐to‐be‐identified carbon sources, potentially produced by their partner SRB. By examining the stepwise¹³C‐enrichment of ANME‐ and SRB‐derived lipids, we further delineate biosynthetic pathways for archaeal and bacterial diether lipid formation and highlight crocetane as a bilayer‐modulating isoprenoid hydrocarbon potentially affecting membrane fluidity and proton permeability. 

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2. Identification of two archaeal GDGT lipid–modifying proteins reveals diverse microbes capable of GMGT biosynthesis and modification

   https://doi.org/10.1073/pnas.2318761121  

   Garcia, Andy A; Chadwick, Grayson L; Liu, Xiao-Lei; Welander, Paula V (June 2024, Proceedings of the National Academy of Sciences)

   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. 

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3. GDGT cyclization proteins identify the dominant archaeal sources of tetraether lipids in the ocean

   https://doi.org/10.1073/pnas.1909306116  

   Zeng, Zhirui; Liu, Xiao-Lei; Farley, Kristen R.; Wei, Jeremy H.; Metcalf, William W.; Summons, Roger E.; Welander, Paula V. (November 2019, Proceedings of the National Academy of Sciences)

   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. 

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4. Membrane lipid and expression responses of Saccharolobus islandicus REY15A to acid and cold stress

   https://doi.org/10.3389/fmicb.2023.1219779  

   Chiu, Beverly K.; Waldbauer, Jacob; Elling, Felix J.; Mete, Öykü Z.; Zhang, Lichun; Pearson, Ann; Eggleston, Erin M.; Leavitt, William D. (August 2023, Frontiers in Microbiology)

   Archaea adjust the number of cyclopentane rings in their glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipids as a homeostatic response to environmental stressors such as temperature, pH, and energy availability shifts. However, archaeal expression patterns that correspond with changes in GDGT composition are less understood. Here we characterize the acid and cold stress responses of the thermoacidophilic crenarchaeonSaccharolobus islandicusREY15A using growth rates, core GDGT lipid profiles, transcriptomics and proteomics. We show that both stressors result in impaired growth, lower average GDGT cyclization, and differences in gene and protein expression. Transcription data revealed differential expression of the GDGT ring synthasegrsBin response to both acid stress and cold stress. Although the GDGT ring synthase encoded bygrsBforms highly cyclized GDGTs with ≥5 ring moieties,S. islandicus grsBupregulation under acidic pH conditions did not correspond with increased abundances of highly cyclized GDGTs. Our observations highlight the inability to predict GDGT changes from transcription data alone. Broader analysis of transcriptomic data revealed thatS. islandicusdifferentially expresses many of the same transcripts in response to both acid and cold stress. These included upregulation of several biosynthetic pathways and downregulation of oxidative phosphorylation and motility. Transcript responses specific to either of the two stressors tested here included upregulation of genes related to proton pumping and molecular turnover in acid stress conditions and upregulation of transposases in cold stress conditions. Overall, our study provides a comprehensive understanding of the GDGT modifications and differential expression characteristic of the acid stress and cold stress responses inS. islandicus. 

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5. Differential effects of lysophospholipid headgroups, acyl chain length and saturation on vacuole acidification, Ca2+ transport, membrane fluidity, and fusion

   https://doi.org/10.3389/fcell.2026.1805164  

   Zhang, Chi; Feng, Yilin; Calderin, Jorge D; Failla, Vito; Balutowski, Adam; Ahmed, Razeen; Knapp, Charlie; Shah, Ved; Grudzien, Daniel; Karat, Jahnavi M; et al (April 2026, Frontiers in Cell and Developmental Biology)

   SNARE-mediated membrane fusion is regulated by the lipid composition of the engaged bilayers. Lipids impact fusion through direct protein-lipid interactions or through modulating the physical properties of membranes to affect protein function. Lysophospholipids (LPLs) can affect membrane curvature, fluidity and energy of deformation. Their effects are due to their head group, and the length and saturation of their single acyl chains. Here we examined how the properties of LPLs affect yeast vacuole fusion, membrane fluidity and ion transport. We found that lysophosphatidylcholine (LPC) with 14–18 carbon acyl chains inhibited fusion with IC₅₀values of ≅ 40–120 µM. While acyl chain length moderately affected fusion, the head group played a major role. Unlike LPCs, Lysophosphatidic acid (LPA 18:1) failed to fully inhibit fusion, while lysophosphatidylethanolamine (LPE 18:1) had no effect. Unlike fusion, fluidity was sensitive to head group type. Fluidity was significantly decreased by LPA 18:1 but not LPCs. Separately we found that changes in acyl chain length and saturation differentially affected Ca²⁺transport and vacuole acidification. Together these data show that the effects of LPLs on membrane fusion, fluidity, and ion transport were due to a combination of head group type and acyl chain length and saturation. 

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