Reappraisal of archaeal C20-C25 diether lipid (extended archaeol) origin and use as a biomarker of hypersalinity

•Extended archaeol (C20-C25) occurs in different saline aquatic settings.•C20-C25 is not restricted to extreme saline environments.•C20-C25 can be used as a biomarker of hypersalinity down to ca 50 psu (5% NaCl w/v).•Haloarchaea of the order Natrialbales are the main source of C20-C25.•Taxonomy and adaptation to salinity and pH may explain C20-C25 production. The diether core membrane lipid sesterterpanyl-phytanyl-glycerol (so-called extended archaeol and often abbreviated C20-C25) is considered as a hallmark of Haloarchaea, a clade of archaea thriving under extreme high salinities. We here report about extended archaeol occurrence in different saline aquatic settings with salinity ranging from ca. 50 psu (5 % NaCl w/v) to saturation (ca. 350 psu). This demonstrates that this lipid is not restricted to extreme saline environments but suggests a minimum salinity threshold of ca. 50 psu above which C20-C25 is most commonly produced. The proportion of C20-C25 relative to that of archaeol (C20-C20) did not appear linearly dependent on the salinity of the site and was potentially also influenced by pH and temperature, preventing its direct use as a quantitative salinity proxy based on the present data set. An extensive literature review of archaeal membrane lipid compositions further highlighted that taxonomy also contributes to the distribution of this lipid in the environment and identifies Natrialbales (one of the three orders of Haloarchaea) as the main source. Statistical analysis showed that, among Haloarchaea, C20-C25 producers display pH and salinity growth optima slightly higher than non-producers and are distributed within two distinct groups, one composed mostly of neutrophiles and one of alkaliphiles. In contrast, the presence of C20-C25 was not correlated to the optimal growth temperature of the strains. This suggests that two confounding parameters, i.e., taxonomy and adaptation to changes in salinity and/or pH, contribute to the distribution of C20-C25 within Haloarchaea.