Structural characterization of archaeal GDGT cyclization: Linking physiological adaptation to paleotemperature reconstruction

Glycerol dialkyl glycerol tetraethers (GDGTs), membrane lipids produced by archaea, have been widely utilized as biomarkers for paleotemperature reconstructions. While the relationship between GDGTs and temperature adaptation has been studied, the effects of structural modifications, specifically cyclopentane and cyclohexane rings, on membrane properties remains insufficiently understood. In this study, molecular dynamics simulations were employed to examine how these structural modifications influence GDGT membrance fluidity, with an emphasis on high-temperature adaptation in archaea. Our results demonstrate that an increasing number of cyclopentane rings is assoicated with reduced membrane fluidity, highlighting their role in facilitating high-temperature acclimation. Additionally, cyclohexane modifications in crenarchaeol, along with its isomerization, further reduce membrane fluidity. These findings indicate a clear link between lipid cyclization and thermal adaptation in archaea. Furthermore, the significant differences in membrane fluidity between GDGT-1 and GDGT-2 are consistent with the theoretical basis of the TEX₈₆ temperature proxy. Interestingly, while the cyclohexane modification of crenarchaeol in environmental samples suggests cold adaptation, this observation contrasts to findings from culture data and molecular dynamic simulations, suggesting the influence of additional factors. Based on these insights, we propose a novel sea surface temperatures reconstruction metric, TEX₈₆^MD, which enhances the accuracy of the TEX₈₆ proxy, and provides broader global applicability, especially in the polar regions.