The impact of increasing temperature on microbial lipid distributions in the Nankai Trough subduction zone, IODP Exp. 370

Site C0023 in the Nankai Through subduction zone was established in 2016 to investigate the temperature limits of deep subsurface life. Here, we report on the detection of intact polar lipids and their degradation products in sediments recovered from this site where temperatures reach values close to the known limit of life of ca. 120°C at the bottom of the core at 1170 m. Lipid biomarkers characteristic for sedimentary methanogens and unclassified heterotrophic archaea were detected within the methanogenic zone where temperatures transition from mesophilic to thermophilic conditions. These include glycerol, butanetriol and pentanetriol dialkyl glycerol tetraethers (GDGT, BDGT, PDGT) and archaeol with glycosidic headgroups. In this zone, elevated intact polar lipid concentrations compared to vegetative cell numbers suggest substantial accumulation of fossil lipids over time at temperatures of ca. 45 to 50°C. A stark decline in both intact lipids and their degradation products below this depth coincides with the onset of the catagenic zone and the subsequent thermal breakdown of organic matter. The detected lipids are degraded at different reaction rates, reflecting their different thermal stabilities. We also observe a selective removal of GDGTs according to the number of rings in their core structure with important implications for the application of these compounds as paleoenvironmental proxies. This study suggests that intact polar lipids provide a time-integrated signal on microbial community distributions and provides insights into abiotic processes affecting their preservation.