Detection of Microbial Lipids as Life Markers and Their Response to Increasing Temperatures in the Nankai Trough Subduction Zone

Abstract:

Microbial life has been found under the most adverse conditions posing the question under which conditions life ceases to exist. Exploration pertaining to the limits of life requires the development of ultra-senstitive detection techniques. In this study, we employed targeted lipid biomarker searches to explore the presence of intact polar lipids (IPL) as life markers for sedimentary microbial communities and their remnant fossil core lipids (CL) at Site C0023 (established during IODP Exp. 370) in the Nankai Trough subduction zone. At this location, temperatures at the bottom of the retrieved core at 1170 m reach values close to the known limit of life of ca. 120°C. We observe distinct IPL and CL patterns that respond to biogeochemical processes and to temperature regimes at Site C0023. Within the methanogenic zone, where temperatures transition from mesophilic to thermophilic conditions, we detected lipid biomarkers characteristic for sedimentary methanogens and unclassified heterotrophic archaea. These include glycerol, butanetriol and pentanetriol dialkyl glycerol tetraethers (GDGT, BDGT, PDGT) and archaeol with glycosidic headgroups. In this zone, elevated IPL concentrations compared to the detected cell counts suggest substantial accumulation of fossil IPL over time at temperatures of ca. 45 to 50°C. A stark decline in both IPL and CL pools below this depth coincides with the onset of the catagenic zone and the subsequent thermal breakdown of organic matter. IPL and CL 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 IPL provide a time-integrated signal on microbial community distributions and provides insights into abiotic processes affecting their preservation.