From the Biosphere to the Geosphere: Assessing Lipid Biomarker Taphonomy Through a Lithification Gradient

The alteration of biomass into simpler molecular remnants is relevant for the search for ancient and extraterrestrial life, where identifying recurrent taphonomic pathways is crucial for the attribution of biogenicity to otherwise nonbiological molecules. This work evaluates the alteration of lipids-recalcitrant biomarkers derived from cell membranes-across a lithification gradient, from a biologically active microbial mat, through a lithifying mat, to a fully lithified microbialite. Lipids from these samples, obtained from the high-altitude, hypersaline lake of Pozo Bravo (Argentinean Andes), were analyzed at molecular and isotopic levels to reconstruct biological sources and assess preservation along a bio-to-geo transition. Lipids from the lithifying mat and microbialite retained molecular features from the soft microbial mat (e.g., cyano- and purple sulfur bacteria), albeit at lower concentrations and diversity. Moreover, our analysis revealed preferential alteration of labile structural features such as unsaturations, methyl-, and pentacyclic structures, which decreased by ≥91% from soft to lithifying mat and ≥68% from lithifying mat to microbialite. Saturated and linear chains were more resistant, decreasing by ≥64% and ≥29%, respectively. These findings highlight how lipid preservation varies during lithification; thus, they provide valuable insights for biogenicity assessments and can help guide future efforts aimed at detecting ancient life.