Fossil steroid acids can arise from microbial alteration of steranes

Steroid acids are unique molecular fossils composed of a steroid carbon skeleton linked to a carboxyl group. These biomarkers contain special geological and biological clues in their structures. However, the origin of steroid acids remains a subject of debate, and there is a lack of direct evidence in previous studies. Here, we present experimental evidence for the origin of steroid acids. The origin of steroid acids during biodegradation in crude oil was observed for the first time. Molecular structures of these steroid acids reveal that they arose from steranes via bacterial oxidation of sidechain terminal methyl groups. Our results suggest that microbial activities indeed contribute to the formation of carboxyl groups on the eukaryotic steroid skeletons, and we believe steroid acids are a promising biomarker for both geomicrobial processes and ancient eukaryotic life in the rock record.