Lipid biomarkers and their indications in the hydrothermal iron oxides of the Indian Ocean Ridge

Submarine hydrothermal systems provide a valuable window into crustal biogeochemical transformations and microbial metabolic pathways. However, the preservation potential of lipid biomarkers in hydrothermal iron oxide (oxyhydroxide) deposits is not well understood. Here, we aimed to provide insights into the distribution of core lipids (CLs) and intact polar lipids (IPLs) as biomarkers of microbial communities that are associated with biogenic Fe oxides and vent sulfides that were collected from the Longqi hydrothermal field in the Southwest Indian Ridge and the Edmond and Kairei hydrothermal fields in the Central Indian Ridge. Our results show that a fraction of the organic carbon may have been derived in situ and this correlates with chemoautotrophic communities that were supported by vent fluids based on the analysis of the 16S ribosomal RNA genes and lipid biomarkers, including archaeal-CLs, archaeal-IPLs, and bacterial branched glycerol dialkyl glycerol tetraether lipids (brGDGTs). The local origin of the lipid biomarkers is also supported by the comparison of the different geological environments. The distribution and index characteristics of the biomarkers suggest that archaea may contribute more to lipid pools than bacteria. Differences in the major compositional contents of the archaeal-CLs, archaeal-IPLs, and brGDGTs were observed among the vent fields. The relationship between the geochemical signatures and biomarkers suggests that the microbial communities and diagenetic environments of iron oxide deposits may play an important role in the production and preservation of specific lipid compounds. Our results provide unique insights into the community composition that is associated with geologically significant biomarkers in modern and ancient hydrothermal environments and information on environmental evolution that was recorded in the depositional record of the oceanic crust.