Search for Preserved Methanogenic Organisms in Ophiolitic Rocks through Lipid Biomarker Analysis – Implications for Astrobiological Explorations

Abstract: Serpentinization is a metamorphic low-temperature geological process, involving a series of chemical reactions that hydrate ultramafic rocks into serpentines and other minerals such as magnetite and brucite. Moreover, the process produces heavily reducing and highly alkaline fluids together with molecular hydrogen (H2) and methane (CH4), which have the capacity to support chemosynthetic microbial life. Serpentine-hosted ecosystems are commonly associated with slow-spreading mid-ocean ridges although comparable sites have been discovered on land. This work presents one of the first lipid biomarker record of the terrestrial, active serpentinization system at the Chimaera Seeps in Turkey. Analysed rock samples were serpentinized peridotites with secondary-formed carbonate veins, where isoprenoidal lipid biomarkers associated with methanogenic archaea, archaeol and glycerol dialkyl glycerol tetraether 0 (GDGT-0), were identified. The presence of archaeol has already been reported in an earlier study of the site, while the obtained GDTG-0 is a novel entry in the lipid biomarker record of the Chimaera Seeps. Along with the archaeal lipidome, bacterial biomarkers were indicated, which most probably originated from sulphate-reducing bacteria. The findings strengthen the theory for the existence of archaeal methanogenesis at the onshore serpentinite-hosted seep sites and show similarities with the Samail Ophiolite, in the Sultanate of Oman, and the Lost City Hydrothermal Field (LCHF) near the mid- Atlantic Oceanic Ridge. Serpentine-hosted ecosystems on Earth serve as analogues to other terrestrial planets such as Mars and icy moons as Enceladus. Molecular studies of such environments can provide an insight into putative biosignatures that may be encountered on Mars and other celestial bodies in the Solar System. As lipids have the potential for a longer preservation compared to DNA and other easily-altering organics, lipid biomarker analysis can indeed be a more promising molecular tool for search of microbial activity in future astrobiological missions.