Hydrogen and reduced carbon compounds generated during the hydration of iron-rich ultramafic rock (“serpentinization”) are commonly considered as sustained sources of reductants for microbial energy metabolism on early Earth, and possibly on other rocky extraterrestrial bodies. However, the availability of oxidants and nutrients for life in serpentinizing settings is likely limiting, particularly in the subsurface. In collaboration with the Oman Drilling Project and the “Rock-Powered Life” team, I have had the unique opportunity to collect deep biomass from fluid and rock in the subsurface of a serpentinite aquifer in Oman to (1) study how life could function near its energetic limit and (2) what robust biosignatures may be produced due to biogeochemical activity. Microbial community composition in serpentinite-hosted fluids, as inferred by 16S amplicon gene sequencing, was strongly correlated with fluid geochemistry, likely driven by differential energy availability that results from fundamental differences in both host rock lithology and the extent of water-rock reaction. Microbial richness corresponded with greater concentrations of nitrate in shallow fluids; this nitrate is attributed to primarily an atmospheric origin due to its relict meteoric oxygen isotopic signature (δ18O ~ 22‰, Δ17O ~ 6‰). Groundwater nitrate was subsequently reduced to ammonium in deeper fluids, retaining essential fixed nitrogen in the subsurface. I conducted an intact polar lipid (IPL) analysis of the microbial biomass extracted from rocks and fluids in order to identify organic signatures that may be representative of microbial communities active under the diverse environmental conditions. To identify IPLs produced by microorganisms functioning under extreme conditions, I established a theoretical database of >106 IPL compounds which enabled me to identify a predominance of unusual glyco- and amino-lipids in subsurface fluids that I interpret as representing a physiological adaptation to phosphate limitation. Despite contamination introduced during drilling, robust (sample/contamination ratio of >100) signatures of non-isoprenoidal ether-linked glycolipids likely produced by endemic sulfate-reducing bacteria were detected in intervals of serpentinite rock core characterized by sulfide mineralization, thus providing a promising target for future efforts to detect organic biosignatures preserved in serpentinite-hosted systems, such as those potentially left on early Earth, Mars, or similar planetary systems.</p
