Temporal Stability and Implications of Phospholipid Biosignatures for Subsurface Microbial Communities

Abstract

Through study of microorganisms living within the terrestrial subsurface deeper understanding of life and its limits on Earth, and beyond, can be obtained. Integral membrane phospholipids and their derivatives can act as biosignatures for microbial life and its processes. However, our understanding of the carbon source for, and rates of cycling of, lipids in low biomass environments where interactions with mineral matrices will be highly important, remains limited. Characterization of phospholipid fatty acids and their isotopic compositions, combined with DNA analysis, identified an ancient, isolated subsurface microbial community dominated by a single taxon, Candidatus Frackibacter that is hypothesized to subsist off host-rock-derived carbon enabling survival on geologic timescales. Thus, deep subsurface groundwaters may provide a refuge for life’s long-term survival on Earth, and potential astrobiological targets like Mars. Interpreting low biomass lipid signatures requires understanding of the temporal stability of lipids and organisms. The implications of association with mineral matrices to lipid stability was characterized via abiotic hydrolysis experiments at low and high pH with either basalt or bentonite. Lipid lifetimes are increased by up to one order of magnitude in the presence of bentonite under acidic and basic conditions, and basalt under highly acidic conditions. The temporal stability of lipid signatures of a low biomass community under growth suppression indicated community survival through a prolonged starvation state. By acquiring a growth advantage in stationary-phase phenotype (GASP), organisms may remain viable beyond their expected lifetimes. Overall, this research demonstrates that low-biomass communities and associated lipids can persist beyond abiotic limits and utilize geologic carbon sources to support metabolism. These results contribute to our understanding of all subsurface life and have implications to the timeframe represented by lipid biomarkers and the potential survival of life in deep subsurface environments.