Lipids biomarkers of the deep terrestrial subsurface biosphere

Abstract:

Lipid biomarkers are key tools for the interpretation of past and present environments, and specifically, intact polar lipids (IPLs) reflect contributions from the living biosphere. While several studies have documented intact polar lipid (IPL) distributions in the marine subsurface, the organic geochemistry of terrestrial subsurface communities remains poorly characterized. Here we present IPL distributions from a portal into deep terrestrial biosphere, the former Homestake Mine, SD USA. Interpretation of IPL distributions can be hampered by a lack of comparative pure culture information or comparative molecular and geochemical data, thus we performed IPL analysis in conjunction with detailed geochemistry and DNA sequencing. A large diversity of lipid structures is observed including phospholipids, aminolipids, glycolipids, GDGTs, and a number of unidentified compounds. Variability in lipid distributions is not random with individual samples clustering based on physical and geochemical parameters. For instance, biofilm samples contain abundant aminolipids relative to filtered subsurface fluid samples. Does this difference reflect phosphorus scarcity in the biofilms, or production of aminolipids by specific microbial phyla? Using comparative analysis between the IPL, molecular, and geochemical datasets we address this, and similar questions, as well as identify potential microbial sources of unknown biomarkers. In the case of the aminolipids, we observe strong covariation between the lipid distribution and that of the bacteroidetes, epsilonproteobacteria, and spirochaetes, but no correlation with P concentration. We can also extend this comparison globally; asking how similar IPL distributions of the deep terrestrial subsurface are previously studied sites. A surprising finding from this work is the total lack of similarity between deep marine and terrestrial subsurface sites. This contribution will help to define the phylogenetic and geochemical mechanisms driving the lipid biomarker signature of the deep terrestrial subsurface biosphere.