Stable Carbon Isotope Depletions in Lipid Biomarkers Suggest Subsurface Carbon Fixation in Lava Caves

Lava caves, formed through basaltic volcanism, are accessible conduits into the shallow subsurface and the microbial life residing there. While evidence for this life is widespread, the level of dependence of these microbial communities on surface inputs, especially that of organic carbon (OC), is a persistent knowledge gap, with relevance to both terrestrial biogeochemistry and the characterization of lava caves as Mars analog environments. Here, we explore carbon cycling processes within lava caves at Lava Beds National Monument, CA. We interrogate a range of cave features and surface soils, characterizing the isotopic composition (δ¹³C) of bulk organic and inorganic phases, followed by organic geochemical analysis of the distribution and δ¹³C signatures of fatty acids derived from intact polar lipids (IPLs). From these data, we estimate the carbon sources of different sample types, finding that surface soils and mineral-rich speleothems incorporate plant-derived biomass (δ¹³C_VPDB ∼ −30‰), whereas biofilms are dominated by strongly ¹³C-depleted lipids (minimum δ¹³C_VPDB −45.4‰) specific to bacteria, requiring a significant proportion of their biomass to derive from in situ fixation of inorganic carbon from previously respired OC. Based on the prevalence and abundance of these ¹³C-depleted lipids, we conclude that biofilms here are fueled by in situ chemolithoautotrophy, despite relatively high concentrations of dissolved OC in colocated cave waters. This unexpected metabolic potential mirrors that found in other deep subsurface biospheres and has significant positive implications for the potential microbial habitability of the Martian subsurface.