RECOGNIZING SIGNS OF LIFE IN OPALINE SILICA HYDROTHERMAL DEPOSITS: A COMPARISON OF BIOSIGNATURE SUITES IN TWO GEOCHEMICALLY DISTINCT HOT SPRING DEPOSITS IN YELLOWSTONE NATIONAL PARK, USA

Though a reducing environment can enhance the preservation of organic matter, its effect on the type and fidelity of microbial biosignatures that form in silica-depositing hot springs has not been studied systematically. Complications arise when characterizing biosignature preservation in natural ecosystems due to spatial and temporal variations in microbial populations, geochemistry, hydrodynamics, and ambient weather conditions. The integration of physical, geochemical, and biological datasets collected over three subsequent years have allowed us to develop preservation potential models for distinct, yet cosmopolitan, microbially dominated communities that occupy terrestrial hot spring ecosystems. Our datasets include water chemistry, microbial community composition, and high-resolution visual, structural, and geochemical analyses of siliceous sinters that form in the presence of the microbial communities analyzed.

In this study, we report on the types of biosignatures that form near the upper-temperature limit of life in two hot spring pools that are characterized by similar dissolved silica concentrations but exhibit significantly different reducing gas concentrations (high vs. low hydrogen sulfide). We found that microbial diversity varied (1-4 taxa) from year-to-year in both pools, and Thermocrinis ruber was present in every high-temperature sample studied. The fluid temperature in the main pools was similar (87-92°C) and remained constant (within a few degrees) throughout our study. Both hot springs deposited opaline silica throughout the entire length of their outflow channels, even within the main pools where dissolved silica concentrations were found to be undersaturated with respect to amorphous silica. Optical and analytical electron microscopy revealed similar, yet distinctive, biosignature suites preserved in the high-temperature geyserite samples of both pools. Microbial mineralization occurred primarily by encrustation, though permineralization and mineral replacement of cells and EPS was found. Geyserite fabrics in both pools displayed evidence of microbial input. Our study indicates that rapid mineralization and temporal variations are also essential factors in the preservation of biosignatures in siliceous hot spring sinters.