TEMPORAL AND SPATIAL VARIATION IN ACTIVELY SILICIFYING HYDROTHERMAL MICROBIAL COMMUNITIES AND RAMIFICATION IN THE ROCK RECORD

Hydrothermal ecosystems have the potential to preserve evidence of life within the rock record because steep thermal gradients lead to rapid mineralization, promoting the entrapment of surrounding biota. One method of lithification, silicification, is a mechanism by which microbiota may be lithified and preserved as resilient microfossils, making silicified environments ideal research locations for the identification of ancient life and possibly life on other planets; although how completely silicified life portrays a complete microbial ecosystem is actively disputed. The silicified rock record is dominated by both filamentous and coccoidal cells, indicating certain microbial morphologies silicify more readily than others representing a biased view based on preferential silicification. The focus of this work was to advance our understanding of microbial silicification at an actively silicifying hot spring. This project encompassed biological (16S rRNA and SEM) and water chemistry sampling of lithifying and actively growing microbial mats at Steep Cone Geyser (Yellowstone National Park) over spatial and temporal transects along an outflow channel of the spring. Variation in flow and a steep thermal gradient from ~94°C to ~55°C along a 5m outflow transect dictated the observable microbial community composition and distribution. The decreasing thermal gradient correlated with an increase in microbial diversity that was composed of select thermotolerant lithotrophic genera such as Thermocrinis and Thermus which were abundant in the boiling source waters to a more diverse microbial community consisting of phototrophs, heterotrophs, and chemolithotrophs such as Leptococcus, Chloroflexus, Chloracidobacterium, and Teipidimonas. In samples distributed spatially and temporally, rod shaped morphotypes inferred from 16S rRNA data were identified in actively growing as well as lithified mat including Thermoflexibacter and compared to SEM results suggesting that at Steep Cone Geyser the presence of rod and coccoidal morphologies is lost during the silicification and entrainment processes. The silicification processes occurring at Steep Cone serve as an excellent active analog for silica rich environments, both here on earth and extraterrestrial.