Hydrothermal Hypoliths: A Comparison of Extremophilic Rock-Dwelling Microorganisms with Other Hydrothermal Photo- and Chemotrophic Communities Across Acidic and Alkaline pH Ranges

Modern extremophiles are ideal microbial communities to study and compare with early Earth environments. Recent evidence suggests that terrestrial hot springs were present 3.5 billion years ago, which necessitates the study and characterization of modern analog hydrothermal microbial communities . Most current work focus es on subaqueous microbial communities within hot springs , however, this study investigates modern hypolithic microbial communities living below siliceous rock at the soil-air interface around the edges of hot springs in Yellowstone National Park. Hypolithic communities are understood to utilize their siliceous covering as UV protection within extreme environments. Here we compare primary productivity of hypolith microbial communities to subaerial phototrophs, subaqueous phototrophs, and subaqueous chemotrophs across a pH range of 2 to 11. In addition, we report the microbial community composition across this pH range. Overall carbon uptake ( m g carbon uptake/g biomass/hr) ranged from 5-1000 (n=16) for hypoliths, 15-2750 (n=8) for subaerial phototrophs, 5-10000 (n=10) for subaqueous phototrophs, and 0-1400 (n=7) for subaqueous chemotrophs. Hydrothermal hypolithic productivity and composition are overall similar to the other hot spring microbes. Notable variations for the hypolithic communities are seen when comparing the acidic and alkaline microbial compositions. This work lays the foundation for subsequent studies into the characterization of the soil chemical environment of these microbial communities and to further understand the capabilities of the microbial communities to survive or transform their localized environments.