Acidic geothermal springs are often inhabited by microorganisms that obtain their primary source of energy from the oxidation of reduced inorganic chemical constituents such as H₂, H₂S, S⁰, Fe^II, and H₃As^IIIO₃⁰.  The geochemical habitats commonly observed in YNP geothermal systems may be similar to those present under early earth conditions or to those in other planetary systems. Consequently, an understanding of these unique metabolisms and their connection with specific geochemical attributes and processes provides important clues regarding the distribution and evolution of microbial species.
Primary productivity in most environments is generally thought to be dominated by phototrophic organisms capable of capturing light energy for metabolic processes.  However, chemotrophic organisms often dominate high temperature (> 70 °C) geothermal systems, where conditions are not favorable for photosynthesis.

Many acidic geothermal source waters contain relatively high concentrations of reduced inorganic species and these constituents play an important role in driving primary productivity in numerous geothermal pools, hot springs and fumaroles. Dr. Inskeep’s research efforts have provided detailed descriptions of geochemical processes and associated microbial populations in YNP thermal habitats (Langner et al., 2001; Macur et al., 2004).
Results from our work in Norris Geyser Basin are being used to develop conceptual models for defining relationships among geochemical processes and the distribution of microbial populations.  For example, the source waters of acid-sulfate-chloride geothermal springs contain a suite of reduced constituents that may serve as electron donors for microbial growth (e.g., H₂, H₂S, S⁰, Fe^II, and As^III). Immediately after discharge, these springs generally exhibit a yellow, S⁰ depositional zone comprised of spheres and rhombohedral elemental So, and where several microbial populations are involved in the oxidation and reduction of S species. As dissolved H₂S concentrations decline, the oxidation of Fe^II and As^III becomes important, resulting in the formation of an Fe-As rich, highly filamentous brown mat. Detailed characterization of these solid phases reveals x-ray and electron amorphous Fe^III-oxyhydroxides containing adsorbed or coprecipitated As^VI. The microbial filaments associated with this zone are heavily encrusted with the As-rich Fe-oxides, suggesting that microorganisms play an important role in the initial nucleation, biomineralization and deposition of terraced Fe mats in acidic systems.

The multiple gradients in temperature and geochemistry provide diverse oligotrophic and extreme environments to study the genetics and physiologies of novel archaeal and bacterial species in relation to their geochemical environments, and their uniqueness in both time and place. Abrupt geochemical gradients in Dragon Spring reveal microbial populations specialized to high H₂ and S environments, as well as high As-Fe environments.

Many high-temperature, low pH (2.5-3) springs such as Beowulf Spring of Norris Basin contain reduced inorganic species such as CH4, H₂, As(III) H₂S, S, and Fe(II) that drive primary productivity via chemolithoautotrophic organisms. Temperature and geochemical gradients in Beowulf Spring define diverse habitats that are associated with a distribution of specialized microbial populations.

Scanning electron micrograph of crystalline phases rich in Fe, S, and K (likely jarosite) forming in an acidic spring in Norris Basin

Scanning electron micrograph of a Si-rich diatom shell in an acid sulfate spring of Norris Basin.

Scanning electron micrograph of filamentous bacteria with solid phase sheaths comprised of Fe, Al and Si, from the outflow channel of Perpetual Spouter, Norris Basin.

Current Laboratory Personnel

Natsuko Hamamura, Postdoctoral Associate
Rich Macur, Postdoctoral Associate
Mark Kozubal, Ph.D. Student
Dustin Morse, Ph.D. Student
Galena Ackerman, M.S. Student
Deanne Masur, M.S. Student
W. Peyton Taylor, M.S. Student
Amanda Nagy, Undergraduate Student
Sarah Korf, Research Associate

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