Exobiology and Evolutionary Biology

Quantifying the Habitability of Low-Temperature Serpentinizing Systems (2)

PI: Tori Hoehler

The overarching goal of the proposed work is to develop a quantitative energy-based metric for habitability, and apply that metric to determine the potential habitability of low-temperature serpentinizing systems. The concept of habitability offers a practical means of prioritizing targets for detailed astrobiological exploration, based on their potential to support life. The “follow the water” approach has highlighted a range of potentially habitable niches across our solar system, but further constraints are required to go beyond the simply binary indicator (“life +” or “life -”) offered by water and develop the discerning metric of habitability that will be an essential tool for planning and conducting future astrobiology missions. We propose to develop a means by which to quantify and resolve the degree of habitability in any given environment, by considering the balance between the biological demand for energy and the corresponding potential for meeting that demand by transduction of energy from the environment into biological process. This approach is metabolism-specific, and factors in the effects of physical and chemical environment, energy supply mechanisms, and organismal/biochemical specifics. The energy balance approach will be applicable across a broad range of environments and metabolisms, but the present work will focus on quantifying the habitability of low-temperature serpentinizing systems. The capacity for generation of significant quantities of H2 via water-rock reactions in serpentinizing systems has led to considerable interest in their potential to support photosynthesis-independent biological communities and, therefore, their potential importance as long-term habitable niches within the terrestrial and Martian crust. The apparent presence of microbiology associated with H2-rich, serpentinite-sourced hydrothermal fluids indicates that at least some serpentinizing
environments are habitable. However, we presently lack the observational data and predictive capacity required to infer potential habitability in low-temperature systems, which likely predominate in terrestrial or Martian crustal settings.
Through coordinated pursuit of the following objectives, our work will fill these knowledge and methodological gaps, resulting in quantitative assessment of potential habitability for low-(e.g., biological)-temperature serpentinizing systems.
Objectives:
1. Develop a numerical reaction-transport model that quantifies energy balance at the single-cell scale, and maps the boundaries of habitability as a function of temperature, pH, and substrate availability. The initial focus of this work will be to quantify habitability for H2-based methanogenesis (considered to be among the most viable metabolisms for such systems), but an important goal is to ensure that the model construct is readily extensible to a wide range of metabolisms and environments.
2. Use laboratory experiments to quantify the rates and levels of H2 production, and characterize fluid chemistry evolution, during hydrous alteration of ultramafic minerals at temperatures within the biologically-tolerated range. There are presently no observations of H2-producing capacity or rates within the biological range of temperature, but provision of H2 is among the most critical determinants of potential methanogen metabolic energy yield.
3. Quantify the effect of variable alkaline pH on the biomass-normalized maintenance energy requirements of H2-utilizing methanogens in laboratory cultures. The alkaline pH that develops during serpentinization likely represents a key control on biological energy demand, but any such effect has yet to be quantified experimentally.
Progress in each of these areas will, individually, fill critical gaps in our knowledge base. When integrated, they will yield a first-of-its-kind, quantitative assessment of energy balance and habitability.