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Life Underground

On Earth, microorganisms appear to inhabit all physical space that provides the minimum requirements for life. These include the availability of water, carbon, nutrients, and light or chemical energy. While these are generally abundant in surface or near-surface environments, their mode and distribution in the subsurface are poorly constrained. Nevertheless, it has now been shown unequivocally that archaea and bacteria inhabit deeply buried rocks and sediments where they contribute to biogeochemical cycles. All evidence suggests that these subsurface ecosystems are spatially enormous and diverse. On other planets, at least in our solar system, putative extant or extinct life would most likely reside underground or in massive ice shells.

The cross-disciplinary team from the University of Southern California (USC), the California Institute of Technology (Caltech), the Jet Propulsion Laboratory (JPL), and the Desert Research Institute (DRI) is developing field, laboratory, and modeling approaches aimed at detecting and characterizing microbial life in the subsurface — the intra-terrestrials. This research will inform the astrobiology community and guide future missions in the search for extraterrestrial life.

The research of the USC team takes advantage of access to a variety of geologically representative deep subsurface sites on Earth, both continental and marine, and targets a set of boreholes (and corresponding samples) drilled into a range of geological provinces. The team is deploying downhole monitoring equipment, obtaining samples for chemical analyses and laboratory experiments, and setting up long-term in situ microbial activity experiments in what they refer to as Tier 1 sites. These include:

• A well (BLM1) in a fractured rock aquifer in Paleozoic carbonate,
• A deeply-sourced artesian well (Nevares Deep Well 2) in a travertine spring mound,
• Sedimentary rock, granite, and volcanic tuff at the former Nevada Test Site,
• A former gold mine (Homestake) in a metamorphic complex now called SURF; there, the teams plans to drill custom holes specifically for the NAI,
• Igneous basement in young Mid-Atlantic ridge flank basalt at a place called ‘North Pond’.

These sites provide samples and opportunities around which the USC team will conduct three comprehensive, interdisciplinary, coordinated, and complementary research efforts.

In Situ Life Detection and Characterization: The goal of this research theme is to integrate a multi-scalar approach to life detection and characterization where the USC team will couple: a) newly developed in situ instruments for biomass detection and distribution directly in the subsurface boreholes, b) in situ deployments of stable isotope probing experiments for tracking the metabolic potential of deep biosphere microorganisms for meter scale analysis, and c) microbial characterization in terms of metabolic potential, colonization patterns, and transformation of minerals at the centimeter and micron scales using laboratory based microanalytical techniques. The team seeks to answer three fundamental sets of questions.

• How do we search for microbes in the subsurface? And once detected, how do we understand their ecophysiology?
• What is the distribution of microbial biomass and activity in the subsurface? What physicochemical factors influence their distributions?
• What biosignatures or evidence of subsurface environments stand the test of time? Are there novel preservation possibilities in the subsurface?

Guided Cultivations of Intraterrestrials: Characterization of the subsurface biosphere is hindered because the majority of the resident microorganisms appear to be ‘unculturable’. The USC team posits that a significant number of these are not unculturable, but rather uncultured. They are investigating the microbial growth, metabolism, and physiology of organisms from low-energy environments using several modified traditional and novel culturing platforms:

• Down-flow hanging sponge reactors to investigate attached communities in porous media,
• Optically accessible diffusion chambers with established electrochemical gradients,
• Continuously stirred tank reactors for ultimate control of cultivation parameters,
• On-chip biofilm reactors to evaluate charge transfer from/to microfabricated electrode arrays,
• Traditional batch reactors as a screening tool and to establish baseline understanding.

Energy Flow and Metabolic Modeling: All life requires energy, and, that energy is ultimately harvested through the catalysis of electron transfer (redox) reactions. The amount of energy that is available from these reactions determines how fast microorganisms grow and thus the rate and quantity of biomass produced in a given setting. The USC team is using numerical energy modeling to map out subsurface habitability on a regional and global scale. They seek to answer the questions:

• How much metabolic energy is available in different deep subsurface environments?
• What are the likely source regions of this metabolic energy?

In a coupled modeling-experimental study, using gradient diffusion chambers, the USC team is also quantitatively investigating the relationship between energy availability, cell maintenance, and active growth. Key questions are:

• Given the same amount of energy, what determines which electron donor or acceptor a microorganism uses?
• How does energy availability in low-energy settings translate into growth, if at all?