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Executive Summary

Life Underground

Our crossdisciplinary team from the University of Southern California, the California Institute of Technology, the Jet Propulsion Laboratory, the Desert Research Institute, and the Rensselaer Polytechnic Institute is developing and employing field, laboratory, and modeling approaches to detect and characterize microbial life in the subsurface the intraterrestrials. On Earth, microorganisms appear to inhabit all 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 nearsurface environments, their mode and distribution in the subsurface are poorly constrained. Nevertheless, it has been shown that archaea and bacteria inhabit deeply buried rocks and sediments where they contribute to biogeochemical cycles. All evidence suggests that these subsurface systems are enormous and diverse. On other planets in our solar system, putative extant or extinct life would most likely be found underground. NASA has declared the extraterrestrial search for life a priority. With a focus on nearby planets where landed missions are more than just a possibility, access to the subsurface will be highly desirable. Robust strategies for subsurface life detection on Mars, Europa, or other potential targets are poorly developed. The search for extant life or its biosignatures is scientifically and technologically extremely difficult; on Earth, it is a formidable but tractable challenge.

To expand our understanding of the subsurface biosphere on Earth and to inform future exploration of subsurface ecosystems elsewhere, our Life Underground project focuses on several accessible and geologically diverse subsurface sites, both continental and marine. During this reporting period, we initiated several field projects, most notably at the Sanford Underground Research Facility (SURF) in South Dakota and boreholes in/near Death Valley. At SURF, reconnaissance of the 300’, 800’, 1700’, 2000’, 4700’, and 4850’ levels identified numerous potential fluid sampling ports. A first round of in situ measurements (e.g., temperature, pH, Eh) was carried out, and samples were collected for geochemistry analysis, DNA sequencing, and microbial culturing experiments. As part of the in situ life detection and characterization activities, a sample analysis protocol was developed using a series of lab standards, samples incubated in local field sites, and previously obtained subsurface materials. A deep UV Raman/fluorescence instrument is part of the analysis suite to detect organics and microbes living in and on rocks. Also part of the analysis protocol is the development of a multiinstrument database for Raman, fluorescence, visible images, XRD, and GC/MS that organizes data obtained from a specific sample. As part of the guided cultivation theme, several techniques were employed that focused on mimicking the naturally occurring complex interactions and energy gradients. Downflow hanging sponge bioreactors were built, tested, and operated in the Orphan and Amend labs to enrich for anaerobic methane oxidizers, iron and manganese reducers, and manganese oxidizers. One of the enrichments from the sponge reactors was then used in a novel electrode cultivation reactor at four different potentials (50, 150, 250, 350 mV). Several lines of evidence point to the growth of two different metal reducers at the highest potentials. Accomplishments in the energy/metabolism modeling theme included the application of a fully coupled biogeochemical reactiontransport model for a hydrothermal vent chimney wall that quantifies the rates of microbiallymediated catabolism. In addition, Gibbs energies and chemical composition data were combined to constrain likely metabolic strategies and rates in marine sediments at several sites. Lastly, at a deeplysourced, highly alkaline, serpentinitehosted spring (the Cedars), high numbers of diverse microbes were found where little or no life as expected. These are now being characterized using physiologic and phylogenetic approaches.