CAREER: A Triple-Isotope Approach to Unraveling Subsurface Food Webs

A vast ecosystem of bacteria and other microorganisms live beneath your feet. This biome extends kilometers deep into the Earth's crust and contains as much organic carbon as surface life. While scientists know a lot about who these microbes are, understanding what they do for a living is a harder question.

Ecologists use chemical signatures of plants and animals to map food chains. In this work, Prof. Osburn will develop a new tool to similarly map microbial food webs, focusing on those found 100s to ~10,000 feet underground.

Just like the signatures in fingernails distinguish carnivores from vegetarians, this tool will reveal which carbon and nitrogen sources are uses in the shallow vs. ultra-deep places underground. The implications of what subsurface microbes eat are surprisingly global. If they eat mostly organic matter made by surface plants, then they are sequestering carbon in huge amounts and depend on surface ecosystems.

If these microbes instead eat and breathe inorganic materials and use CO2, then these ecosystems could be very ancient indeed. The scientific concepts underlying this work (food chains, microbial ecology, earth system science, and field-based inquiry) are well suited to be adapted for high school classrooms. Prof.

Osburn will develop activities featuring these concepts in collaboration with Science in Society and Chicago Public Schools, with the ultimate goal of introducing Earth Science and field-based inquiry to students with a diversity of backgrounds and perspectives. She will also host some of these students as interns for immersive laboratory experiences.

The continental subsurface harbors vast and active microbial biospheres; however, the net dependence of these microbes on surficial inputs remains unknown. This work will interrogate the dependency of subsurface microbial food webs on surface carbon, within a gradient of subsurface environments (shallow to 2.9 km deep). Ultimately the questions are: Are subsurface ecosystems dependent on surficial inputs, how does this change with depth, and how might it have changed with time?

Ultra-deep continental ecosystems may live independently of surficial organic matter and record signatures of dark primary productivity in their biomass. Prof. Osburn will develop a novel amino acid (AA) triple isotope fingerprint and apply it to deconvolve trophic relationships in a range of deep subsurface environments.

To accomplish these ends, first she will calibrate the isotopic fingerprint with relevant heterotrophic and chemoautotrophic microbes (Objective 1), then analyze subsurface biomass from three well-characterized localities including shallow (1-150 m), intermediate (250-1500 m), and ultra-deep (2.4-2.9 km) environments (Objective 2). Education and outreach activities center on addressing the grave problem of diversity in Geoscience by building interactive, analytical content modules for diverse Chicago public high school classrooms (Objective 3) and creating immersive laboratory experiences for high school students via internships and field opportunities (Objective 4).

By incorporating research products into the K-12 classroom and inviting high school interns into research arenas, she directly links research and education objectives.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.