Astrobiology Science and Technology for Exploring Planets (ASTEP)

PI: Zerkle, Aubrey

The proposed research will focus on collecting sulfur multiple isotope data for laboratory culture experiments with phototrophic and non-phototrophic S oxidizing organisms, and evaluating the experimental results in the context of metabolic fractionation models for these S oxidizing organisms. Our findings will be relevant for studies of sulfur ecosystems that use sulfur cycle models and isotopic data to characterize the transformations and transport of sulfur, and should have specific applications to modern and ancient ecosystems where biological S oxidation plays a role, on earth and elsewhere. In this manner, we anticipate that this research will directly contribute to NASA studies aimed at both: 1) understanding biogeochemical cycling in modern environments and in Earth’s early biosphere; and 2) interpretation of biological versus abiotic chemical signatures on earth and other planets.

The primary objectives of the proposed research are:
1) to place basic limits on the magnitude of multiple sulfur isotope fractionations associated with biological S oxidation;
2) to reinforce the understanding of the metabolic steps that control the isotopic fractionations for all four stable isotopes in S oxidation metabolisms; and
3) to quantify the contribution of S oxidation pathways to sulfur isotope signatures produced in models of the sulfur cycle relevant to the early Earth and other planets.

These research objectives will be met by performing laboratory culture studies on phototrophic and non-phototrophic S oxidizing bacteria, and analyzing ³³ S / ³² S , ³⁴ S / ^{32^S , and ^36} S / ³² S in the resulting sulfur products. This work builds on a series of studies conducted in collaborations between Co-I Farquhar and other members of the research team involved in this proposal (also including a substantial body of related work on sulfate reduction and sulfur compound disproportionation by Johnston and others), and extends a database of results in progress by PI Zerkle. Laboratory culturing experiments will be conducted in collaboration with researchers at the University of Southern Denmark in the departments of Biology and Biochemistry & Molecular Biology. All analytical instrumentation and protocols for multiple sulfur isotope measurements are well established at the University of Maryland. Isotope measurements are undertaken by converting extracted sulfur to silver sulfide and then fluorinating this to sulfur hexafluoride, which is purified and analyzed using a ThermoFinnigan MAT 253 dual inlet mass spectrometer.