NASA Astrobiology Institute (NAI)

1. Microbial pyrite oxidation in nature and the lab: sulfate mineral biosignature investigation

   Project Investigators: Max Coleman

   Other Project Members

   Randall Mielke (Research Staff)

   Ed Young (Collaborator)

   Karen Zeigler (Research Staff)

   Summary

   In the laboratory the project aims to culture bacteria that oxidize iron and sulfur under various conditions (a range of temperatures and different degrees of acidity). The goal is to develop criteria to distinguish minerals that formed as indirect result of bacterial activity from those that form with no biological association. It s anticipated that such distinctions may involve subtle, but distinctive differences in chemical and isotopic compositions, including variations in the ratios of the naturally occurring stable (non-radioactive) isotopes of iron, sulfur and oxygen. Analyses will be made of similar minerals, formed naturally in an acid mine drainage area of SW Spain, that may be an analog for processes that are believed to have happened on the surface of Mars four billion years ago.

   Astrobiology Roadmap Objectives:

   • Objective 2.1: Mars exploration
   • Objective 4.1: Earth's early biosphere
   • Objective 7.1: Biosignatures to be sought in Solar System materials

   Project Progress

   We have cultured two strains of bacteria, Acidithiobacillus ferrooxidans and Leptospirrilum ferrooxidans at room temperature and have adapted cultures to grow as low as 4°C. Both strains have been adapted to grow in sulfate-free medium so that biogenic sulfate may be uniquely characterized. To identify the source of oxygen used in the production of sulfate, either an atmospheric source or oxygen from water, we used two types of media, one with “normal” water and the other spiked with excess oxygen-18. The isotopic spike produces water that has relatively less oxygen-17 relative to O-18, which normally correlate, but can be distinguished by comparing the results to mass-dependent relations. Bacteria appear to use atmospheric oxygen to initiate oxidation of pyrite sulfur but subsequently an inorganic process becomes dominant that uses oxygen in water. The switch in oxygen sources and utilization is accompanied by a rapid increase in oxidation kinetics and transition to entirely biologic oxidation of iron by bacteria. Previously, ¹⁸O/¹⁶O analyses using non-spiked isotope compositions have shown qualitative involvement of atmospheric oxygen but these earlier studies could not explain all aspects of the isotopic data, nor could they quantify contributions from different oxygen reservoirs. Analysis of all three oxygen isotopes in microbially produced sulfate has provided the first quantitative constraints on the extent and duration of microbial utilization of atmospheric oxygen to produce sulfate. Future work will explore a large range in environmental conditions to determine other fractionation processes and constrain changes in the oxygen utilization pathways.

   Cross-Team Collaborations

   We have collaborated with the PI of the UCLA node, Prof. Ed Young, and a member of the UCLA Research Staff, Dr. Karen Ziegler, who performed triple oxygen isotope analyses to complement our conventional 18O/16O measurements. These collaborations have been key to our new understanding of the oxygen utilization pathways and relations to oxidation kinetics, as described above. The collaborative work has not yet been published but will be reported under our joint authorship at the Goldschmidt Geochemistry Conference to be held in Vancouver, Canada, July 2008.

Publications

Bonifacie, M., Jendrzejewski, N., Agrinier, P., Humler, E., Javoy, M. & Coleman, M.  (2007).  The chlorine isotope composition of Earth's mantle.  Science, 319:1518-1520.