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Project Progress

Task 1. Molecular Spectroscopic Investigations into Ancient Biochemistry (Cody, Fogel, Hazen)

Cody, Fogel, and Hazen have collaborated with NAI postdoctoral fellow Kevin Boyce and Andrew Knoll at Harvard University and scientists in Australia and South Africa on studies of the chemical, isotopic, and molecular characterization of a variety of ancient organic fossils. Recent results include isotopic, chemical, and morphological evidence for the oldest known microbial mats from a sandy marine environment and chemical evidence of specific structural biopolymers in Earth’s most ancient vascular plants (Figure 1).

Boyce and CIW colleagues have made a preliminary determination of the biological affinities of a bizarre Paleozoic fossil, Prototaxites, which produced enormous tree-like trunks composed of intertwined, 50-micrometer-diameter tubes. On the basis of carbon and nitrogen isotopic analyses, specimens cluster into two discrete populations of carbon isotopic values. Pending further organic analyses to rule out the possibility of diagenetic artifacts, this isotopic disparity may reflect heterotrophy upon isotopically distinct sources, suggesting that Prototaxites was a twenty-foot-tall fungus.

Task 2. Life Detection in Extraterrestrial Materials (Steele, Fogel, Toporski)

Steele, in collaboration with postdoctoral fellow Jan Toporski, has continued to explore the use of commercial antibodies for printing protein microarrays and testing these microarrays against defined antigens and unknown samples of solar system exploration relevance such as Martian regolith simulant and samples from Haughton crater and the Enspel Formation. Steele and collaborators are characterizing populations of bacteria in the above samples as well as terrestrially contaminated meteorites (Figure 2) by use of polymerase chain reaction (PCR), cloning, and sequencing. Furthermore, Steele and Toporski continue to refine extraction protocols for biomarkers from the above samples and to explore the survival of biomarker molecules through the fossilization process. Fogel, Steele, and Toporski have applied stable isotopes (carbon and nitrogen) to unravel the trophic structure of the ancient ecosystem preserved in the Enspel Formation, Germany.

Task 3. Mass-Independent Sulfur Isotope Fractionation in Archean Rocks (Farquhar, Rumble, J. Scott, Wang)

Sulfur isotopic geochemistry enjoys the unique power to record information on both ancient biochemistry and ancient atmospheric photochemistry. Experimental data are needed, however, in order to interpret isotope compositions in terms of atmospheric versus metabolic mechanisms. The fractionation of sulfur isotopes by microbial enzymatic catalysis has been well-studied in terms of ³⁴S/³²S fractionations, but published data on ³³S and ³⁶S are lacking. Rumble, J. Scott, and postdoctoral fellow Pei-Ling Wang are measuring the fractionation of multi-sulfur isotopes by pure cultures of 11 species of microbes. The study is focused on microbes that are able to metabolize elemental sulfur, because native sulfur is believed to be an important component of the Archean atmosphere. Preliminary results on five species show no evidence that microbes imitate the “mass-independent” fractionation characteristic of atmospheric photochemistry. This is an important result, because it simplifies the interpretation of sulfur isotope analyses of ancient rocks. Additional experiments on the full range of microbial species and under a variety of environmental conditions are continuing.

A highly resolved stratigraphic record of sulfur isotope fractionations transecting such pivotal geologic epochs as the Archean-Proterozoic boundary and the episode of Paleo-Proterozoic glaciation is sought, because it will provide insight into evolutionary changes in Earth’s atmosphere. Drill core samples have been collected from several continents covering these time intervals. Farquhar, Rumble, postdoctoral fellows Wang and Shuhei Ono, together with collaborators from the Harvard NAI team, have been analyzing the samples. A study of the Fortescue and Hamersley Groups, Australia, shows strong correlation between depositional environments and sulfur isotope systematics. There is evidence of microbial activity as an important factor causing the precipitation of pyrite and thus preserving an isotopic record of atmospheric photochemistry.

Farquhar’s group has described the mass transfer of sulfur through biosynthetic networks associated with sulfate reduction and disproportionation. They demonstrated that sulfur isotope fractionations for ³²S, ³³S, and ³⁴S during dissimilatory sulfate reduction can be explained as a direct consequence of distillation effects associated with the sulfate reduction pathways. They further carried out vibrational calculations for equilibrium sulfur isotope fractionations to provide a reference frame for evaluating biological and abiological sulfur isotope fractionation effects.

Farquhar’s laboratory at the University of Maryland has installed and is in the final stages of testing a Finnigan MAT 253 gas-source mass spectrometer for sulfur isotope analyses. Coupled to this instrument is an SF6 preparation manifold for fluorination of samples by laser and conventional techniques.

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