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Prof. Meinrat Andreae



Geology & Geophysics Department, College of Science, Building (4)

Office: 2 B 147

Telephone: 76198

Project Title

Composition and Structure of Desert Rock Crusts (“Varnishes”) by Microanalysis

Project Summary

The first scientific descriptions of desert varnish go back to Darwin and Humboldt, who characterized it as thin shiny black crust on desert rocks (Figure 1). Intensive studies on these crusts began in the second half of the 20th century. The discovery of bacteria and fungi as components of desert varnish led to a controversial discussion about the role of microbes in its formation. Did the microbes just colonize an available habitat, or were they instrumental in its formation? Either way, the presence of live microbes in the coatings provides evidence for the possibility of life under most extreme conditions of radiation, temperature, and desiccation. The discovery of varnish-like structures on Mars re-ignited scientific interest, since such varnishes could provide an exobiological habitat.

Desert varnishes have a thickness of up to 200 µm, and usually consist of alternating light and dark layers with a thickness of ~1 µm. Their main inorganic constituents are opaline silica, clays, and iron and manganese oxides. They contain remarkably large amounts of carbon and nitrogen, on the order of 10-30 and 1-3 atom-%, respectively, in close association with the inorganic structures. Analyses of abraded varnish material have shown the presence of amino acids, ATP, and DNA, but because of the destructive type of sample preparation, the association of these biogenic compounds with potential biological structures has been obliterated.

In order to reveal the relationships between biological and abiotic components we intend to use microchemical techniques to obtain spatially resolved information of the distribution of elements and chemical species within the varnishes. From the observed chemical-structural relationships we expect clues about the potential biogenic contributions to the varnish formation process and about the structure of the microbial habitats. This will deepen our understanding about biogeochemical processes and survival of microbes in extreme environments on Earth and possibly on other planets.