Microbial Fossils Detected in Desert Varnish

Introduction: Mars Global Surveyor Thermal Emission Spectrometer data indicate regions with significant levels of hematite (_Fe2O3). Fe-oxides, like hematite, can form as aqueous mineral precipitates and as such may preserve microscopic fossils or other biosignatures. Several potential terrestrial analogues to martian hematite like hydrothermal vents have preserved microfossils [1]. Microbial fossilization in Fe-oxides is often a function of biomineralization. For example, goethite (FeO2H) encrustation of fungal my-celia from the mid-Tertiary preserved fungal morphologies such that their genera could be determined [2]. Another terrestrial analogue to martian hematite may be is desert varnish. Desert varnish is a thin coating on rocks in semi-arid to arid regions [3,4]. The coloration, thickness, texture, and chemical composition of desert varnish varies spatially down to the nanometer-scale. Additionally, regional variations also exist. Desert varnish is composed primarily of Mn-oxides, Fe-oxides, and clays but also contains trace amounts of organic and detrital components. The most likely elemental source for desert varnish is wind-blown dust. The metal constituents of the dust are taken into solution and then selectively precipitated onto the substrate or attached clays at concentrations orders of magnitude higher than normal dust concentrations. This process may be aided or completely mediated by a variety of microorganisms that inhabit varnish surface and matrix [3,4]. The most common desert varnish inhabitants are epilithic, slow-growing, melanin-pigmented micro-colonial fungi (MCF, also known as meristematic fungi which includes black yeasts), and typical soil inhabiting actinomycetes and nonmotile endospore-forming gram-positive cocci [4,5]. Dozens of cultured strains of varnish microorganisms oxidize Mn and/or Fe [3,5,6]. These microorganisms include members of the bacterial genera Micrococcus, Arthrobacter, Bacil-lis and the actinomycetes Geodermatophilis. Unfortunately , the study of MCF has proven to be inherently difficult and little is known about them [3,5]. One SEM study [7] found Mn present only within the center of an MCF and not in the surrounding varnish. However, they were unable to culture the specimen for further investigation. Speculation exists that the mela-nin-pigmented, thick and multi-layered walls of MCF would enhance the biosorbtion of metals [5]. The purpose of this study is the examination of the potential of desert varnish in preserving microfossils or other biosignatures. Two previous studies have found evidence of fossilization. Krinsley's study [4] deter