Fossils of Prokaryotic microorganisms in the Orgueil meteorite

The Orgueil CI1 meteorite, which fell in southern France on the evening of May 14, 1864, has been one of the most extensively studied of all known carbonaceous meteorites. Environmental and Field Emission Scanning Electron Microscopy (ESEM and FESEM) studies of freshly fractured interior surfaces of the Orgueil meteorite have resulted in the detection of the fossilized remains of a large and diverse population of filamentous prokaryotic microorganisms. The taphonomy and the modes of the preservation of these remains are diverse. Some of the remains exhibit carbonization of a hollow sheath and in other cases the remains are permineralized with watersoluble evaporite minerals, such as magnesium sulfate or ammonium salts. Images and EDS spectral data are provided documenting a variety of Orgueil microstructures: abiotic evaporite minerals; pre-solar grains; framboids, ovoids and fluorine-rich filaments of indeterminate biogenicity. These results provide information regarding the size, size range, morphologies and chemical compositions of abiotic microstructures found in native cryptohalite and crystalline and fibrous epsomites from Poison Lake, Washington, USA and Catalayud, Zaragoza, Aragon, Spain. High resolution ESEM and FESEM secondary and backscattered electron Images and Energy Dispersive X-ray Spectroscopy (EDS) data will also be presented of recognizable filamentous cyanobacteria and other prokaryotic microfossils. Many of the microfossils found embedded in the meteorite rock matrix are consistent in size and microstructure with known genera and species of filamentous cyanobacteria. Many of these forms can be recognized as morphotypes of cyanobacteria belonging to the Order Oscillatoriaceae. These large, complex microstructures are clearly distinguishable from the abiotic minerals examined - cryptohalite and fibrous epsomites. Many of the well-preserved filamentous forms in the Orgueil meteorite are embedded in the rock matrix and exhibit identifiable biological characteristics and EDS elemental compositions that clearly differentiate them from abiotic microstructures. They exhibit branched and unbranched filaments; uniseriate and multiseriate ensheathed trichomes with specialized cells and structures for reproduction (e.g. hormogonia and akinetes) and nitrogen fixation (heterocysts). The meteorite forms also show evidence of motility (emergent hormogonia and hollow, coiled empty sheaths) and they are often found in mats, consortia and microbial assemblages that are consistent with the known associations and ecologies of modern genera and species of cyanobacteria. Smaller filaments, consistent with the phototrophic filamentous prokaryotes typically present in anoxic layers associated with modern cyanobacterial mats, have also been found in the Orgueil meteorite.