In situ silicification of an Icelandic hot spring microbial mat: implications for microfossil formation
Transmission electron microscopy and energy-dispersive x-ray analysis revealed that filamentous phototrophic bacteria resembling Chlorojlexus aurantiacus underwent rapid silicification in an Icelandic hot spring microbial mat. The mineralization associated with the cells occurred both extracellularly, within and on the external sheaths of the bacteria, and intracellularly, within the cytoplasm. The exceptional preservation of the bacterial sheaths is due to the presence of distinct mineral nucleation sites. This results in the production of silica casts of the bacteria, which bear a striking resemblance to microbial remains in ancient microfossil assemblages. RCsumC : Les analyses par microscopie Clectronique i transmission et par 1'6nergie dispersive des rayons-x ont rCvCl6 que les bactkries filamenteuses phototrophes, ressemblant B Chlorojlexus aurantiacus, ont subi une silicification rapide au sein d'un tapis microbien de source therrnale, en Islande. La mineralisation associee aux cellules se presente sous forme extracellulaire, B l'interieur de et sur la membrane externe des bacteries, et sous forrne intracellulaire, i l'interieur du cytoplasme. La preservation exceptionnelle de la membrane des bacteries est due B la presence de sites de nucleation minerale disperses. Le resultat est la production des moulages siliceux des bactkries, affichant une resemblance remarquable avec les vestiges microbiens d'anciens assemblages microfossiles. (Traduit par la raaction) Fossil evidence of the Earth's earliest biosphere shows that complex communities of microorganisms were abundant and well distributed throughout the Precambrian Era. Among the richest beds of microbial fossils are those found in the Gunflint Iron Formation north of Lake Superior in Canada (Barghoorn and Tyler 1965), the early Archaean Apex chert of Australia (Schopf and Packer 1987; Schopf 1993), and the Riphean section of the Anabar uplift in Russia (Sergeev 1993); all ranging in age from 0.8 to 3.5 Ga. The distribution and types of microorganisms preserved as cellularly intact microfossils are remarkably similar to those in modern microbial mats, in which organic-rich layers formed by cyanobacteria and (or) other types of filamentous bacteria alternate with layers that are comprised of detrital inorganic sediment and authigenic mineral precipitates (Morse and Mackenzie 1990). These benthic microbial communities are considered to be analogous, and possibly homologous, with those that were preserved billions of years ago as stromato-