THE SIMULATED SILICIFICATION OF BACTERIA – NEW CLUES TO THE MODES AND TIMING OF BACTERIAL PRESERVATION AND IMPLICATIONS FOR THE SEARCH FOR EXTRATERRESTRIAL MICROFOSSILS

Evidence of early life on Earth in the form of silicified microorganisms has been reported from rock formations throughout the geological and fossil record from as early as 3.5 billions years ago to recent silicification in hot spring environments. In order to better understand the mechanisms of silicification and thus improve our search patterns for evidence of fossil microbial life in rocks, a series of laboratory controlled experiments were designed to simulate the silicification of microorganisms. The facultative aerobic bacterial strain Pseudomonas fluorescens and the obligate anaerobic sulphate reducing bacterial strain Desulphovibrio indonensis were exposed to silicifying media. The experiments were designed to obtain information on the influence of exposure time of the biofilms to silica-rich aqueous solutions, as well as to elucidate the effects of ion availability. Biofilms of both bacterial species were exposed to a 1000 ppm silica solution for 4 hours, 24 hours, 1 week, 1 month, 3 months, and 6 months, and also for 1 week to 1000, 3000, and 5000 ppm silica solutions. Subsequent to the silicification processes the bacterial biofilms were analysed using Transmission Electron Microscopy (TEM) in combination with Energy Dispersive X-ray Analysis (EDX). Both bacterial species showed evidence of silicification after only 4 hours in 1000 ppm silica solution, although it emerged that D. indonensis were less prone to silicification. High ion availability resulted in better preservation of cellular detail; the concentration of silica therefore is more important than the duration of exposure to a silicifying agent. Cells became permineralised and no evidence of massive amorphous silica precipitation was observed. High-resolution TEM analyses revealed the presence of nanometer-sized crystallites within the bacterial cells after one week in the 5000 ppm silica solution of possible siliceous nature. The mechanisms of silicification under laboratory-controlled conditions and the implication for silicification in natural environments, the search for early Earth life and possible evidence of extraterrestrial life are discussed.