Article: The experimental silicification of microorganisms

A number of samples containing microorganisms was silicified at atmospheric and deep-sea pressures with the aim of studying the process of fossilization using SEM and TEM techniques. The samples included a bacteria-fungi-diatom culture, a bacteria-diatom culture, a microorganism-rich water sample from the interface of south-eastern Atlantic deep-sea sediments, and a microbial mat from the surface of other southeastern Atlantic deep-sea sediments. Silicification commenced with the impregnation of organic material (e.g. cell walls, cytoplasm) by subelectron-microscope-sized crystallites, and the nucleation of spheres of porous hydrated silica within the mucus (extracellular polymeric substances, EPS) of the groundmass. With increasing silicification time the silica precipitates became more electron dense (due to hydrolysis and polymerization) and larger, forming thicker deposits which resulted in an encrusted mammillated surface on the microbial fossils. Eventually, the microorganisms were completely engulfed by the siliceous deposits. The formation of artefacts during fossilization, such as collapsed, silicified cytoplasm, looking like false nuclei in the bacteria, was common. These experiments were undertaken in order to understand the process of silicification in nature. In particular, with respect to the exceptionally well-preserved, silicified microbial mat communities in uppermost Oligocene to middle Miocene sediments from the south-eastern Atlantic, they demonstrated that both the microorganisms and the polymeric slime in which they lived were mineralized. The degree of mineral impregnation and encrustation is related to the availability of the mineralizing ions. The heavily encrusted microorganisms in each of the upper Oligocene to middle Miocene samples from the south-eastern Atlantic Ocean were thus probably subjected to mineralization for a longer period than weakly impregnated/encrusted microorganisms from the same sample.