The silica cycle in the Precambrian

A new method for controlling leaching through permeable channels

Microbial mats: Stromatolites: Edited by Yehuda Cohen, Richard W. Castenholz and Harlyn O. Halvorson. Alan R. Liss, New York, 1984, 498 pp

Microbial mats, located along the margins of hot-spring pools and outflow channels at Lake Bogoria, Kenya, are commonly silicified forming friable laminated crusts Columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray Silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small (< 2 μm) silica spheroids Rapid silicification of the microbes, which may begin while some are still alive, can preserve sheaths and in some examples, the filaments, capsules and cells Although this provides evidence of their general morphology, the biological features that are required for taxonomic identifications are commonly poorly preserved



The silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes Almost all the silica at the Loburu springs nucleates on microbial substrates This affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts

Rapid in situ silicification of microbes at Loburu hot springs, Lake Bogoria, Kenya Rift Valley

Evidence of microbial life on Earth has been found in siliceous rock formations throughout the geological and fossil record. To understand the mechanisms of silicification and thus improve our search patterns for evidence of fossil microbial life in rocks, a series of controlled laboratory experiments were designed to simulate the silicification of microorganisms. The bacterial strains Pseudomonas fluorescens and Desulphovibrio indonensis were exposed to silicifying media. The experiments were designed to determine how exposure time to silicifying solutions and to silicifying solutions of different Si concentration affect the fossilization of microbial biofilms. The silicified biofilms were analyzed using transmission electron microscopy (TEM) in combination with energy-dispersive spectroscopy. Both bacterial species showed evidence of silicification after 24 h in 1,000 ppm silica solution, although D. indonensis was less prone to silicification. The degree of silicification of individual cells of the sam...

The simulated silicification of bacteria--new clues to the modes and timing of bacterial preservation and implications for the search for extraterrestrial microfossils.

Bacterial sulfate reduction and pH: Implications for early diagenesis

Selective replacement of sulfate-evaporite minerals by silica and the precipitation of silica in association with sulfide mineral phases in banded iron formations may be mediated by the metabolic a...

Sulfate-reducing bacteria and silica solubility: a possible mechanism for evaporite diagenesis and silica precipitation in banded iron formations

Growth experiments utilizing the sulfate reducing bacterium, Desulfovibrio desulfuricans, suggest a microbial influence in. Experiments conducted with 400 ppm silica produced a dense precipitate of silica within 72 hours in which cells were entombed and caught in cellular division. Experiments conducted in the absence of cells required one month or longer for a loose flocculant of silica to develop; in some samples a precipitate did not form over a 3 month duration. The silicification phenomenon greatly altered the morphology of the bacteria causing deformation of rod-shaped vibrios into partially collapsed cocci and intermediate morphological varieties. This alteration was probably caused by osmotic shock influenced by crystallite accumulation at the cell wall and concomitant retardation of solute transport during silica polymerization. It is suggested that these experiments more closely reproduce natural conditions of silicification of microorganisms than have previous studies and that the morphological change may have implications for interpreting the Precambrian microbial fossil record.

Silica Precipitation Induced by the Anaerobic Sulfate Reducing Bacterium Desulfovibrio Desulfuricans: Effects Upon Cell Morphology and Implications for Preservation

John W. Wireman

Papers

Bacterial sulfate reduction and pH: Implications for early diagenesis

Sulfate-reducing bacteria and silica solubility: a possible mechanism for evaporite diagenesis and silica precipitation in banded iron formations

Silica Precipitation Induced by the Anaerobic Sulfate Reducing Bacterium Desulfovibrio Desulfuricans: Effects Upon Cell Morphology and Implications for Preservation