ABSTRACT
The growth rate of silica sinters in active geothermal hot springs as seen in New Zealand or Yellowstone National Park is controlled both by inorganic and biogenic factors. However, the mechanisms that lead to the precipitation of amorphous silica from geothermal waters, or the silicification of the ubiquitously present microorganisms, are still controversial. Geothermal fluids contain high concentrations of silicic acid (H4 Si04) derived from water-rock interaction at depth. Upon reaching the surface, geothermal fluids cool and silicic acid begins to polymerize. Experimental evidence shows that the ratio of monomeric to polymeric silica plays an important role in silica precipitation rates (Carroll et al. 1998). It has also been recognized that mesophilic and thermophilic organisms can act as substrates for silica precipitation (e.g. Cady & Farmer 1996; Mountain et al. 2003 and references therein). Nevertheless, the question as to whether microorganisms play an active or passive role in silica precipitation is still unresolved. Walter et al. (1972, 1976) concluded that silicification of microbial material was a passive process. Later studies have postulated that microbial material, such as cell walls and extracellular
polysaccharides, can provide preferred sites for silica nucleation and that some bacteria can be preserved (Ferris et al. 1986; Konhauser & Ferris 1996; Phoenix et al. 2000, 2001; Konhauser et al. 2001). Whether silicification is an entirely passive or microbiallypromoted process remains to be answered and is currently a subject of continued study (Benning et al. 2004a).
