Many of the siliceous hot springs in Yellowstone National Park contain subaqueous, spinose siliceous precipitates up to 5 cm high that occupy shallow terracettes in siliceous terraced mound accumulations, discharge channels, etc. These siliceous “shrubs” are composed of opal-A with an arborescent or branching pattern and have strong morphological similarities to bacterial shrubs from carbonate-precipitating hot springs. Siliceous shrubs constitute a major precipitate style associated with discharge channel – flow-path facies throughout most of the 20 m of flow path at Cistern Spring, Norris Geyser Basin. They are found in siliceous spring waters ranging in temperature from 76.4 to 16.2 °C and pH from 6.0 to 7.4. At every scale, siliceous shrubs contain abundant evidence of microbial life in the form of bacterial body fossils and extracellular polymeric substances. The presence of relict organic constituents and bacterial morphological fossils indicates that the shrub fabric and architecture are dominated by bacteria, i.e., there is potentially a strong biotic effect on the precipitation process. Precipitation of opal in siliceous shrubs is very likely the result of either active bacterially induced precipitation or passive mediation through organic templates. On a larger scale, siliceous shrubs contain abundant evidence of former microbial activity in hot springs, thus they are good microbial biomarkers.

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