Numerous rod-shaped bacterial cells were present in amorphous silica which had formed on the copper plate placed in geothermal hot water, under conditions of pH 7.2 and 85 ± 2°C. Bulk genomic DNA in the siliceous deposit was extracted, using lysozymes and the freeze-thaw method. The volume of siliceous deposit formed on one copper plate and the amount of genomic DNA extracted from siliceous deposits exponentially increased with the time of incubation. The phylogenetic diversity in these DNA extracts was investigated by cloning and sequencing of partial 16S rRNA genes obtained by PCR. The bacterial community was composed mainly of three phylogenetic types in domain Bacteria. Cluster I (8 clones) was affiliated with the Aquificales and cluster II (15 clones) was closely related to the genus Thermus. Cluster III (2 clones), the sequences of which were homologous with Gram-positive anaerobic thermophilic bacteria, was also detected. These extremely thermophilic bacteria may possibly contribute to the rapid aggregation of amorphous silica.
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