Microbial–silica interactions in Icelandic hot spring sinter: possible analogues for some Precambrian siliceous stromatolites

• Published in: Sedimentology
• Main Authors: Konhauser, Kurt O., Phoenix, Vernon R., Bottrell, Simon H., Adams, David G., Head, Ian M.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2001
• Subjects: Iceland
• Online Access: http://dx.doi.org/10.1046/j.1365-3091.2001.00372.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1046%2Fj.1365-3091.2001.00372.x; https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3091.2001.00372.x

Silicified deposits, such as sinters, occur in several modern geothermal environments, but the mechanisms of silicification (and crucially the role of microorganisms in their construction) are still largely unresolved. Detailed examination of siliceous sinter, in particular sections of microstromatolites growing at the Krisuvik hot spring, Iceland, reveals that biomineralization contributes a major component to the overall structure, with approximately half the sinter thickness attributed to silicified microorganisms. Almost all microorganisms observed under the scanning electron microscope (SEM) are mineralized, with epicellular silica ranging in thickness from < 5 μm coatings on individual cells, to regions where entire colonies are cemented together in an amorphous silica matrix tens of micrometres thick. Within the overall profile, there appears to be two very distinct types of laminae that alternate repeatedly throughout the microstromatolite: ‘microbial’ layers are predominantly consisting of filamentous, intact, vertically aligned, biomineralized cyanobacteria, identified as Calothrix and Fischerella sp.; and weakly laminated silica layers which appear to be devoid of any microbial component. The microbial layers commonly have a sharply defined base, overlying the weakly laminated silica, and a gradational upper surface merging into the weakly laminated silica. These cyclic laminations are probably explained by variations in microbial activity. Active growth during spring/summer allows the microorganisms to keep pace with silicification, with the cell surfaces facilitating silicification, while during their natural slow growth phase in the dark autumn/winter months silicification exceeds the bacteria’s ability to compensate (i.e. grow upwards). At this stage, the microbial colony is probably not essential to microstromatolite formation, with silicification presumably occurring abiogenically. When conditions once again become favourable for growth, recolonization of the solid silica surface by ...