MINERALIZATION OF MICROBES IN MODERN HOT SPRINGS

Siliceous sinters that are found in modern hot spring systems within the Kenya Rift Valley and the Taupo Volcanic Zone on the North Island of New Zealand, commonly contain numerous well-preserved microbes. Conversely, calcitic and aragonitic deposits from hot springs in those same areas are generally devoid of mineralized microbes. This preservational bias may carry important implications for the assessment of ancient microbiotas. Microbes will only be preserved in the rock record if they are mineralized before they are degraded by their own autolytic enzymes or consumed by chemoheterotrophs. Thus, the common observation of three-dimensionally intact morphologies in sinters implies that they were rapidly mineralized. In this work we highlight the speed with which mineralization can take place. In our experiments, consisting of placing glass slides in the hot (> 75°C), silica-saturated waters of Iodine Spring in the Waimangu Geothermal Valley (North Island, New Zealand), we observed the completed silicification of microbes in less than 96 hours. It might be expected that such rapid mineralization would produce perfectly preserved microbes that would be easy to identify in terms of extant taxa. This is not the case. Indeed, silicification disguised the microbes to such an extent that it was difficult even to match the silicified microbes with the extant, non-mineralized microbes found on the same glass slide. The mineralization style of some microbes seems, in some cases, to be taxa specific. Such a relationship, if real, might carry important implications for the recognition and interpretation of ancient mineralized microbes that survive diagenesis. Two or more layers of opal-A precipitates characterize some of the silicified microbes found on the glass slides placed in Iodine Pool. The innermost layer is typically formed of very small opal-A spheres that are arranged in well-defined rows, whereas the outermost layers are formed of featureless masses and spheres of opal-A. It is possible that a regularly spaced feature in the original microbe controlled the location of the spheres in the innermost layer. If so, that factor only controlled precipitation of the innermost layer. The local physicochemical environment, not the microbe, controlled precipitation of the opal-A that forms the outer layers.