Taxonomic fidelity of silicified filamentous microbes from hot-spring systems in the Taupo Volcanic Zone, North Island, New Zealand

Modern, silica-precipitating hot springs, like those found in the Taupo Volcanic Zone (TVZ) on the North Island of New Zealand, are natural laboratories for assessing microbial silicification. Many of the silicified microbes found in the siliceous sinters of these spring systems seem to be life-like replicas of the original microbes. Such preservation reflects the fact that many of the microbes are replaced and encrusted by opal-A before they are destroyed by desiccation and decay. The taxonomic fidelity of these silicified microbes depends on the preservation potential of those features which are needed to identify them. For example, identification of extant cyanobacteria, relies on as many as 37 different features, most of which are not preserved by silicification. In the hot-spring systems of the TVZ, characterisation of cyanobacteria which have been replaced and encrusted by opal-A is typically restricted to colony morphology, the length, diameter and morphology of the filament, and the presence/absence of septa, branching or a sheath. In many cases, description is limited to a subset of these parameters. Such a limited set of morphological characteristics severely impedes identifications in terms of extant taxa. The physical changes which accompany the stepwise diagenetic progression from opal-A to opal-CT +/- opal-C to microcrystalline quartz may lead to further degradation of the silicified microbes and the loss of more taxonomically important features. Clearly, considerable care must be taken when trying to name silicified microorganisms and make palaeoenvironmental inferences.