MICROBIALLY DOMINATED TERRESTRIAL HOT SPRING MINERAL ASSEMBLAGES IN KAMCHATKA RUSSIA: THE PRESENT IS THE KEY TO PAST

The Uzon Caldera constitutes a large geothermal region with varied high-temperature, acidic, and strong redox environments hosting novel life forms. Spring waters, microbial mats and sediments were sampled within this context to characterize the thermophilic and hyperthermophilic microbial populations and their associated mineral assemblages. Carboxydotrophs, iron-, sulfate-, and arsenate-reducers among others were targeted through isolation of novel species, MPN field incubations, 16S rDNA clone libraries, metagenomic libraries, enrichment culturing, and radiotracer experiments. We have isolated new species/genera related to Carboxydocella, Dictyoglomus, Thermoanaerobacter, Thermobaculum, Geobacillus, Bacillus, and Anoxybacillus. Minerals in sediments, sinters, and mats were studied using X-ray diffraction, TEM, SEM/EDS, and stable isotope geochemistry. Electron microscopy has revealed intimate and potentially functional relationships between nano-crystalline surfaces (alunite, smectite, halloysite, opal, pyrite, and ferrihydrite) and thermophilic microbes. Extracellular silicification and intracelllular pyritization are common mineral forming processes known to interact with microbial cells and cellular polymeric substances. The advantage of remnant silicified microbial tubes is that they act as a protective containment to microorganisms, which reside within tubes. In the recent geologic record (rocks thousands of years old) these structures will likely be recognized as microbial remnants. These structures will unlikely be preserved in the long term geologic record (millions of years) as for example, opal-A dissolves back into solution altering and/or destroying silicified molds. As microbial morphological preservation decreases throughout time, one of the remaining markers of biological activity will be in the associated mineral assemblage. Here we demonstrate a mineral assemblage clearly forming in the presence of a microbial community. Finding the same mineral assemblage forming today in the absence of a microbial community is the evidence needed to suggest that life is not a key factor. One can conclude that similar mineral assemblages found in ancient rocks owe their genesis to similar microbial processes in hot reducing fluids rich in silica and metals.