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Evaporative silicification can drive microbial fossilization within diverse natural habitats. Research into this process is pivotal to understanding the terrestrial fossil record and the preservation of biomarkers within extreme environments. We employed laboratory experiments to silicify the polyextremotolerant bacteriumHalomonas hydrothermaliscultured at low, intermediate and high salinities (1, 3.5 or 11.8% w/v of NaCl) under iron-rich or iron-deprived conditions. Silicification was achieved by adding sodium silicate solution (30 or 150 ppm of Si) onto cultures, followed by evaporation. Scanning electron microscopy demonstrated the presence of mineralized bacteria with intact morphology across all culture conditions. However, multivariate analysis of the attenuated total reflectance Fourier-transform infrared (ATR-FT-IR) spectra of silicified cultures showed significant differences between the examined salinities, most notably between cultures silicified after incubation at high salinity and those at lower salinities. Although the spectra of mineralized low- and intermediate-salinity cultures appeared distinct from their nonsilicified counterparts, these differences were less pronounced at high salinity. By showing that differences in salinity can influence microbial responses to mineralization at the molecular level, these data indicate that the potential for evaporative silicification to contribute to microbial fossilization may differ between freshwater and hypersaline environments. [ABSTRACT FROM PUBLISHER]

The weathering of silicate in the world's critical-zone (rock-soil interface) is a natural mechanism providing a feedback on atmospheric CO2concentrations through the carbonate-silicate cycle. We examined culturable bacterial communities from a critical-zone in western Iceland to determine the optimum growth temperature and their ability to solubilize phosphate-containing minerals, which are abundant within the critical-zone area examined here. The majority of isolated bacteria were able to solubilize mineral-state phosphate. Almost all bacterial isolates were mesophilic (growth optima of 20–45°C), despite critical-zone temperatures that were continuously below 15°C, although all isolates could grow at temperatures associated with the critical-zone (−2.8–13.1°C). Only three isolates were shown to have thermal optima for growth that were within temperatures experienced at the critical-zone. These findings show that the bacteria that inhabit the western Icelandic critical-zone have temperature growth optima suboptimally adapted to their environment, implying that other adaptations may be more important for their long-term persistance in this environment. Moreover, our study showed that the cold basaltic critical-zone is a region of active phosphate mineral-weathering. [ABSTRACT FROM PUBLISHER]

Dark-colored rocks that attenuate photosynthetically active radiation (PAR) are generally thought to be poor substrates for endolithic colonization. In this study we show that the internal space of the common dark-colored mica mineral, biotite, is a favorable microhabitat for the growth of filamentous cyanobacteria. Laboratory incubation experiments demonstrated that filamentous cyanobacteria with a cell diameter of ≤3 μm were able to colonize biotite interlayers. Light transmittance measurements on solid biotite sheets showed that PAR (400–690 nm) transmission is sufficient to allow for cyanobacterial growth in the interlayer regions of the mineral. Of note is that damaging UV radiation is attenuated by more than one order of magnitude at the corresponding depth, most likely by the iron contained within the mineral, showing that the interlayer region represents a UV protected microhabitat with respect to the mineral surface. The interlayer region is therefore a favorable environment for small cells that cannot rely only on UV screening compounds. These results show that biotite, and by extension other layered silicate minerals, are a hitherto unrecognized habitat suitable for endolithic microorganisms with potential implications in research fields such as astrobiology. [ABSTRACT FROM PUBLISHER]