Salinity Influences the Response of Halomonas hydrothermalis to Artificial Fossilization by Evaporative Silicification

Abstract

ABSTRACTEvaporative 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 bacterium Halomonas hydrothermalis cultured 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.