Biosignatures and bacterial diversity in hydrothermal deposits of Solfatara

We have combined mineralogy, organic geochemistry and mole-cular microbiology to study hydrothermal deposits from Solfatara Crater, a geologically young volcanic formation (∼4,000 years old) displaying hot (45–95◦C) and acidic (pH 1.7) mud pools and fumaroles. The search for inorganic (mineral) biosignatures re-vealed the presence of delicate structures, most likely mineral-ized extracellular polymers (EPSs), and the presence of potential biologically induced minerals: sulfides, sulfates (barite and alu-nite), elemental sulfur, and iron oxides. Geochemical analyses re-vealed a low total organic carbon content, 0.13 to 0.53%, displaying δ13C values from −17.09 to −27.39‰, and total nitrogen con-tents from 0.03 to 0.12%, which are characteristic of hydrother-mal systems and suggest the presence of autotrophic carbon fixa-tion. Lipid biomarker analysis showed the presence of hopanoids and linear alkanes, and the absence of detectable steroids, imply-ing the occurrence of bacteria in our samples. We constructed 16S rRNA gene libraries from the environmental samples. Most environmental sequences obtained were affiliated to the Alpha-and Betaproteobacteria (Hydrogenophilus-like), the Acidobacteria, and to a lesser extent, the Gammaproteobacteria and Actinobacte-ria. When known, the closest cultivated relatives were often ther-mophilic or thermotolerant bacteria oxidizing iron, hydrogen, o