Unravelling extremophiles diversity of volcanic habitats

Volcanic habitats are classified as extreme environments due to their values of pH, temperature and concentration of chemicals, like heavy metals, permanently close to the extreme limits known for life. Due to the potential biotechnological applications of extremophile microorganisms, the investigation of microbial life in volcanic environments is of great interest. Moreover, volcanic environments are dominated by physiochemical conditions considered analogue to those present during the first stages of life on Earth and represent key habitats for astrobiology. Six volcanic lakes, dominated by different geochemical parameters such as concentration of dissolved gas and salt, were explored. Acidic waters of the El Chichón crater lake and related thermal springs were collected in Mexico, whereas the ultra-acidic lake of Poas volcano, the acidic Botos lagoon, the Irazù crater lake, the Laguna Hule and Rio Cuarto were sampled in Costa Rica. DNA-fingerprinting techniques allow to overcome the limitations imposed by the low level of cultivability of environmental bacteria. Denaturing Gradient Gel Electrophoresis (DGGE) fingerprinting applied on the metagenome extracted from the lake waters demonstrated that i) the sites were colonised by bacteria, ii) the different lakes hosted a peculiar microbiota, likely adapted to the specific environmental conditions. Cultivation-dependent techniques were also applied in order to explore the physiological and metabolic properties of the organisms able to colonize thermal spring field related to the El Chichón volcanic system, where pioneer vascular plants were observed. Special attention has been given to plant growth promoting (PGP) bacteria, with the aim of revealing bacteria able to aid the plant to cope with the harsh environmental conditions. Many different bacterial species were associated to plant roots, some of them known for their capability to grow in extreme conditions. Several strains showed 1-aminocyclopropane-1-carboxylate (ACC)-deaminase activity, one of the best studied PGP activity, with a role in protecting the plant from environmental stress. The isolates showed, besides ACC-deaminase, also other PGP activities such as phosphate solubilization, dinitrogen fixation, and the production of siderophores, potentially promoting plant growth by supplying mineral elements. The results of molecular fingerprinting and cultivation analyses demonstrated that the analyzed volcanic lakes and the associated thermal springs are colonised by a rich microbiota and that the specific geochemical context is the main driver of the microbial community composition. Moreover, plants adapted to live in the extreme environment of El Chichón lake are associated with several bacterial species having potential activity of plant growth promotion and stress protection.