Microbiological survey of el Chichon volcanic lake by cultivation - dependent and - indipendent techniques

Volcanic environments belong to the so-called “extreme environments” where the environmental stressors for the living biota are represented by acidic pH, moderate to high temperatures and high concentrations of chemicals like heavy metals. The investigation of microbial life in volcanic environments is of great interest. Thermoacidophilic microorganisms have potential biotechnological applications and, moreover, their ecophysiological study can give insights about the origin of life on our planet, since volcanic environments are considered analogous to the early Earth. In June 2009 five sites located into and in proximity of the volcanic lake El Chichón (Chiapas, Mexico) have been sampled from the acidic (pH 2.3) SO4-Cl type crater lake (LE), from the acidic (pH 4.1) SO4-type steam-heated pools (T 42°C) within the Soap Pool field (SP) and from acidic (pH 2.6-3.0) Cl-SO4 type saline thermal springs (T 53-59°C), located ~2 km outside the crater (AS1 and AS8). Molecular microbiology techniques are independent from cultivation and have the potential to overcome the limitations imposed by the low level of cultivability of environmental bacteria. Denaturing Gradient Gel Electrophoresis (DGGE) applied on the total DNA extracted from the samples demonstrated that i) all the sites were colonised by bacteria, ii) the different environments studied in El Chichón area contained a peculiar microbiota, probably adapted to the specific geochemical context. Basing on DGGE data, the microbiota inhabiting the crater lake resulted dominated by the species Acidithiobacillus thiooxidans, which has sulphur-oxidation energetic metabolism and plays a key role in the sulphur biogeochemical cycle. Sulphur-iron oxidizing bacteria were also cultivated from the SP and AS thermal springs. Bacteria of the order Aquificae were specifically retrieved in the thermal springs external to the crater, represented by Sulfurihydrogenibium azorense, a thermophilic, strictly chemolithoautotrophic species previously isolated from terrestrial hot springs in the Azores, Portugal. The molecular study was integrated by a cultivation-dependent approach in order to explore the physiological and metabolic properties of the organisms able to colonize such an extreme environment, with particular emphasis on bacteria having plant growth promoting (PGP) activity, since pioneer vascular plants were observed in SP site. Several bacterial species were associated to plant roots, some of them known for their capability to grow in extreme conditions. Tests about their survival at in situ conditions and potential PGP activity are at present undergoing. 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-deamminase, also other PGP activities such as phosphate solubilization, dinitrogen fixation, and the production of siderophores. Molecular and cultivation-based approaches demonstrated that El Chichón crater lake and the associated thermal springs are colonised by a rich microbiota mainly involved in the sulphur and iron biogeochemical cycles, which diversity is selected by the specific geochemical context. Plants adapted to live in this extreme environment are associated with several bacterial species having potential activity of plant growth promotion and stress protection.