Constraining Microbial Community Composition, Structure, and Function Within Acidic Geothermal Springs on São Miguel Island

The study of polyextremophiles is critical for understanding both the limits of life and the adaptations that enable life under extreme conditions. Thermoacidophiles, which grow under co-occurring low pH (>3) and high temperature (>45C) conditions in geothermal springs, provide an ideal opportunity to investigate both. This thesis characterises the composition, structure, and function of thermoacidophilic communities living in acidic geothermal springs on São Miguel island, Azores, Portugal. Results from 16S rRNA gene sequencing showed that the polyextreme springs (pH 70°C) were dominated by archaea, had low microbial diversity, and supported divergent microorganisms. Overall, and in common with other similar studies, the results showed that community composition was largely driven by pH rather than temperature. No clear statistical link was found between the chemical composition of the springs and the microbial community composition. Detailed bioinformatics studies to assess the community structure and function were hampered by the available software performing poorly on divergent microorganisms. New software tools were developed during this study to effectively assess such divergent communities; these tools allowed accurate constraint of the polyextremophile community composition, structure and function. One tool, MetaAcc allowed the composition of simulated microbial communities with highly divergent microorganisms to be accurately determined. A second tool, MicroState, allowed for the approximation of community structure by predicting trophic states using only genomic data. In addition, the composition, structure, and function of a sampled thermoacidophilic biofilm was investigated using a combination of available and new bioinformatics tools. Results showed that the community contained several divergent community members, and was dominated by heterotrophs. The biofilm microorganisms showed adaptation that promoted survival in low pH and high temperature conditions. Functional potential data suggested the presence of complex carbon and iron biogeochemical cycles within the biofilm community that promoted the formation of green rust. Microbially-driven green rust formation has only previously been observed in moderately acidic or neutrophilic conditions