This thesis, supervised by Manuel Porcar, Albert Quintana and Juli Peretó, studies the microbial diversity of several dry and irradiated environments, explores several approaches to increase the number of strains of potential interest and investigates some biotechnological and biomedical applications. The research results have been published in the journals Microbial Biotechnology, Frontiers in Microbiology, Microorganismes, International Journal of Systematic and Evolutionary Microbiology, Scientific Reports and Frontiers in Bioengineering and Biotechnology. The thesis was defended on November 3, 2023.

Bioprospecting is the search for microorganisms, or their parts, to develop biotechnological tools. Extremophilic environments, such as highly irradiated and dry environments host interesting microbes from the biotechnological perspective. The work described in the thesis "Light and Drought. Bioprospecting the microbiota of highly irradiated and dry environments" aims at characterizing the microbial profiles associated to natural and artificial surfaces subjected to irradiation (the Mediterranean rocky-shore, the Tabernas Desert and hospital UV cabins), through a holistic strategy including culture-independent and -dependent techniques. This work emphasizes that improving the recovery of the maximum microbial diversity and the description of novel taxa may be achieved through the combination of simple culturing strategies. In parallel, the biotechnological and biomedical applications of these microorganisms were investigated. Furthermore, the use of alternative culture configurations of solid-state fermentation through the use of 3D matrixes has been studied.

The combination of culture media and incubation conditions has resulted in the isolation of a high number of different genera, potentially new taxa and biotechnologically-relevant strains, as evidenced by the description of three new Kineococcus and two Belnapia species. Moreover, new generation sequencing (NGS) allowed the determination of differences and similarities among the studied sites. Specifically, the communities of the three highly-irradiated environments were dominated by members of Pseudomonadota, Actinomycetota and Bacteroidota. Moreover, NGS revealed that the low diversity found in the collection of isolates from the Mediterranean rocky shore was influenced by culturing methods and PCR performance. Moreover, genera such as Hymenobacter, Rubellimicrobium, Paracoccus and Corynebacterium were abundant in NGS yet completely absent in the collections.

In a standardized approach to microbial growth under low water activity, the design of 3D-printed solid matrixes was aimed at studying whether these structures may influence yeast growth and their impact on biotechnology. The growth of Saccharomyces cerevisiae revealed that these 3D matrixes dramatically fostered cell-yields compared to the equivalent static cultures and appeared to favour alcoholic fermentation. Moreover, highly-differentiated proteomic profiles indicated drastic metabolic adjustments.

Finally, the antioxidant potential strains isolated from the Mediterranean supratidal zone was assessed through in vitro and in vivo assays. Moreover, the antioxidant potential of a strain of Micrococcus luteus, a carotenoid-synthetizing bacteria, was tested in a murine model of a human mitochondrial disease (Leigh Syndrome), as a collaboration with the group of Mitochondrial Neuropathology in the Universitat Autònoma de Barcelona. Although the administration of a microbial preparation had no beneficial effects on the diseased mice, serendipity led to the identification of maltodextrin (used as a cryoprotectant) as responsible of a reduction in neuroinflammation, an increased lifespan and a variability in the abundance of Akkermansia spp. in the gut of mice.

Esther Molina-Menor conducted her doctoral research in the Biotechnology and Synthetic Biology group under the supervision of Manuel Porcar, a University of Valencia researcher at the Institute for Integrative Systems Biology I2SysBio (UV-CSIC), Albert Quintana, Associate professor at Universitat Autònoma de Barcelona and researcher at the Institut de Neurociències (INc, UAB) and Juli Peretó, UV Professor and researcher at I2SysBio (UV-CSIC). During the development of the project Esther Molina-Menor has enjoyed a contract within the University Teacher Training Program (FPU, Ministry of Universities) and carried out research stays at the Max Plank Institute for Terrestrial Microbiology (Marburg) and at INc (UAB). The examining board was formed by Carles Pedrós-Alió (CSIC), Rosa Aznar Novella (UV) and Juana María Navarro Llorens (Universidad Complutense de Madrid), who graded the thesis as outstanding.