Biodiversity of Cultivable Actinobacteria from Moroccan Extreme Ecosystems and Their Plant Growth-Promoting Potential

Extreme ecosystems represent valuable reservoirs of microbial biodiversity, harboring unique microorganisms with remarkable ecological and biotechnological properties. Morocco encompasses a wide range of extreme habitats, including arid deserts, high-altitude mountains, polymetallic mining areas, marine environments, and specialized rhizospheres, which remain largely unexplored for their actinobacterial diversity. This study investigated the diversity of cultivable actinobacteria isolated from five contrasting Moroccan extreme ecosystems and evaluated their plant growth-promoting (PGP) potential. Twenty-three actinobacterial isolates were obtained and identified through 16S rRNA gene sequencing. The isolates belonged predominantly to the genus Streptomyces, while representatives of the rare genera Nocardioides, Saccharomonospora, Actinomadura, and Prauserella were also recovered, highlighting the taxonomic richness of these environments. Functional characterization revealed that all isolates exhibited at least one beneficial trait associated with plant growth promotion. Several strains showed high capacities for phosphate and potassium solubilization, atmospheric nitrogen fixation, indole-3-acetic acid production, and siderophore synthesis. Principal coordinate analysis identified isolates from desert, marine, and rhizospheric habitats as the most promising multifunctional strains. The coexistence of high microbial diversity and multiple plant-beneficial functions demonstrates that Moroccan extreme ecosystems constitute an important source of novel actinobacteria with considerable ecological and agricultural value. These findings emphasize the importance of conserving microbial biodiversity in extreme environments and support the exploitation of native actinobacteria as eco-friendly biofertilizers and biostimulants for sustainable agriculture under changing environmental conditions.