Microbial Communities in Natural Mineral Waters of Bulgaria: Diversity and Biotechnological Potential

Mineral waters represent unique limnological ecosystems with stable physicochemical conditions and specialised microbial communities adapted to extreme environments. Bulgarian mineral waters remain comparatively underexplored despite their considerable ecological and biotechnological significance. These studies present a systematic narrative review of microbial diversity, ecological functions, and biotechnological potential of microbial communities from Bulgarian mineral springs. A total of 233 scientific sources published between 1990 and 2026 were analysed, of which 33 focused on Bulgarian sites. Data were retrieved from major scientific databases, regional reports and grey literature. Due to strong methodological heterogeneity, a qualitative synthesis was conducted, supported by bibliometric summaries of research focus and environmental context. The available evidence demonstrates that microbial communities in Bulgarian mineral waters include diverse bacteria, archaea, cyanobacteria, and microalgae that adapt to broad thermal and geochemical gradients. These microorganisms actively participate in element cycles, form complex biofilms, and show numerous physiological adaptations to oligotrophic and extreme temperature conditions. Bulgarian systems broadly reflect global microbial patterns but exhibit additional variability linked to contrasting hydrogeological settings. Many taxa produce thermostable enzymes, antimicrobial compounds, and exopolysaccharides with significant biotechnological potential. The review identifies significant research gaps and emphasises the importance of integrated multi-omics approaches for future exploration of Bulgarian mineral water ecosystems.