Beyond the boundaries of microbial diversity in radioactive environments: novel microbial species isolated from a former silver-uranium mine’s radon-saturated waters

The Svornost mine in Jáchymov (Joachimstahl), Czech Republic, historically the world’s first radium mine and formerly a silver-uranium mine, is currently the deepest facility exploiting radon-saturated waters. These geologically ancient waters, approximately 13,000 years old, exhibit unique physicochemical properties and are generally considered inhospitable to life. Nevertheless, they harbor phylogenetically and evolutionarily distinct microorganisms. A previous culturomic survey yielded nine Gram-stain-negative, rod-shaped, aerobic bacterial isolates. In this study, a polyphasic taxonomic approach, including 16S rRNA gene phylogeny, core-genome analyses, and comparisons of genome indices such as average nucleotide identity (ANI), average amino acid identity (AAI), and percentage of conserved proteins (POCP), together with physiological and chemotaxonomic characterization, confirmed the taxonomic novelty at species level of these nine strains. The novel strains span five bacterial classes (α-, β-, and γ- Proteobacteria , Cytophagia , and Flavobacteriia ) and eight genera, and are herein proposed as Hymenobacter abyssi J193 T (= DSM 113871 T =CCM 9301 T ), Rhizobacter abyssi J219 T (= DSM 113781 T =CCM 9293 T ), Aquabacterium abyssi J223 T (= DSM 113847 T =CCM 9303 T ), Aquincola abyssi J276 T (= DSM 113828 T =CCM 9294 T ), Flavobacterium abyssi J372 T (= DSM 113872 T =CCM 9299 T ), Phenylobacterium abyssi J367 T (= DSM 113838 T =CCM 9302 T ), Phenylobacterium fontinale J426 T (= DSM 113839 T =CCM 9296 T ), Mesorhizobium abyssi J428 T (= DSM 114114 T =CCM 9297 T ) and Pseudomonas fontinalis J452 T (= DSM 113783 T =CCM 9298 T ). These findings highlight the potential of radon-saturated ancient waters as a source of novel microbial diversity, offering insights into the evolution of bacteria under extreme, spatially isolated environmental conditions.