Nadezhda Chernikova

Five Gram-negative, rod-shaped, motile microorganisms were obtained from root nodules of Arctic and Subarctic Oxytropis species. The strains were designated as RCAM 04772^T, RCAM 04855, RCAM 06350^T, RCAM 05645, and RCAM 04822^T. Phylogenetic and phylogenomic analyses placed them in the genus Mesorhizobium. Genome sizes ranged from 7.70 to 8.45 Mb, with a DNA G + C content of 62.2–62.5%. ANI and dDDH values with the closest type strains (89.85–94.29% and 38.2–56.7%, respectively) were below the species delineation thresholds. Two strain pairs shared values above these thresholds—RCAM 04772^T with RCAM 04855 (99.65% ANI, 97.9% dDDH), and RCAM 05645 with RCAM 06350^T (97.50% ANI, 77.6% dDDH), while RCAM 04822^T remained distinct, indicating three novel species. The cellular fatty acid profile was dominated by C16:0 and C18:1Δ9c. The strains were psychrotolerant and capable of growth at 1 °C. Analysis of the symbiotic genes nodC and nifH placed all five strains in a one clade together with previously described Arctic isolates, and they were therefore assigned to symbiovar oxytropidis. All strains nodulated Oxytropis spp. and several other legumes from the tribes Galegeae, Hedysareae, and Robinieae. Based on genomic, phenotypic, and chemotaxonomic evidence, we propose Mesorhizobium kamtschaticum sp. nov. (RCAM 04772^T = B-2194^T and RCAM 04855 = B-2195), Mesorhizobium oxytropidicola sp. nov. (RCAM 06350^T = B-2197^T and RCAM 05645 = B-2198), and Mesorhizobium subarcticum sp. nov. (RCAM 04822^T = B-2203^T).