A novel nitrogen-fixing bacterium, Propionivibrio soli sp. nov. isolated from paddy soil.

A facultative anaerobic nitrogen-fixing bacterium, designated SG131, was isolated from paddy soil. Strain SG131 showed high 16S rRNA gene sequence similarities with type strains Propionivibrio limicola DSM 6832 (96.9%), Propionivibrio pelophilus asp 66 (96.0%) and Propionivibrio dicarboxylicus DSM 5885 (95.7%). The phylogenetic trees (based on 16S rRNA gene sequences and 120 conserved genes from genomes, respectively) indicated that strain SG131 clustered with members of the genus Propionivibrio. Growth of strain SG131 was observed at 25-40 °C, pH 5.5-10.5 and 0-0.5% (w/v) NaCl. The quinone was Q-7, and the main fatty acids were C ω6c and/or C ω7c (25.9%), C (23.3%), C-cyclo (11.7%), C (6.0%) and C (5.9%). The genomic DNA G + C content of strain SG131 was 60.3%. The average nucleotide identity (ANI) values between strain SG131 and its most closely related species P. limicola DSM 6832, P. pelophilus DSM 12018 and P. dicarboxylicus DSM 5885 were 74.4%, 74.9% and 75.6%, respectively. The digital DNA-DNA hybridization (dDDH) values between strain SG131 and its most closely related species P. limicola DSM 6832, P. pelophilus DSM 12018 and P. dicarboxylicus DSM 5885 were 19.9%, 20.6% and 20.5%, respectively. All these values were lower than the recommended species delineation thresholds of ANI (95-96%) and dDDH (70%). Strain SG131 possessed core genes (nifHDK) of nitrogen fixation and was confirmed its nitrogen-fixing ability by the ARA method. According to the above-described analysis, strain SG131 represents a novel species of the genus Propionivibrio, for which the name Propionivibrio soli sp. nov. is proposed. The type strain is SG131 (= GDMCC 1.3313 = JCM 35595).