Genomic insights into a free-living, nitrogen-fixing but non nodulating novel species of Bradyrhizobium sediminis from freshwater sediment: Three isolates with the smallest genome within the genus Bradyrhizobium

Abstract

Three bacterial strains isolated from a sediment sample collected at a water depth of 4 m from the Huaihe River in China were characterized. Phylogenetic investigation of the 16S rRNA gene and concatenated housekeeping gene sequences assigned the three novel strains in a highly supported lineage distinct from the published Bradyrhizobium species. The sequence similarities of the concatenated housekeeping genes of the three novel strains support their distinctiveness with the type strains of named species. Average nucleotide identity values of the genome sequences (79.9？82.5%) were below the threshold value of 95？96% for bacterial species circumscription. Close relatives to the novel strains are Bradyrhizobium erythrophlei, Bradyrhizobium jicamae, Bradyrhizobium lablabi, Bradyrhizobium mercantei, Bradyrhizobium elkanii and Bradyrhizobium japonicum. The complete genomes of strains S2-20-1T, S2-11-2 and S2-11-4 consist of single chromosomes of size 5.55, 5.45 and 5.47 Mb, respectively. These strains lack a symbiosis island, key nodulation and photosystem genes. Based on the data presented here, the three strains represent a novel species for which the name Bradyrhizobium sediminis sp. nov. is proposed for S2-20-1T as the type strain. Those three strains are proposed as novel species in free-living Bradyrhizobium isolates with the smallest genomes so far within the genus Bradyrhizobium. A number of functional differences between the three isolates and other published genomes indicate that the genus Bradyrhizobium is extremely heterogeneous and has roles within the community including non-symbiotic nitrogen fixation.