Alpha-, gamma- and beta-proteobacteria detected in legume nodules in Latvia, using full-length 16S rRNA gene sequencing

ABSTRACT Rhizobia are soil bacteria that form nodules on legumes and fix atmospheric nitrogen, reducing the need for chemical fertilisers. In most studies rhizobia are described as alpha-proteobacteria and have been studied extensively for several decades. However, in recent years an increasing number of studies present beta- and gamma-proteobacteria strains as potential rhizobia-like bacteria. In this study we identify bacteria isolated from nodules of various legumes grown in Latvia, using a full-length 16S rRNA gene sequence, used for more precision, compared with a partial sequence of the gene. Bacterial strains in this study have been isolated from 1962–2019, part of which are rhizobia strains from a unique historical collection. This is the first time bacterial strains isolated from nodules in Latvian soils have been identified with molecular biology methods. A wide taxonomic diversity was detected – bacteria species from α-, β-, γ-proteobacteria classes and Paenibacillus polymyxa strains from the Bacilli class. P. polymyxa strains were detected only in the historical collection, while β- and γ-proteobacteria strains were obtained only from the newly isolated specimens, uncovering new potential strains for a commercial legume seed inoculum. Soil samples were collected, and phosphorus levels were determined. Several strains indicate phosphate solubilising properties.