Reclassification of a Polynucleobacter cosmopolitanus strain isolated from tropical Lake Victoria as Polynucleobacter victoriensis sp. nov.

Abstract

The genus ; Polynucleobacter; (family ; Burkholderiaceae; ) is phylogenetically subdivided into at least four subclusters. One of those, subcluster PnecC, was recognized as a cryptic species complex. Here we test by comparative genome analyses whether subcluster PnecD, currently solely represented by the species ; Polynucleobacter; cosmopolitanus; , also represents such a cryptic species complex. The genome sequences of the two ; P. cosmopolitanus; strains, MWH-MoIso2T and MWH-VicM1, were determined. The latter strain was also characterized in the previous description of ; P. cosmopolitanus; . These two strains originate from a temperate lake located in Austria and from the large tropical Lake Victoria located in East Africa, respectively. Strains MWH-MoIso2T and MWH-VicM1 possess quite small genomes of 1.78 and 1.63 Mbp, respectively, and share similar G+C values of 44.1 and 43.1 mol%, respectively. Both strains encode only a single copy of the ribosomal operon, and their 16S rRNA genes differ only in four positions, equalling a sequence similarity of 99.74 %. Both genomes possess characteristics indicating evolutionary genome streamlining, such as high coding densities of 93.9 and 94.6 % of bases, respectively. Average nucleotide identity (ANI) comparisons of the genomes of the two strains resulted in a value of 78.4 %, suggesting that each of the strains represents a separate species. Our investigation suggests that PnecD represents an additional cryptic species complex within the genus ; Polynucleobacter; that was not resolved by 16S rRNA gene sequence analyses. We propose reclassification of strain MWH-VicM1 as ; Polynucleobacter; victoriensis sp. nov., with type strain MWH-VicM1T(=DSM 21486T=JCM 32005T).