An updated outlook at the planctomycetal genus Singulisphaera: New members, genomes, and functional roles

The genus Singulisphaera accommodates stalk-free planctomycetes with spherical cells, which multiply by budding, possess complex cell organization, large genomes, and colonize a wide spectrum of terrestrial environments. Two described species of this genus, S. acidiphila and S. rosea, were isolated from acidic peatlands and characterized as mildly acidophilic bacteria. Here, we characterize two neutrophilic Singulisphaera isolates from a boreal fen, strains Ch08^T and PoT^T. We also obtained and analyzed the previously unavailable genome sequence of S. rosea S26^T. As confirmed by genome analyses and substrate utilization tests, all strains of Singulisphaera species were capable of growth on amorphous chitin due to the possession of chitinases affiliated with the glycoside hydrolase family GH18. The gene encoding chitinase from strain PoT^T was expressed in Escherichia coli, and the endochitinase activity of the recombinant enzyme was confirmed. The genomes also contained gene clusters encoding Planctomycetal-type bacterial microcompartment (BMC) organelles, suggesting participation of Singulisphaera species in degradation of plant-derived polysaccharides. Transcriptome analysis performed with glucose- and L-fucose-grown cells of strain PoT^T showed up-regulation of these BMC-encoding genes during growth on L-fucose. Tyrosinases involved in the oxidative removal of phenolic compounds were present only in S. acidiphila MOB10^T. All Singulisphaera genomes contained the genes of the recently identified pathway of ornithine lipid biosynthesis. Based on the reported results, we describe two novel species, S. photorubra and S. chitinilytica with the type strains Ch08^T (=KCTC 102485^T = LMG 34153^T) and PoT^T (=KCTC 102290^T = VKM B-3855^T), respectively. The emended description of the genus Singulisphaera is also proposed.