Genome-based reclassification of Metallosphaera prunae as a later heterotypic synonym of Metallosphaera sedula

The present study aimed to clarify the taxonomic assignment of two closely related Metallosphaera species using genome-based taxonomic approaches, and comprehensive phylogenomic and comparative analyses. Genomic information for six type strains was available at the time of conducting this analysis. One group of type strains was found to be conspecific, namely Metallosphaera sedula Huber et al. 1989 and Metallosphaera prunae Fuchs et al. 1996. Genomic relatedness analyses between both type strains yielded a blast-based average nucleotide identity of 99.8%, an average amino acid identity of 99.9% and an estimated digital DNA-DNA hybridization of 98.4%, indicating a shared taxonomic position for M. sedula and M. prunae. These values exceed the threshold values for bacterial species delineation, suggesting that they belong to the same species. Furthermore, 16S rRNA gene sequences of M. sedula and M. prunae were highly similar (99.8%), and in the phylogenetic trees constructed using 16S rRNA gene sequences and whole-genome sequences, M. sedula and M. prunae clustered together and formed a monophyletic clade. Based on this evidence, we propose the reclassification of M. prunae Fuchs et al. 1996 as a later heterotypic synonym of M. sedula Huber et al. 1989.