A hotspot of diversity: novel species isolated from Baltic Sea sediments delineate a sympatric species complex.

Two bacterial strains, SP1S1-4 and SP2S1-2, were isolated from sediment samples collected in the Stockholm archipelago in November 2021. Following whole-genome sequencing, these strains were identified as tentatively belonging to two novel genospecies, based on digital DNA-DNA hybridization, as implemented in the Type Strain Genome Server. , and were, in this order and within a narrow genomic relatedness range, their closest genotypic relatives. Additional sampling and sequencing efforts led to the retrieval of distinct isolates that were monophyletic with SP1S1-4 and SP2S1-2, respectively, based on phylogenomic analysis of whole-genome sequences. Comparative analyses of genome sequence data, which included blast-based average nucleotide identity, core genome-based and core proteome-based phylogenomics, in addition to MALDI-TOF MS-based protein profiling, confirmed the distinctness of the putative novel genospecies with respect to their closest genotypic relatives. A comprehensive phenotypic characterisation of SP1S1-4 and SP2S1-2 revealed only minor differences with respect to the type strains of , and . Based on the collective phylogenomic, proteomic, and phenotypic evidence presented here, we describe two novel genospecies within the genus , for which the names sp. nov. and sp. nov. are proposed. The type strains are, respectively, SP2S1-2 (=CCUG 76457=CECT 30688), with a draft genome sequence of 5 041 805 bp and a G+C content of 46.3 mol%, and SP1S1-4 (=CCUG 76453=CECT 30684), with a draft genome sequence of 4 920147 bp and a G+C content of 46.0 mol%. Our findings suggest the existence of a species complex formed by the species , , sp. nov., and sp. nov., with falling in the periphery, where distinct genomic species clusters could be identified. However, this does not exclude the possibility of a continuum of genomic diversity within this sedimental ecosystem, as discussed herein with additional sequenced isolates.