In the footsteps of Heinz Schlesner and Peter Hirsch: Exploring the untapped diversity of the phylum Planctomycetota in isolates from the 1980s to the early 2000s

Recent sampling and strain isolation campaigns have accelerated research on the bacterial phylum Planctomycetota. The contribution of more than 100 novel isolates to the open collection of currently 123 described planctomycetal species in the last decade benefited greatly from pioneering work conducted in the second half of the last century. One of those pioneers was Heinz Schlesner, who investigated budding and prosthecate bacteria from habitats world-wide during his time at Christian-Albrechts-University Kiel. An outcome of his research was a strain collection with more than 500 isolates belonging to different bacterial phyla, many of which are uncharacterised members of the phylum Planctomycetota. Due to the lack of affordable genome sequencing techniques at the time of their isolation, most of them were characterised based on phenotypic features and DNA-DNA hybridisation experiments. After the retirement of Heinz Schlesner in 2002, the collection was stored for several years and transferred to Jena in 2019. To get a glimpse on the diversity of members from the phylum Planctomycetota in Schlesner’s collection, we here summarised from his records and publications all available information about the collection regarding sampling habitat and phylogeny. Furthermore, we conducted an updated phylogenetic analysis for a representative excerpt of the collection based on the 16S rRNA gene sequence of 59 strains Schlesner deposited in the NCBI database during strain characterisation studies published in the 1980s until the early 2000s. The results support that strains from his collection are still a valuable contribution to expand the cultivated diversity of the understudied phylum Planctomycetota.