Planctomycetes are very intriguing bacteria that impassion, inspire and stimulate the scholars that study them. Discovered in the beginning of the last century, they were initially confused with 'floating fungus', hence the genus name Planctomyces (planktos, wandering, floating; Gr. masc. n. mukês, fungus; https://lpsn.dsmz.de/genus/planctomyces). However, it was only in the second half of the twentieth century that their diversity started to be unveiled (Devos and Ward 2014; Lage et al. 2019; Dedysh et al. 2020a). Together with Chlamydiae, Verrucomicrobia and other poorly described phyla, such as Lentisphaerae, Kiritimatiellaeota and other candidate phyla, they form the Planctomycetes-Verrucomicrobia-Chlamydiae (PVC) superphylum (Wagner and Horn 2006). Planctomycetes are distinctive bacteria and their divergent characteristics have confused both those scientists analysing them and also those considering them as interesting curiosities (Wiegand et al. 2018; Rivas-Marín and Devos 2018). For example, they have been mistakenly proposed to form a third cell type organisation, neither Gram-negative nor Gram-positive (Santarella-Mellwig et al. 2013; Devos 2014a, b; Boedeker et al. 2017). Indeed, they have even been called the 'nucleated' bacteria (Fuerst 2005). For a long time, they were thought to lack peptidoglycan, an anomaly considered as shared with the Chlamydiae but recently reappraised (Jeske et al. 2015; van Teeseling et al. 2015). These and other misconceptions have pervaded the PVC field for a long time. With the advances in genomics, molecular and microscopy techniques, combined with more sampling, Planctomycetes researchers have begun to fix some of the most obvious misconceptions in the field (reviewed in Devos 2014a, b; Wiegand et al. 2018; Rivas-Marín and Devos 2018; Lage et al. 2019). However, two of the real divergent features of Planctomycetes, also shared with Chlamydiae, are their asymmetrical division and their lack of the division protein FtsZ, unique in the free-living bacterial world (Rivas-Marín et al. 2016, 2020).

In the last 20 years, a great expansion of studies on the biology of Planctomycetes has occurred, showing the great importance and applied potential of these bacteria (Ward 2012; Devos et al. 2013; van Niftrik and Devos 2017; Lage and Devos 2018). Novel species began to be revealed but these were typically reported on an ad hoc basis.

We are now facing exciting times for exploring, discovering and comprehending these fascinating bacteria. Indeed, significant steps in exploring the diversity of Planctomycetes have been achieved recently. Among these, significant insights have been made by Dedysh and colleagues including the recognition of three new orders within the class Planctomycetia and novel insights into the first described Planctomycete, Planctomyces bekefii (Dedysh et al. 2020a, b). In addition, answering the appeal of Christian Jogler, many of the major scientists in the Planctomycetes field, joined forces to report the genome sequencing and analysis of 79 cultivated bacterial strains from all major taxonomic clades of the phylum Planctomycetes, most of them not yet described (Wiegand et al. 2020). This project has been a landmark in understanding the systematics of the Planctomycetes and has provided a platform for a significantly expanded taxonomic census of the phylum. Jogler, his team and collaborators have collected, cultivated, sequenced and characterized close to 100 species in order to provide a world-class reference study on one bacterial phylum (Wiegand et al. 2020). The main work described the broad classification, phylogeny and genome analyses of the strains, confirming most of the previous progress in correcting misconceptions about the phylum and also describing novel surprises in the molecular cell biology of these bacteria, providing the most up-to-date phylogeny.

Antonie van Leeuwenhoek Journal of Microbiology has for a long time supported the community of researchers studying PVC bacteria, with special issues and papers drawn from meetings of this group (Devos et al. 2013; Lage and Devos 2018). Continuing this relationship, in this special issue, with the publication of 21 articles, Jogler, his team and collaborators go a long way towards complementing their groundbreaking work enlarging the description of a number of species in the genera Blastopirellula, Rhodopirellula, Rubinisphaera, Rubripirellula and Tautonia. Furthermore, novel genera have been created to accommodate previously unclassified Planctomycetes, including Alienimonas, Aureliella, Bremerella, Calycomorphotria, Crateriforma, Lignipirellula, Novipirellula, Polystyrenella, Symmachiella and others. Many of these new taxa originated from diverse marine environments, different seas and oceans as well as habitats such as a hydrothermal vent system, active volcanic sites, marine sediments, natural or artificial marine surfaces, biofilms, kelp forest and other algae and a jellyfish, thus showcasing a huge variability in terms of their biology and ecology. The reports described in this Special Issue are fascinating and demonstrate the huge diversity of the Planctomycetes phylum. The diversity of morphological shapes and features is unequaled in microbiology.

Jogler and his team are world-wide recognised experts in advanced microscopy and this is obvious when looking at the figures accompanying the articles. The diversity of methods and quality of the pictures is stunning. The micrographs, including light, phase contrast (Phaco), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), are beautiful and expose the diversity revealed from the genomic data. Some of the characteristics of these planctomycetes are given in Table 1. Species described in these articles have a genome size from 5 to almost 10 Mb, with a G + C content from 55 to 71%, and pH range for growth from 5.5 to 10 (Table 1). They also include species with special features such as irregular shapes and consecutive budding (Alienimonas californiensis), or strains containing conspicuous subcellular filamentous structures, possibly related to cytoskeletal elements (e.g. Calycomorphotria hydatis). This work is also important for its contribution to the phylogenetic description of the phylum. In one article, the authors emend the description of the genus Rubripirellula, in another they split the current genus Rhodopirellula into a more strictly defined genus, while in yet another, they reclassify strains.

In conclusion, the work by Jogler, his team and collaborators is striking in its depth, size, and number of novel species described. The isolation and description of these novel Planctomycetes opens the possibility for many future studies of young scientists that will unveil many still unknown features of these unique bacteria. There is no doubt that this is only the preliminary characterisation of these strains and that much great microbiology will be extracted from this great collection in the future.

We are thankful to Christian Jogler for his vision and efforts, resulting in this great contribution and amazing work.

Table 1 Summary of the properties of the novel taxa described in this Special Issue
Full size table