A century of research on the Planctomycetota bacterial phylum, previously known as Planctomycetes.
One hundred years after planctomycetes were discovered and 50 years since the first isolate was successfully cultured, this bacterial phylum remains enigmatic in many ways. In the last few decades, a significant effort to characterize new isolates has resulted in >150 described species, allowing a more comprehensive analysis of their features. However, metagenomic studies reveal that a diverse group of planctomycetes has yet to be cultured and characterized, and that many biological surprises are yet to be revealed. This is the case for the recently discovered phagotrophic Candidatus Uabimicrobium, which challenges our understanding of the distinction between prokaryotes and eukaryotes. The unique biology of planctomycete cells, such as their ability to divide without the FtsZ protein, their complex structure and characteristic morphology, their relatively large genomes containing many genes with unknown function, and their variable metabolic capabilities, imposes significant barriers for researchers. Although ubiquitous, the precise ecological roles of planctomycetes in various environments are still not fully understood. However, their distinctive metabolism opens the door to a large number of potential biotechnological applications, which are beginning to be unveiled. In this article, we first review the historical milestones in planctomycetes research and describe the pioneers of the field. We then describe the controversies and their resolutions, we highlight the past discoveries and current interrogations related to planctomycetes, and discuss the ongoing challenges that hinder a comprehensive understanding of their biology. We end up with directions for exploring the biology and ecological roles of these fascinating organisms.