The unique cell biology presented by members of the phylum Planctomycetota has puzzled researchers ever since their discovery. Initially thought to have eukaryotic-like features, their traits are now recognized as exceptional but distinctly bacterial. However, recently discovered strains again added novel and stunning aspects to the planctomycetal cell biology-shapeshifting by members of the "Saltatorellus" clade to an extent that is unprecedented in any other bacterial phylum, and phagocytosis-like cell engulfment in the bacterium "Candidatus Uabimicrobium amorphum." These recent additions to the phylum Planctomycetota indicate hitherto unexplored members with unique cell biology, which we aimed to make accessible for further investigations. Targeting bacteria with features like "Ca. U. amorphum", we first studied both the morphology and behavior of this microorganism in more detail. While similar to eukaryotic amoeboid organisms at first sight, we found "Ca. U. amorphum" to be rather distinct in many regards. Presenting a detailed description of "Ca. U. amorphum," we furthermore found this organism to divide in a fashion that has never been described in any other organism. Employing the obtained knowledge, we isolated a second "bacterium of prey" from the harbor of Heligoland Island (North Sea, Germany). Our isolate shares key features with "Ca. U. amorphum": phagocytosis-like cell engulfment, surface-dependent motility, and the same novel mode of cell division. Being related to "Ca. U. amorphum" within genus thresholds, we propose the name "Ca. Uabimicrobium helgolandensis" for this strain.IMPORTANCE"Candidatus Uabimicrobium helgolandensis" HlEnr_7 adds to the explored bacterial biodiversity with its phagocytosis-like uptake of prey bacteria. Enrichment of this strain indicates that there might be "impossible" microbes out there, missed by metagenomic analyses. Such organisms have the potential to challenge our understanding of nature. For example, the origin of eukaryotes remains enigmatic, with a contentious debate surrounding both the mitochondrial host entity and the moment of uptake. Currently, favored models involve a proteobacterium as the mitochondrial progenitor and an Asgard archaeon as the fusion partner. Models in which a eukaryotic ancestor engulfed the mitochondrial ancestor via phagocytosis had been largely rejected due to bioenergetic constraints. Thus, the phagocytosis-like abilities of planctomycetal bacteria might influence the debate, demonstrating that prey engulfment is possible in a prokaryotic cellular framework.
Keywords: endocytosis; evolutionary biology; planctomycetes.