This taxonomic study demonstrates that Planctomyces bekefii (originally described as a fungus in 1924) and Blastocaulis sphaerica (described as a bacterium in 1935) are identical organisms found in freshwater planktonic habitats. Both are spherical, budding bacteria with distinctive stalks radiating from a common holdfast, often encrusted with iron depositions. Through detailed morphological comparison and observations of living material from enrichment cultures and Michigan marl lakes, the author establishes that differences between the two descriptions are insignificant. Because these organisms represent the type species of their respective genera, both the species and generic names are considered subjective synonyms, with Planctomyces having priority based on earlier publication date. Despite acknowledging that Blastocaulis had a more rigorous description, nomenclatural rules require using the older name. The author presents an amended genus description and recognizes four valid Planctomyces species: P. bekefii, P. gracilis, P. condensatus, and P. kljasmensis (new combination). Several other named organisms are reclassified or excluded from the genus based on morphological criteria.

Key findings

• Planctomyces bekefii and Blastocaulis sphaerica are identical organisms and must be considered subjective synonyms, with Planctomyces having nomenclatural priority
• Both organisms are freshwater, planktonic, budding bacteria with spherical cells (1.4-1.7 μm diameter) attached to slender stalks radiating from a common center, frequently with iron-containing deposits
• Four valid Planctomyces species are recognized: P. bekefii, P. gracilis, P. condensatus, and P. kljasmensis, distinguished by cell size, stalk morphology, and growth characteristics
• The genus Planctomyces is established as the valid name despite Blastocaulis having a more detailed original description, based on nomenclatural priority rules