The Planctomycetota are a phylum of widely distributed bacteria, occurring in both aquatic and terrestrial habitats. They play a considerable role in global carbon and nitrogen cycles, with many species of this phylum capable of anaerobic ammonium oxidation, also known as anammox. Many Planctomycetota occur in relatively high abundance as biofilms, often associating with other organisms such as macroalgae and marine sponges. Planctomycetota are included in the PVC superphylum along with Verrucomicrobiota, Chlamydiota, Lentisphaerota, Kiritimatiellaeota, and Candidatus Omnitrophica. The phylum Planctomycetota is composed of the classes Planctomycetia and Phycisphaerae. First described in 1924, members of the Planctomycetota were identified as eukaryotes and were only later described as bacteria in 1972. Early examination of members of the Planctomycetota suggested a cell plan differing considerably from other bacteria, although they are now confirmed as Gram-negative bacteria, but with many unique characteristics. Bacteria in the Planctomycetota are often small, spherical cells, but a large amount of morphological variation is seen. Members of the Planctomycetota also display distinct reproductive habits, with many species dividing by budding, in contrast to all other free-living bacteria, which divide by binary fission. Interest is growing in the Planctomycetota regarding biotechnology and human applications, mainly as a source of bioactive molecules. In addition, some Planctomycetota were recently described as human pathogens. The species Gemmata obscuriglobus has been identified specifically as comprising bacteria with unique characteristics among the Planctomycetota, such as their ability to synthesize sterols. Structure and morphology Cell shape and appendages The distinct morphological characteristics of bacteria in the Planctomycetota have been discussed extensively. The common morphology is often spherical cells roughly 2 μm in diameter, as observed in the species Aquisphaera giovannonii. However, the diversity in cell shape often varies greatly in them. Ovoid and pear-shaped cells have been described in some species, and often occur in rosettes of three to 10 cells. Gemmata obscuriglobus is a well studied species in the Planctomycetota with spherical cells. In contrast, bacteria in the species Planctopirus limnophila have ovoid cells. Many Planctomycetota species display structures and appendages on the outer surface of the cell. Flagella, common in most bacteria, have also been observed in the species P. limnophila. Many Planctomycetota also have a holdfast, or stalk, which attaches the cell to a surface or substrate. Members of some species, though, such as Isosphaera pallida lack a holdfast. Unique appendages known as crateriform structures have been observed in species of Planctomycetota