Planctomycetota

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 most 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. Anammox, a highly promising process for biological wastewater treatment, is performed by biofilm-forming members of the phylum.

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 belonging to the class Planctomycetia. The outer surface of cells in the species P. limnophila display both large and small crateriform structures. Large crateriform structures often cover the cell surface, while small crateriform structures are often only at the end of the cell. Light microscopy demonstrated fibers of both stalk and pili type in P. limnophila and G. obscuriglobus. The pili fibers in both these species were often associated with large crateriform structures; in contrast, the stalk fibers were associated with small crateriform structures.

Cell wall composition Early examination of the Planctomycetota suggested that their cell plan differed considerably from both Gram-positive and Gram-negative bacteria. Until recently, bacteria in the Planctomycetota were thought to lack peptidoglycans in their cell walls, and were instead suggested to have proteinaceous cell walls. Peptidoglycan is an essential polymer of glycans, present in all free-living bacteria, and its rigidity helps maintain integrity of the cell. Peptidoglycan synthesis is also essential during cell division. Recently, those in the species G. obscuriglobus were found to have peptidoglycan in their cell walls.

Internal cell composition Planctomycetota were once thought to display distinct compartmentalization within the cytosol. Three-dimensional electron tomography reconstruction of G. obscuriglobus displayed varying interpretations of this suggested compartmentalization. The cytosol was suggested to be separated into compartments, both the paryphoplasm and pirellulosome, by an intracytoplasmic membrane. This interpretation has since been demonstrated to be incorrect. In fact, the intracytoplasmic membrane is well known to be the cytoplasmic membrane which displays unique invaginations, giving the appearance of compartmentalization within the cytosol. Planctomycetota therefore display the two compartments typical of Gram-negative bacteria, the cytoplasm and periplasm. The excess membrane observed in G. obscuriglobus triples the surface area of the cell relative to its volume, which is suggested to be associated with sterol synthesis.

Pigments Many Planctomycetota species display pink or orange coloring, suggested to result from the production of carotenoid pigments. Carotenoids are produced by plants and fungi, and by some heterotrophic bacteria to protect against oxidative stress. Three different carotenoid pigments have been identified in two different strains of the Planctomycetota. In marine environments, Planctomycetota are often suspended in the water column or present as biofilms on the surface of macroalgae, and are often exposed to harmful ultraviolet radiation. More highly pigmented species of the Planctomycetota are more resistant to ultraviolet radiation, although this is not yet well understood. It has since been shown that Planctomycetota synthesize C30 carotenoids from squalene and that this squalene route to C30 carotenoids is the most widespread in prokaryotes.

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