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. Unique characteristics of anammox cells Bacteria in the