The oxygenic photosynthetic procaryotes comprise two separate groups. The most thoroughly studied and understood group is the cyanobacteria. This is a widely distributed and diverse collection of multicellular photosynthetic bacteria that possess chlorophyll a and carry-out oxygenic photosynthesis. A more recently discovered group of oxygenic photosynthetic procaryotes is placed in the order prochlorales. these bacteria share many features with the cyanobacteria, but they also contain chlorophyll b as well as chlorophyll a, lack phycobilin pigments, and differ in some other features. This group appears to be heterogeneous phylogenetically and related to diverse groups of the cyanobacteria on the basis of 16S rRNA data, but their exact relationships have not yet been determined.