It is necessary to realize that the term "photosynthetic prokaryotes" encom passes the widest and most diverse grouping of bacteria, but in itself has no taxonomic or phylogenetic significance. It means those organisms, other than eukaryotes, which require, obligately or facultatively, light for growth. The re cent application 16S rRNA sequencing to microbial phylogeny, asso ciated mainly with the work of Woese, emphasizes the evolutionarily dis persed nature of purpIe and green photosynthetic bacteria as weIl as the rather coherent phylogenetic connections of the cyanobacteria. It is not...