Definition: An order of GRAM-NEGATIVE OXYGENIC PHOTOSYNTHETIC BACTERIA containing CHLOROPHYLL, a and b, but lacking PHYCOBILISOMES (light-harvesting antennae). There are three genera: PROCHLORON; PROCHLOROCOCCUS; and PROCHLOROTHRIX. This order was thought to be the missing link between CYANOBACTERIA and plant CHLOROPLASTS, hence the name.
