Although there had been several earlier records of very small chlorophyll-b-containing cyanobacteria in the ocean,<unk>  was actually discovered in 1986 by Sallie W. (Penny) Chisholm of the Massachusetts Institute of Technology, Robert J. Olson of the Woods Hole Oceanographic Institution, and other collaborators in the Sargasso Sea using flow cytometry. The first culture of<unk>  was isolated in the Sargasso Sea in 1988 (strain SS120) and shortly another strain was obtained from the Mediterranean Sea (strain MED). The name<unk>  originated from the fact it was originally assumed that<unk>  was related to Prochloron and other chlorophyll b containing bacteria, called prochlorophytes, but it is now known that prochlorophytes form several separate phylogenetic groups within the cyanobacteria subgroup of the bacteria kingdom.
Marine cyanobacteria are to date the smallest known photosynthetic organisms:<unk>  is the smallest at just 0.5 to 0.8 micrometres across. Possibly they are also the most plentiful species on Earth: a single millilitre of surface seawater may contain 100,000 cells or more. Worldwide, there are estimated to be 100 octillion individuals.<unk>  is ubiquitous between 40°N and 40°S and dominates in the oligotrophic (nutrient poor) regions of the oceans.
The light harvesting pigment complement of<unk>  is unique, consisting predominantly of divinyl derivatives of chlorophyll a (Chl a2) and b (Chl b2) and lacking monovinyl chlorophylls.<unk>  occupies two distinct niches, leading to the nomenclature of the low light (LL) and high light (HL) groups , which vary in pigment ratios (LL possess a high ration of chlorophyll b2: a2 and HL low b2: a2), light requirements, nitrogen and phosphorus utilization, copper and virus sensitivity. These "ecotypes" can be differentiated on the basis of the sequence of their ribosomal RNA gene. Recently the genomes of several strains of<unk>  have been sequenced .
Prochlorococcus
Actinomyces
Human papillomavirus
Peptococcus
Mycobacterium smegmatis
Mycoplasma
Candida glabrata
Candida 