''Prochloron'' (from the Greek ''pro'' (before) and the Greek ''chloros'' (green) ) is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of unicellular oxygenic photosynthetic prokaryotes commonly found as an extracellular symbiont on coral reefs, particularly in didemnid

ascidians Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" ...

(sea squirts). Part of the phylum

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

, it was theorized (

endosymbiotic theory Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...

) that ''Prochloron'' is a predecessor of the photosynthetic components, chloroplasts, found in photosynthetic eukaryotic cells. However this theory is largely refuted by

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

studies which indicate ''Prochloron'' is not on the same line of descent that lead to chloroplast-containing algae and land plants. ''Prochloron'' was discovered in 1975 by Ralph A. Lewin of the

Scripps Institution of Oceanography The Scripps Institution of Oceanography (sometimes referred to as SIO, Scripps Oceanography, or Scripps) in San Diego, California, US founded in 1903, is one of the oldest and largest centers for oceanography, ocean and Earth science research ...

. ''Prochloron'' is one of three known prochlorophytes, cyanobacteria that contain both chlorophyll ''a'' and ''b'' bound to a special light-harvesting protein. Surprisingly, unlike most cyanobacteria Prochloron do not contain the red or blue pigments called Phycobilin's, seen in many species of cyanobacteria. Repeated unsuccessful attempts to culture ''Prochloron'' outside a host make them the only known obligate photosymbionts in the phylum Chordata.

Species

The only taxonomically valid species is ''P. didemni''.

References

* * Waterbury, John, et al. Little Things Matter A Lot. Oceanus Magazine. 2004, 43(2). * Synechococcales Cyanobacteria genera {{cyanobacteria-stub