Prochloron

Prochloron (from the Greek pro (before) and the Greek chloros (green) ) is a genus of unicellular oxygenic photosynthetic prokaryotes commonly found as an extracellular symbiont on coral reefs, particularly in didemnid ascidians (sea squirts). Part of the phylum cyanobacteria, it was theorized (endosymbiotic theory) that Prochloron is a predecessor of chloroplasts, components found in photosynthetic eukaryotic cells. However, this theory is largely refuted by phylogenetic studies which indicate Prochloron is not on the same line of descent that lead to the chloroplasts of algae and land plants.

Prochloron was discovered in 1975 by Ralph A. Lewin of the Scripps Institution of Oceanography. Prochloron is one of three known prochlorophytes – cyanobacteria that contain both chlorophyll a and b bound to light-harvesting proteins. The other prochlorophytes are Prochlorococcus marinus, an open-ocean species, and Prochlorothrix hollandica, a filamentous freshwater species. Surprisingly, unlike most cyanobacteria, Prochloron do not contain the red or blue pigments called phycobilins, 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.