Laboratory cultivation of prochloron, a tryptophan auxotroph.

Laboratory cultures have been established of the didemnid symbiont Prochloron, a unique prokaryotic alga that synthesizes chlorophylls a and b but no phycobilin pigments. Cell division in Prochloron cultures occurs under acidic conditions (pH 5.5) in the presence of tryptophan. The alga is a naturally occurring tryptophan auxotroph that survives in nature by close association with the host, Diplosoma similis. The metabolic dysfunction that renders Prochloron auxotrophic may involve only the initial step of the tryptophan biosynthetic pathway.