Nine strains of Chlorella protothecoides and 43 strains representing the five species of Prototheca were screened in flask culture for their ability to synthesize L-ascorbic acid (AA). Ascorbic acid was detected in all strains, ranging from 4.8 to 0.38 mg AA g x (-1) of dry cells. Organisms selected for further study grew well and maintained their AA productivity above a pH of 3.5. They can produce AA using a variety of carbon and nitrogen sources. Aerobic fermentation of selected strains resulted in extracellular accumulation of AA up to 76 mg x l(-1). By classical mutagenesis and selection methods, we created mutants of Prototheca moriformis ATCC 75669 that produced greater quantities of AA than the wild-type strain (78.4 vs 21.9 mg AA g x (-1) of cells). A process based on extracellular production could greatly reduce the cost of AA manufacture by eliminating the need for extraction of the AA from the cells.

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