One of the recently described mutant strains of chlorella (strain 5/520) accumulates -in the dark phytoene, phytofluene, ζ-carotene, protetrahydrolycopene and prolycopene and forms α- and β-carotene, xanthophylls and appreciable amounts of chlorophyll in light only 1, 2. The experiments described below indicate that 1. O2 and light are indispensable for xanthophyll and appreciable chlorophyll synthesis. Both syntheses appear to be linked somehow. 2. Carotene synthesis depends upon light but not

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... on O2 and is not linked to xanthophyll synthesis. 3. Carotenoid synthesis in light is always accompanied by a decrease of polyenes of the Porter-Lincoln sequence, which have been accumulated in the cells during previous cultivation in the dark. Evidence seems strong that these polyenes are direct precursors of the normal carotenoids. 4. An as yet unidentified carotene is formed in appreciable amounts along with the normal carotenes in the light, probably as byproduct, not a precursor.