1. Biogenesis of secondary carotenoids (SC) has been studied with special reference to species of the genera Ankistrodesmus, Chlorella, and Scenedesmus. The biosynthesis of SC differs from that of the primary carotenoids. There is a close connection between the production of these oxopolyenes and the decomposition of the chlorophylls. According to our working hypothesis, decomposition products of chlorophyll or precursors of its biogenesis may act as precursors of SC. The following facts support this conclusion. 2. The production of SC runs synchronously with the decomposition of chlorophylls. 3. The original concentration of chlorophylls is usually sufficient for the formation of SC, there may be used the phytol as well as the porphyrin ring. 4. Green algae are able to assimilate the nitrogen of derivatives of porphyrins, pyrrols, and indols. 5. After an increase of chlorophyll concentration in the cells, caused by a better supply with iron, also the amount of SC increases under conditions of nitrogen deficiency. 6. Under iron-deficient conditions precursors of chlorophyll synthesis may be accumulated and used for the production of SC. 7. Chloramphenicol which inhibits chlorophyll decomposition and SC synthesis, blocks the first process only. Nitrogen-deficient cultures of Chlorella zofingiensis, poor in chlorophylls and rich in SC, are influenced by chloramphenicol to small extent only. In this species formation of chlorophylls depends on pH; below pH 6.0 it is inhibited. Under these conditions, precursors are perhaps accumulated and used for the synthesis of SC. 8. By means of a partial inhibition by chloramphenicol of chlorophyll formation, we have been able to identify pheophytin and hematoporphyrin as possible links in the chain of reactions leading from chlorophylls to SC. 9. These and some further unidentified derivatives of porphyrin have been found in mutants of Ankistrodesmus braunii which did not form SC. 10. Autotrophic and heterotrophic green mutants of Ankistrodesmus, Chlorella, and Scenedesmus are neither bleached completely nor do they synthesize SC under nitrogen-deficient conditions. 11. Heterotrophic and apochlorotic algal strains, i.e. Hyaloraphidium and Prototheca descending from Ankistrodèsmus and Chlorella, as well as all studied white and yellow mutants of Chlorella and Ankistrodesmus are unable to form SC. 12. Some heterotrophic mutants show a stepwise reversion to the wild type after addition of chloramphenicol to the culture medium. Intermediate stages of this process have been isolated. There is a close connection between the ability to form and to decompose chlorophylls and the property of synthesize SC. 13. These facts have been discussed with regard to our working hypothesis that phytol or/and the porphyrinring of chlorophyll may act as precursors of SC. A scheme for the transformation of porphyrin to SC has been proposed. © 1968 Springer-Verlag.
