Chlorophyll synthesis and degradation were analyzed in the cyanobacterium Synechocystis sp. PCC 6803 by incubating cells in the presence of 13 C-labeled glucose or 15 N-containing salts. Upon mass spectral analysis of chlorophyll isolated from cells grown in the presence of 13 C-glucose for different time periods, four chlorophyll pools were detected that differed markedly in the amount of 13 C incorporated into the porphyrin (Por) and phytol (Phy) moieties of the molecule. These four pools represent (i) unlabeled chlorophyll ( 12 Por 12 Phy), (ii) 13 C-labeled chlorophyll ( 13 Por 13 Phy), and (iii, iv) chlorophyll, in which either the porphyrin or the phytol moiety was 13 C-labeled, whereas the other constituent of the molecule remained unlabeled ( 13 Por 12 Phy and 12 Por 13 Phy). The kinetics of 12 Por 12 Phy disappearance, presumably due to chlorophyll de-esterification, and of 13 Por 12 Phy, 12 Por 13 Phy, and 13 Por 13 Phy accumulation due to chlorophyll synthesis provided evidence for continuous chlorophyll turnover in Synechocystis cells. The loss of 12 Por 12 Phy was three-fold faster in a photosystem I-less strain than in a photosystem II-less strain and was accelerated in wild-type cells upon exposure to strong light. These data suggest that most chlorophyll appears to be de-esterified in Synechocystis upon dissociation and repair of damaged photosystem II. A substantial part of chlorophyllide and phytol released upon the de-esterification of chlorophyll can be recycled for the biosynthesis of new chlorophyll molecules contributing to the formation of 13 Por 12 Phy and 12 Por 13 Phy chlorophyll pools. The phytol kinase, Slr1652, plays a significant but not absolutely critical role in this recycling process.