Degradation and Formation of Sulfolipid Occurring Concurrently With De- And Re-Generation of Chloroplasts in the Cells of Chlorella Protothecoides1

It has been demonstrated that when the cells of Chlorella protothecoides are grown mixotrophically under illumination in a medium rich in nitrogen source (urea) and poor in glucose, the normal green cells are obtained, while in a medium rich in glucose and poor in the nitrogen source, entirely chlorophyll-less cells with profoundly degenerated plastids (“glucose-bleached“ cells) are produced, irrespective of whether in the light or in darkness. The “glucose-bleached” cells turn green with regeneration of fully organized chloroplasts when incubated in a nitrogen-enriched medium in the light (“light-greening”), while in the dark they become pale green with formation of only partially organized chloroplasts (“dark-greening”). When, on the other hand, the green cells are transferred into a medium enriched with glucose, they are bleached fairly rapidly with degeneration of chloro-plasts in the light as well as in darkness (“bleaching”). Using 35S as a tracer, investigations were made on the changes of contents of the algal cells in sulfolipid and other sulfur compounds during the processes of the greening and bleaching. By determining the radioactivities of chromatographically separated sulfur-containing compounds of the uniformly 35S-labeled green (“G”) and “glucose-bleached” (“W”) cells, it was found that the concentration of a species of sulfolipid (discovered by BENSON et al.) as well as those of glutathione, sulfotrioses and most of the other sulfur-containing compounds were at least 5 times higher in the “G” cells than in the “W” cells, while sulfoquinovosyl glycerol was present in approximately equal amounts in the two types of cells. Phospholipid contents and compositions in the two types of algal cells were found to be practically identical. The sulfolipid content of algal cells increased and decreased almost in parallel with the processes of greening and bleaching, respectively. Studying the mode of incorporation of radiosulfate into various sulfur compounds of algal cells during the processes of “light- and dark-greening” and “bleaching” (lasting about 70 hr), it was found that active 35S-incorporation into sulfolipid occurred throughout the process of “light-greening,” while in the “dark-greening” and “bleaching” the active incorporation abruptly ceased after the initial 24 hr period of experiments. It was suggested that the biosynthesis of the sulfolipid is closely related to the formation of photosynthetic apparatus in chloroplast. When the 35S-labeled green cells were bleached in a medium containing no radiosulfate, the 35S-sulfolipid and most of other 35S-sulfur compounds decreased markedly but the 35S-sulfoquinovosyl glycerol increased considerably. It was inferred that the deacylation of the sulfolipid, a surfactant lipid, with formation of water soluble sulfoquinovosyl glycerol may be a cardinal event of bleaching process, causing a disintegration of the intact architechture of photosynthetic apparatus. Based on these observations it was concluded that the sulfolipid is an integral component of photosynthetic structure.