In dark-adapting dividing cells of Euglena gracilis var. bacillaris, chloroplast lamellae gradually come apart into constituent discs which are progressively lost from the plastids. The entire process from fully formed chloroplasts containing approximately 14 lamellae, each composed of about three discs, to the proplastid (containing no internal membranes) takes about 144 hours (eight generations). Electron micrographs of the process are presented and also the kinetics of loss of discs, lamellae, and pigments. Chlorophyll is lost, at the outset, at a rate commensurate with simple dilution among chloroplasts of progeny cells. The rate of loss of discs and lamellae, however, is somewhat slower and suggests either that all chloroplasts do not divide at each cell division or that there is a continuing slow production of chloroplast membranes in the dark after pigment synthesis has stopped.In non-dividing cells, essentially no loss of structure occurs over the entire 144 hours of dark adaptation. The plastids appear to lose little or no internal structure over the entire period and never return to the proplastid condition. Chlorophyll a drops to about one-third of the original value in the first 24 hours and is lost slowly from then on.The evolutionary and ecological implications of this type of control for a cell which is a facultative phototroph or organotroph are discussed.
Carotenoids in the eyespot apparatus are required for triggering phototaxis in
Euglena gracilis