Light-Regulated, Circadian Respiration Activity of Euglena gracilis Mutants that lack Chloroplasts

A circadian photosynthesis rhythm was confirmed in Euglena gracilis wild-type cells by measuring up to eight endogenous oscillations of the concentration of free oxygen in the culture medium under constant light conditions. In colourless Euglena mutants lacking functional chloroplasts, the light dependency and the circadian fluctuations of the free oxygen concentration were inverse to those of wild-type cells. The lowest O2 concentrations in cultures of the mutant were observed during the daytime, when photosynthesis is maximal in wild-type cells. After synchronisation by 12 h: 12 h light (L) :dark (D) cycles, the mutant cultures showed circadian fluctuations in their oxygen concentration when transferred to constant light. When subjected to a more complex L: D : L: D : L: D regime of 12h:3h:9h: 12h:3h:9h the mutant cells reacted immediately to the light conditions manifested in varying O2 concentrations. Red and blue light cycles of 8 h : 8 h L: D were equally efficient in regulating respiration activity when compared with white light cycles (8h L:8h D) of 16h duration. These results suggest that the demand for energy is under light and circadian control. Whether the energy is supplied by photosynthesis or respiration depends on the Euglena cells tested.