NIGHT BIOMASS LOSS AND CHANGES IN BIOCHEMICAL-COMPOSITION OF CELLS DURING LIGHT DARK CYCLIC CULTURE OF CHLORELLA-PYRENOIDOSA/

The effects of culture conditions on biomass loss during the dark peri od (night biomass loss) and the biochemical composition of Chlorella p yrenoidosa cells cultured under a repeated light/dark cycle were inves tigated, During the night, decreases mere observed in the biomass conc entration and carbohydrate contents of de cells while their protein co ntent increased. These changes imply that in the absence of light ener gy, intracellularly stored carbohydrate is metabolized as an energy so urce. This energy is used in part for cell maintenance and in part for protein synthesis. The changes were maximum during the exponential gr owth phase, decreased with the age of the culture, and were affected b y the culture conditions during both the day and night. The night biom ass loss decreased with increasing temperature during the day, but was greater in cultures grown under high light intensity. At any given gr owth phase, the night biomass loss increased with increasing cell carb ohydrate content. The culture conditions during the day affected the n ight biomass loss through their influence on the cell biochemical comp osition. Although biomass loss could be reduced by lowering the temper ature and avoiding mixing of the culture during the night, the loss co uld not be completely prevented. Since Chlorella cells can grow hetero trophically on some organic carbon sources during the dark period, a c yclic light-antotrophic/dark-heterotrophic culture was investigated as a means of preventing night biomass loss. By adding glucose to the cu lture during the night, continuous cell growth during both day and nig ht was achieved without adverse effect on the biochemical composition of the cells. Similar results were obtained using ethanol or acetate a s an alternative organic carbon source. These latter compounds are pre ferred to glucose as organic carbon sources in order to reduce the ris k of contamination.