EFFECT OF CELL-MOVEMENT BY RANDOM MIXING BETWEEN THE SURFACE AND BOTTOM OF PHOTOBIOREACTORS ON ALGAL PRODUCTIVITY

Effect of algae movement, as a result of random mixing, between the su rface and bottom zones of shallow, moderately deep and deep photobiore actors (incident light intensities per unit volume were 8125, 4062 and 2031 mu mol m(-3) s(-1), respectively) on the reactor productivity wa s investigated. The results showed that at low cell concentrations, mo vement of cells between the surface and bottom zones of shallow and mo derately deep reactors had no significant effect on Chlorella pyrenoid osa C-212 growth and productivity. However, as the cell concentration in the reactors increased, cell movement between the two zones resulte d in increased productivity of the shallow reactor but decreased produ ctivity of the moderately deep reactor. On the other hand, in the deep reactor, random movement of cells between the two zones resulted in d ecreased Chlorella growth rate regardless of the cell concentration. T his may be attributed to the fact that at high cell concentration or i n a deep reactor, if the cells move between the surface and bottom of the reactor, they spend too long a time in the dark part of the reacto r where there is no cell growth, and endogenous respiration as well as cell death may lead to a decrease in cell concentration. When Spiruli na platensis M-135 cells were cultivated in the deep reactor, even at high cell concentration, movement of cells between the surface and bot tom zones of the reactor led to an increase in the reactor productivit y. The reasons for the difference in the results obtained with these t wo strains of algae could be attributed to the difference in their lig ht requirements since it was found that the saturation light intensity and specific decrease in cell concentration when incubated in the dar k were lower for Spirulina than for Chlorella cells.