OPTIMIZING THE POPULATION-DENSITY IN ISOCHRYSIS-GALBANA GROWN OUTDOORS IN A GLASS COLUMN PHOTOBIOREACTOR

Particularly high population densities are readily sustainable in newl y designed glass column reactors. The optimal density of Isochrysis ga lbana in these columns in summer was 4.6 g L-1 dry algal mass at which value the highest sustainable productivity obtained was a record of 1 .6 g L-1 d-1. The population density exerted a direct effect on produc tivity: The higher the light intensity, the more pronounced was the de pendence of the output rate on the population density, variations of 1 0%+/- from the optimal density resulting in a significant decline in p roductivity. The population density had also a very significant effect on the course of photoadaptation which took place during the first da ys after transferring the cultures from the laboratory to the outdoors . The output rate was lower by 5 to 35% on the first day of such trans fer as compared to the light-adapted control. The higher the cell dens ity, the faster was the process of photoadaptation as indicated by the rise of the productivity and O2 tension to the control level. The pot ential for excess light damages was most prominent in the column react ors used, in which the light path was much reduced compared with that in open raceways. Significant photoinhibition took place at below opti mal population density (2.8-3.8 g L-1), and when cell density was furt her reduced (1.9 to 1.1 g L-1), exposure to full sunlight caused photo oxidative death within a few hours. The pattern of O2 concentration in the culture that emerged along the day served as a useful indicator o f photolimitation.