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

Particularly high population densities are readily sustainable in newly designed glass column reactors. The optimal density of Isochrysis galbana 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 productivity: The higher the light intensity, the more pronounced was the dependence of the output rate on the population density, variations of 10%+/- from the optimal density resulting in a significant decline in productivity. The population density had also a very significant effect on the course of photoadaptation which took place during the first days 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 transfer as compared to the light-adapted control. The higher the cell density, the faster was the process of photoadaptation as indicated by the rise of the productivity and O2 tension to the control level. The potential for excess light damages was most prominent in the column reactors used, in which the light path was much reduced compared with that in open raceways. Significant photoinhibition took place at below optimal population density (2.8-3.8 g L-1), and when cell density was further reduced (1.9 to 1.1 g L-1), exposure to full sunlight caused photooxidative death within a few hours. The pattern of O2 concentration in the culture that emerged along the day served as a useful indicator of photolimitation.