FISHERIES RESEARCH INSTITUTE,MOA,TAIWAN

Being rich in Docosahexaenoic acid (DHA), the microalga Isochrysis aff. galbana (TISO) is regarded as a good live food for rotifer, copepod and oyster larvae, and as an indirect food for grouper larviculture. In order to resolve the insufficiency of CO₂ with high-density algal culture, the effects of pH adjustment or CO₂ addition for small volumes (0.5 or 1.0 L) of stagnant culture and large volume (100 L) of aerated culture under continuous illumination were studied. For the small-volume culture, CO₂ or HCl was initially added to adjust the pH value to 5.5, 6.5, and 7.5. The growth was best for the CO₂-6.5i group, while it was worst for the CO₂-5.5i group. In the daily pH adjustment experiment, the CO₂-6.5e group had the best performance with 221% proliferation compared to the control. The lowest cell density was observed in the culture with 1.0 g/L NaHCO₃, and no significant difference was detected among the other groups. The addition of 0.5 g/L NaHCO₃ and the adjustment of the pH to 6.5 with HCl resulted in 212% proliferation in contrast to the control. For the large-volume culture, CO₂ gas or an HCl solution was used to maintain the pH at 6.5, 7.0, or 7.5. The addition of CO₂ was more effective than HCl. However, there were no significant differences among the groups to which CO₂ was added. The cell density reached 1.35 x 106 cells/ml, which was nearly twice that of the control on day 7. This study revealed that the addition of the carbon source can enhance the growth of I. galbana.