The final concentrations of astaxanthin reached plateaus after about 300 h and this was dependent on the intensity of the incident light. The concentrations were higher under illumination with flashing light than under continuous light at the same incident intensity. For example, under flashing light at an incident intensity of 8 μmol m−2 s−1, the final astaxanthin concentration was 29 μg ml−1, which was equal to that obtained at 12 μmol m−2 s−1 under continuous illumination. As a result, energy consumption was reduced to 2/3. Again it appeared that the high intensity flashing light (12 μmol m−2 s−1) facilitated astaxanthin accumulation, similar to the case of the dry cell weight.
Although under continuous and flashing light at an incident intensity of 2 μmol m−2 s−1, almost the same final dry cell weights were obtained, as shown in Fig. 3, the final astaxanthin concentration under flashing light was higher than that under continuous illumination at the same incident intensity, and was also higher under flashing light at 4 and 8 μmol m−2 s−1, as stated above. From these results, the accumulation of astaxanthin, compared with the cell weight, showed a much stronger dependence on the intensity of the impinging light. It is well known that strong light enhances astaxanthin accumulation (10), because astaxanthin is produced by H. pluvialis cells to protect cells from the strong intensity of the light. Under flashing light, H. pluvialis cells were stimulated to enhance astaxanthin production by the higher intensity of the light during the light period. Therefore, the application of flashing light from blue LEDs can result in higher final astaxanthin concentrations with less consumption of electricity.
The results shown in FIG. 2, FIG. 3 and FIG. 4 were obtained at a frequency of 100 Hz. Figure 5 shows the relationship between final astaxanthin concentration and the frequency of the flashing light. Frequencies over the range of 25–200 Hz had little effect on final astaxanthin concentration at the same incident intensity. In these flashing light experiments, the duration of the light period ranged from 1.7 to 6.8 ms, and an exposure to light of 12 μmol m−2 s−1 during this period was effective for enhancing astaxanthin accumulation.