Under flashing light illumination, the dry cell weight at an incident light intensity of 4 μmol m−2 s−1 was about 0.5 mg ml−1, higher than that under continuous light at the same incident intensity. In the flashing light experiments, light was supplied intermittently, with light and dark periods. In this work, the light intensity impinging on the illuminated surface of the cultivation vessels during the light period was fixed at 12 μmol m−2 s−1 and the incident light intensity was adjusted by the duty cycle. As a result, the cells were exposed to a higher intensity of light during the light period than the incident intensity, i.e., 12 μmol m−2 s−1 multiplied by the duty cycle/100.
Under continuous light, on the other hand, cells were continuously exposed to light at the incident intensity. The high intensity of light during the light period under flashing light illumination, i.e., 12 μmol m−2 s−1, might stimulate light-dependent physiological responses in H. pluvialis compared to continuous light illumination. At an incident intensity of 8 μmol m−2 s−1 the dry cell weight reached a saturated level at around 0.6 mg ml−1 under conditions of both continuous and flashing light.
In the dark, the final dry cell weight was substantially lower than that under illumination, because no morphological changes were induced.
Final astaxanthin concentration in culture broth
Figure 4 shows the time course for astaxanthin concentration under the same experimental conditions as above. The accumulation of astaxanthin in H. pluvialis started 90–100 h from the start of cultivation under any illumination conditions, while the cell number ceased to increase at this stage, as shown in FIG. 2 and FIG. 4. This indicates that, at this stage, the H. pluvialis cells started to increase in size and to accumulate astaxanthin in parallel with the morphological change. H. pluvialis cells appeared to show two different accumulation rates of astaxanthin during 100–200 h. The rate at 4 μmol m−2 s−1 under flashing light was higher, while at 4 μmol m−2 s−1 under continuous light, the rate was low. Such different accumulation rates indicate that the rate of accumulation of astaxamthin is affected by the impinging light intensity and high intensity during the light period under flashing light illumination might induce a high accumulation of astaxanthin at an earlier stage of production.