Measurement of cell and astaxanthin concentrations
The dry cell weight was determined from the equation below, as previously reported (4).
Dry cell weight (mg ml−1)=[3.04×{(OD750-OD680)/OD680}+1.40]×OD680
Dry cell weight (mg ml−1)=[3.04×{(OD750−OD680)/OD680}+1.40]×OD680
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The size, number and color of cells were observed microscopically (Eclipse 80i; Nikon, Tokyo) using a Thoma haemocytometer.
To determine the astaxanthin concentration, 0.5 ml aliquots of cell suspensions were removed from the culture vessel and centrifuged at 9800×g for 10 min. Cell precipitates were resuspended in 0.5 ml of methanol and mixed with 0.40 g of silica particles (particle size = 0.2–1 mm; Kanto Chemical, Tokyo). To extract astaxanthin from cells, the mixture was vigorously mixed with a vibrator for 10 min and then centrifuged for 10 min. The supernatant (0.5 ml) was mixed with 0.1 ml of a methanol solution of NaOH (5 mM) and kept overnight under nitrogen in the dark at 20°C to saponify astaxanthin eaters (15 and 16). The astaxanthin concentration in the prepared samples was determined by HPLC (LC-10; Shimadzu, Kyoto) on a reverse phase column (Cosmosil 5C18-MS-II, 4.6×150 mm; Nacalai Tesque, Kyoto). Methanol was used as the mobile phase at a flow rate of 1 ml min−1, and the absorbance of the effluent solution was monitored at 470 nm with a UV-VIS detector (SPD-10AV; Shimadzu). The concentration of astaxanthin was determined from a calibration curve prepared using an authentic sample of free astaxanthin.
Results and discussion
Increase in number of H. pluvialis under illumination with continuous or flashing light emitted from blue LEDs
Figure 2 shows the time course for the cell number of H. pluvialis when cells were grown mixotrophically under illumination with continuous or flashing light from blue LEDs. The frequency of the flashing light was 100 Hz.