Two strains of chlorella were isolated from Lake Shinji (limno) and Nakanoumi (hyphalmyro), respectively, and cultivated under the laboratory condition (temp. 25℃, aeration of 900 ml/min/l and 6500 lux illumination). Changes in the photoreductive activities of both strains were investigated to elucidate the phase of photosynthetic amount in the limno and hyphalmyro areas. The both strains showed specific circadian rhythms in the photoreductive activities under the regime of light-dark cycle (12 hr each). In 2 hr after the illumination, the activity of the limno chlorella was rapidly enhanced to a maximum value (about 500 μ moles O_2 evolved/mg chl/hr) and decreased with continuous illumination for 4 hr (corresponding to noon). In the afternoon the photoreductive activity was maintained at a level as much as 300 μ moles O_2 evolved/mg chl/hr. At early night the activity decreased further downhill by a level as much as 200 μ moles O_2 evolution. The activity of the chlorella was kept at the low level at night.
The activities of CO_2-fixation of both strains were also investigated. It is noteworthy that the both chlorella showed specific circadian rhythms in the CO_2-fixation, similar to the circadian rhythms in the photoreductive activities. The CO_2-fixation activity of a limno chlorella was rapidly enhanced to a maximum value in 3 hr after the illumination and then decreased with continuous illumination in 6 hr. In the afternoon the CO_2-fixation activity was maintained at a steady level. On light-off, the activity decreased further downhill by a level of about one third of the maximum value. During the darkness the activity was kept at a constant low level.
In a hyphalmyro chlorella, the maximum value of the activity was observed at 6 hr after the light-on, and then the activity decreased rapidly by the low level which was maintained during the night.