Photoacclimation of Scenedesmus protuberans (Chlorophyceae) to fluctuating irradiances simulating vertical mixing

Photoacclimation of Scenedesmus protuberans Fritsch to fluctuating irradiances, simulating vertical mixing, was studied in light-limited continuous cultures. The algae were exposed to a simple sinusoidal light regime simulating diurnal irradiance, and two fluctuating regimes in which light fluctuations resulting from four and eight vertical circulations of the algae through the water column were imposed on the sinusoidal light regime. The total daily light dose (TDLD) was kept constant. Maximum photosynthesis increased, and cellular chlorophyll content and photosynthetic unit size decreased in response to light fluctuations. The efficiency of photosynthesis decreased during the light period in the sinusoidal light regime, whereas it remained at the same level in the fluctuating light regimes. The results suggest that prolonged exposure to intermediate irradiances lowers the photosynthetic efficiency of S.protuberans more than short exposure to high irradiances alternated by low-light periods, and that this species is able to optimize its photosynthesis in fluctuating light.