Growth and light absorption of some planktonic cyanobacteria, diatoms and Chlorophyceae under simulated natural light fluctuations

Specific growth rates of Limnozhrix redekei, Planktothrix agardhii (cyanobacteria), Synedraacus, Stephanodiscus minutulus (diatoms), Scenedesmus acuminatus and Scenedesmus armatus (Chlorophyceae) were compared under different time structures of illumination, but the same daily light exposure, at 20°C. Fluctuating irradiance simulating a uniform rapid transport of the algal cells across the aquatic light field on a cloudless day with Zeu/Zmix=1 was compared with constant irradiance throughout the same photoperiod of 12 h length as well as a photoperiod of 6 h length. Fluctuating light (30 min for a cycle) resulted in a decrease in specific growth rates as compared with constant irradiance at the same photoperiod length. This decrease amounts to 15–20% for diatoms, 20–25% for Chlorophyceae and 35–40% for cyanobacteria, respectively. The decrease is somewhat lower if the fluctuations simulating mixing are slower (60 min for a cycle). The specific growth rate is also decreased by a shorter photoperiod, but this effect is more species specific. Regarding the in vivo absorption spectra, fluctuating light or a shorter photoperiod has little or no effect on the Chlorophyceae and diatoms studied, whereas cyanobacteria show an increase in light absorption by chlorophyll a and phycobilins.