Influence of high frequency light/dark fluctuations on photosynthetic characteristics of microalgae photoacclimated to different light intensities and implications for mass algal cultivation

Oxygen evolution from a Scenedesmus obliquus dominated outdoor culture was followed in a small volume chamber, irradiated either by continuous white light or under light/dark frequencies between 0.05 to 5000 Hz, using arrays of 'high intensity' red light emitting diodes (LED's). By placing neutral density filters in the path of the white light, light saturation curves of the oxygen evolution (P/I curves) were measured using diluted aliquots of algal cultures. The results clearly showed that photosynthetic rates increased exponentially with increasing light/dark frequencies, that a longer dark period in relation to the light period does not necessarily lead to higher photosynthetic rates (efficiencies), and that algae do not acclimate to a specific light /dark frequency. One of the most important factors that influenced photosynthetic rates, either under continuous illumination or intermittent, was whether the algae were dark or light acclimated. Low light/dark frequencies were perceived by the algae as low light conditions, whilst the opposite was true for high frequencies. The light utilisation efficiency in a fluctuating light/dark environment depended on the acclimated state of the algae, the specific frequency of the fluctuations and the duration of the exposure. Since the frequencies determined the 'perceived' quantities of light, dark reactions played an important role in determining the average photosynthetic efficiencies. These results have important implications for algal biotechnology.