The Israeli Journal of Aquaculture - Bamidgeh 55(4), 2003
The 7th Annual Dan Popper Symposium


Avigad Vonshak*

Jacob Blaustein Institute for Desert Research, Microalgal Biotechnology, Ben Gurion University of the Negev, Sde Boker Campus 84990, Israel


Outdoor algal cultures are exposed to two different rhythms of light and dark. The first is relatively fast. It is induced by the mixing and turbulent flow in the culture that dictates the frequency of the cycle. Algal cells shift between full solar radiation when they are located at the upper culture surface and complete darkness when they reach the bottom of the culture, usually at a depth of 12-15 cm. The second regime is relatively slower. It is the daily change in solar radiation from sunrise to sunset. Both light cycles impose unique physiological conditions in terms of adaptation or acclimation of the outdoor grown algal cells. At optimal concentrations for biomass production, light penetrates to a depth of only 3-5 cm, leaving the rest of the culture in complete darkness. In a regular pond with a depth of about 15 cm, more than 50% of the culture is in complete darkness. This situation, together with many other observations reported in the last 15 years, led to the conclusion that algal cultures grown outdoors are light-limited.

More recent findings demonstrate that the maximum photosynthetic activity in outdoor cultures is not reached at the highest light intensity. We have indications that, by shading algal cultures, photosynthetic activity and productivity in outdoor cultures can be increased. Following daily changes in variable fluorescence (Fv/Fm) in algae growing systems that differ in the amount of light to which the algal cells are exposed indicated that PS 2 activity decreases as a function of exposure to high solar radiance. This finding supports the idea that outdoor algae cultures are photo-inhibited. Can algal cultures be light-limited and photo-inhibited at the same time, or is something wrong in our interpretation?