The Israeli Journal of Aquaculture - Bamidgeh 55(4), 2003
The 7th Annual Dan Popper Symposium
ARE OUTDOOR ALGAE CULTURES LIGHT-LIMITED, LIGHT-SATURATED OR LIGHT-INHIBITED?
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?