Photoinhibition in Outdoor Cultures of the Cyanobacterium Spirulina Platensis

The photosynthetic activity, degree of photoinhibition and the output rate of a dense culture of Spirulina platensis strain Art-B, grown outdoors in an open raceway pond was studied. In the morning, when the photon flux density (PFD) was 1,100 uE m-2 s-1, no photoinhibition was detected and the photosynthet ic activity was 109 umols h-1 mg cha. At noon time, when PFD was 2,100 uE m-2 s-1, a photoinhibition level of 25 was estimated and the photosynthetic activity of the culture was 84 umols h-1 mg cha. Shading the outdoor cultures by shade nets resulted in a decrease in photoinhibition of the shaded cultures, followed by an increase in the productivity from 11.5 gr m-2 d-1 at the non-shaded culture to 14.7 gr m-2 d-1 at the 30% shaded one.

Cultures grown under different shade conditions or in the laboratory under continuous illumination varied in their chlorophyll to cartenoids ratio. Exposing each of these cultures to high PFD in the lab indicated a change in the sensitivity to the high light stress, where the unshaded cultures were the most resistant to high PFD and the laboratory cultures being the most sensitive ones.