Allelopathic activity of Spirogyra sp.: stimulating bloom formation and toxin production by Oscillatoria agardhii in some irrigation canals, Egypt

It has often been observed that irrigation canals in Egypt that contain Spirogyra are covered with blooms of Oscillatoria during the summer season. These blooms do not occur in the canals not containing Spirogyra. Thus, two irrigation canals, one containing Spirogyra and another not containing Spirogyra, were chosen to study the possible allelopathic activity of Spirogyra on the growth of Oscillatoria agardhii. The growth of O. agardhii and other associated cyanobacterial species was followed in these two canals during the period May-September 2000. The results revealed that O. agardhii was dominant and formed blooms in the canals containing Spirogyra, while it had a moderate/rare occurrence in those not containing Spirogyra. To confirm the stimulatory allelopathic activity of this green alga on the growth of and microcystin production by O. agardhii, a laboratory experiment was run by growing O. agardhii with different concentrations of Spirogyra aqueous extract. The results showed that the growth of and microcystin production by O. agardhii increased with increasing concentrations of Spirogyra aqueous extract. This finding demonstrates the allelopathic activity of the green alga Spirogyra stimulating the growth of and toxin production by O. agardhii. Such a biotic factor should be taken into consideration when cyanobacterial blooms are monitored in freshwater bodies.