Mass occurrence and toxicity of the cyanobacterium Lyngbya majuscula under phosphorus-limited conditions in the Red Sea

Large mass of Lyngbya majuscula has occurred on the surface of coral reefs in Farasan island in the Red Sea, Saudi Arabia. It was also observed that dermatitis symptoms appeared on people bathing and swimming in this area. The present study describes the toxicity of field samples and batch cultures of L. majuscula to Artemia salina and mice, and investigates effects of inorganic and organic phosphates on the growth and toxicity of this cyanobacterium during batch experiments. The extract of field samples of L. majuscula, was highly toxic to A. salina with LC50 value of 2.8 mg cm−3. This extract also showed a moderate toxic effect to intraperitoneally injected mice (LD50 = 425 mg kg−1), and highly dermatotoxic effect including skin redness and inflammation within 24 hours upon application to shaved back of mice (LC100 = 88 mg dm−3). In addition, the study revealed that the growth of L. majuscula increased by increasing inorganic and organic phosphate concentrations. In contrast, the highest toxicity of this cyanobacterium was obtained in phosphorus -depleted cultures, and decreased by increasing phosphate concentration in the medium. The results of phosphatase activity assay revealed a high enzyme activity (8.6 μmol pNP mg−1 dry wt h−1) of the field samples of L. majuscula. The activity increased by increasing organic phosphate concentrations, but decreased with high inorganic phosphate concentrations during batch experiments. The present study suggests that L. majuscula mats could be the causative agent in dermatitis developed in swimmers and bathers in Farasan Island waters, and that formation of these mats would be enhanced by organic phosphate resulting from human activities and rotted seaweeds.