Microcystin-producing blooms of Anabaenopsis arnoldi in a potable mountain lake in Saudi Arabia

This study reports for the first time the presence of Anabaenopsis arnoldi blooms in Saudi freshwaters. This species has been investigated with high cell densities (3.8 × 10³–264 × 10³ cells mL^?1) during June–November 2007 in Tendaha Lake, one of the major freshwater sources in Saudi Arabia. High temperature and conductivity, and a high concentration of phosphate, and low nitrate concentrations may have contributed to the formation of these blooms. The blooms were found to produce microcystins (MCYSTs) at concentrations up to 364 μg g^?1 dry weight as detected by an enzyme-linked immunosorbent assay. MCYSTs were also detected in the raw and treated water of the lake at concentrations (1.6–8.3 and 0.33–1.6 μg L^?1, respectively) exceeding the World Health Organization guideline level of 1 μg L^?1 for these toxins. HPLC analysis revealed that the extracts of A. arnoldi blooms contained MCYST-RR, -YR and two unidentified MCYSTs, but a pure culture of A. arnoldi isolated from Tendaha Lake during the present study produced MCYST-RR and –YR only. This is the first study to report MCYST production by A. arnoldi . Therefore, this cyanobacterium should be taken into consideration during monitoring of toxic cyanobacterial blooms in drinking and recreational water sources in the world, particularly arid and semi-arid countries including Saudi Arabia.