Indirect photolysis of cyanotoxins: one possible mechanism for their low persistence

We studied the effect of the presence of humic substances on the photochemical degradation of the cyanobacterial hepatotoxin microcystin. The concentration of three microcystin variants (LR, YR and RR) decreased significantly in the presence of humic substances but remained constant in pure water when exposed to natural sunlight. After 8 h of irradiation in quartz tubes, the residual concentrations of the three variants LR, YR and RR were 55±4, 53±3 and 44±5%, respectively, of the initial concentrations. Different concentrations of humic substances (2–16 mg l−1 DOC) did not yield different degradation efficiencies. For microcystin-RR, a degradation rate was determined and a half-life time of 10.5 h was estimated. These results show a potential importance for indirect photolytical degradation for the elimination of microcystins in natural waters.