Rates of humic substance photosensitized degradation of microcystin-LR in natural waters

In the course of the eutrophication of most inland waters, the abundance of cyanobacteria has increased, and consequences of raised cyanotoxin levels gave rise to numerous studies on their occurrence, ecological impacts, and toxicological effects. One of the most potent cyanotoxin, microcystin-LR, has tumor-promoting properties; thus, knowledge of the persistance of the toxin in natural waters is of high interest. The environmental fate of dissolved microcystin is still poorly understood, Microbial degradation is efficient in most inocula but is achieved only after a lag phase of several days to weeks. Under field conditions, the toxin can be alternatively degraded or transformed by indirect photolysis in the presence of humic substances or algal pigments. In the present study, we determined the rates of degradation of microcystin-LR by natural sunlight in the presence of fulvic acids and natural dissolved organic matter. Microcystin-LR was not degraded by sunlight alone but in the presence of photosensitizers. First-order rate constants depended on the optical density of experimental solutions and showed a saturation effect at higher concentrations of fulvic acids. In filtrates of surface waters, first-order degradation rates depended linearly on the absorbance of respective waters at lambda = 350 nm. Compared to solutions of purified fulvic acids, photolyis in natural waters amounted to only one-third at a given optical density. Therefore, rates of photosensitized degradation in natural waters were rather low, and estimates of in situ half-life times were about 90-120 days per meter depth of the water column. Although photosensitized degradation of microcystin occurs in all surface water bodies immediately following their release, it will be of significance only in very shallow water bodies.