M. Welker et C. Steinberg, Rates of humic substance photosensitized degradation of microcystin-LR in natural waters, ENV SCI TEC, 34(16), 2000, pp. 3415-3419
Citations number
33
Categorie Soggetti
Environment/Ecology,"Environmental Engineering & Energy
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, h
as tumor-promoting properties; thus, knowledge of the persistance of the to
xin in natural waters is of high interest. The environmental fate of dissol
ved microcystin is still poorly understood, Microbial degradation is effici
ent 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 o
r transformed by indirect photolysis in the presence of humic substances or
algal pigments. In the present study, we determined the rates of degradati
on of microcystin-LR by natural sunlight in the presence of fulvic acids an
d natural dissolved organic matter. Microcystin-LR was not degraded by sunl
ight alone but in the presence of photosensitizers. First-order rate consta
nts 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 abs
orbance of respective waters at lambda = 350 nm. Compared to solutions of p
urified fulvic acids, photolyis in natural waters amounted to only one-thir
d at a given optical density. Therefore, rates of photosensitized degradati
on in natural waters were rather low, and estimates of in situ half-life ti
mes were about 90-120 days per meter depth of the water column. Although ph
otosensitized degradation of microcystin occurs in all surface water bodies
immediately following their release, it will be of significance only in ve
ry shallow water bodies.