We studied the effect of the presence of humic substances on the photochemi
cal degradation of the cyanobacterial hepatotoxin microcystin. The concentr
ation of three microcystin variants (LR, YR and RR) decreased significantly
in the presence of humic substances but remained constant in pure water wh
en exposed to natural sunlight. After 8 h of irradiation in quartz tubes, t
he residual concentrations of the three variants LR, YR and RR were 55 +/-
4, 53 +/- 3 and 44 +/- 5%, respectively, of the initial concentrations. Dif
ferent 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 microcystin's in natural waters. (C) 1999 Elsevier Science L
td. All rights reserved.