toxicity in rodents. it is apparent that a significant amount of the oral dose passes the intestinal barrier. developmental effects microcystins do not appear to show developmental toxicity. genotoxicity the who states microcystin-lr has no mutagenic effect. however, the induction of dna strand-breaks in lymphocytes has been observed in mice after single oral administration. the effect is time- and dose-dependent. there is no change in the expression of selected genes involved in the cellular response to dna damage after a 4-hour exposure. after 24 hours, the dna damage-responsive genes were upregulated, which indicates that microcystin-lr is an indirect genotoxic agent. in china, the highest incidence of liver cancer occurs in areas with abundant cyanobacteria in the surface waters. tumor development is associated with low-concentration exposure over a long period of time. in vitro studies in vitro studies showed that microcystin-lr is a potent inhibitor of protein phosphatase 1 (pp-1) and pp2a, but has no effect on protein kinase c or cyclic amp-dependent kinase. mutagenicity does not appear to occur for purified toxins derived from microcystis, although the toxins were clastogenic for human lymphocytes. biodegradation a metalloprotease enzyme isolated from bacteria at lake rotorua, among other locations, is called microcystinase, is part of a 3 enzyme biodegradation pathway. this particular enzyme results in a product with 160-fold decrease in toxicity. history the chinese general zhu-ge liang was the first to observe cyanobacteria poisoning about 1000 years ago. he reported the death of troops who drank green coloured water from a river in southern china. the first published report of an incidence of cyanobacteria poisoning dates from the poisoning of an australian lake in 1878. also, in china and brasil, people died after drinking water from a lake. all these incidents have been attributed to cyanobacteria and the toxic compound microcystin-lr. that is the reason why the world health organization (who) issued a guideline for microcystins in drinking water. the who guideline for microcystins in drinking water, based on microcystin-lr, is 1 μg/l. with the high levels of eutrophication in south africa, typical exposures can be as high as 10 μg/l.