pH-Dependent hydrophobicity of the cyanobacteria toxin microcystin-LR

Although the presence of hepatotoxins in surface and drinking water is regarded as an ecotoxicological and human health problem, limited knowledge exists on the fate of these compounds. Therefore the n-octanol/water distribution ratio ( D ow) of the hepatotoxin microcystin-LR (MC-LR) was determined in order to enable the prediction of the fate of this compound in the aquatic environment. The presence of two free ionizable carboxyl groups and one free ionizable amino group in MC-LR suggest that its speciation and partitioning between octanol and water is pH dependent. This was experimentally studied and confirmed in the present study. The log D ow of MC-LR decreased from 2.18 at pH=1 to −1.76 at pH=10. From the low log D ow values, especially at the basic pH region at which the toxin producing cyanobacteria flourish, a low tendency to bioconcentrate from water in biota can be expected.