Topology and enhanced toxicity of bound microcystins in Microcystis PCC 7806

The cyanobacterium Microcystis is a potent producer of microcystins and cyanopeptolins and causes most of the toxicity outbreaks in freshwaters worldwide. Microcystins are mainly stored in the cells and little is found in the water. The intracellular concentration of microcystins in Microcystis PCC 7806 was at least 0.9 mM, although the solubility of microcystins in water was only about 10 microM. This low solubility does not allow the solubilisation of such high amounts of microcystins in the cytosol of Microcystis. Differential fractionation of cell constituents showed that microcystins and cyanopeptolins were bound to a protein fraction primarily composed of phycobilins. The percentage of microcystins and cyanopeptolins found in the thylakoid membranes was very low. Phycobilins may be the major proteins that have binding sites for these oligopeptides. A molar ratio near to 6 was observed for microcystins to the phycobilin (alphabeta) monomer. The binding of the microcystins to the protein was rather weak and allowed rapid dissociation of microcystins from the protein-matrix. Toxicity assays with Thamnocephalus platyurus showed that native microcystin when still bound to cyanobacterial protein was more toxic than an equivalent amount that has been desorbed from the protein by treatment with methanol. It is suggested that phycobilins serve in the gut of grazers as carrier molecules for the rapid transport of microcystin from lysed cells of Microcystis to the epithelium where the uptake of microcystins occurs. Because protein-bound microcystin does not bind to C18 cartridges, this behaviour can be the cause of many analytical discrepancies observed. The blue-coloured water observed upon the collapse of Microcystis blooms may be extremely toxic because the released phycobilins may carry the major fraction of microcystins.