Potenciais mecanismos de redução da toxicidade de microcistinas e sua utilização no tratamento da água

Abstract

Several bloom-forming cyanobacterial genera produce microcystin, a potenthepatotoxin, wich has turned a main concern for people who deal with water treatmentfor human consuption. In Brazil, hepatotoxins are produced especially by species of thegenera Microcystis and Radiocystis. Due to the presence of double bond in the ADDAamino acid, the microcystin molecule is very resistant to chemical hydrolysis and evento boiling. Therefore there is a constant search for effective solutions, at low cost, forthe removal of this cyanotoxin from the drinking water. Although the conventionalprocesses of water treatment can reduce the concentration of microcystins from theparticulate fraction, the soluble fractions can still contain high concentrations of thiscyanotoxin and pass through the treatment systems. The present work shows the resultsof experimentes carried out at a laboratory scale, using blooms samples and cultures ofthe cyanobacteria Microcystis protocystis, microcystin RR producer. The work wasdeveloped in two phases. The first one focused on studying the reduction of microcystintoxicity, based on changes occurred on blooms samples and cultures exposed to lightand submitted to pressure. The second stage focused on applying the results obtained inthe first study, on a bench-scale, with the addition of chemical coagulants. Theconcentration of microcystin in the sludge generated during the process of sedimentationwas also evaluated, as well as some techniques of reduction of these concentrations ofmicrocystin. The experiments were carried out with M. protocystis containing a highconcentration of microcystin. In the first stage, the results showed a production ofhydrogen peroxide after the damage caused to the cells by the induced pressure. Areduction of phycocyanin concentration was also noticed, however no significantmodifications occurred for chlorophyll a and carotene concentration. The microcystinwas degraded showing changes in the peaks of the chromatogram and in the absortionspectrum when compared to those obtained before the treatment. The lethal dosage tothe mice was increased by approximately 10 times after pressurization of the samplesand exposition to light, showing a reduction of toxicity. The experimental conditionsprovided cellular lysis, with the consequent releasing of phycocyanin and microcystin.The phycocyanin, under illumination, was photo-oxidized and activated the productionof ROSs, which caused a reduction of microcystin toxicity. These results seem toindicate the influence of ROS´s in the reduction of microcystin toxicity and suggest apotential method for water treatment during the occurrence of cyanobacteria blooms. Inthe second stage of the experiments with the three coagulants, is was observed that theferric chloride was the most efficient for the removal of cells and of microcystin.Finnaly, for the treatment of the toxic sludge generated, the exposition to light during 12hours was the process that presented a better removal of microcystin.