Quantificaçâo de microcistinas em águas do Rio Tapajós, Amazônia, Brasil

Abstract

Microcystins are hepatotoxic toxins produced by freshwater cyanobacteria (blue-green algae). The greater possibility of microcystin production in alarming concentrations occurs when there are dense concentrations of cyanobacteria cells, also called blooms. The eutrophication of aquatic environments and climate change modulate the formation of blooms causing problems for water supply, recreation, fish consumption, and others. Acute microcystin contamination can cause the death of aquatic and terrestrial animals, and in long-term contamination can promote primary liver cancer in humans. This study aimed to quantify microcystins (MC-RR, MC-YR and MC-LR) in Tapajós river waters between the cities of Santarém and Itaituba-PA, near the riverside communities, and in different fluviometric quotas of the river, as well as correlate the environmental parameters of the waters with the concentration of microcystins. Water samples were collected in 2020, 2021 and 2022, during periods of drought, flood, full and ebb of the Tapajós River. Thirty-eight samples were subdivided into filtrate (extracellular) and biomass (intracellular). The microcystins were extracted and purified through Solid Phase Extraction and analyzed by High Performance Liquid Chromatography with detection in photodiode arrangement. The analysis was validated according to the criteria of Resolution No. 166 of the National Health Surveillance Agency. The method was linear and selective for the three microcystins. We obtained accuracy values (recovery) between 80.01 and 107.69%, precision values with relative standard deviation between 1.43 and 9.53%, detection limit values between 0.104 and 0.174 μg/mL and quantification limit between 0.167 and 0.205 μg/mL. Ten filtrate subsamples and four biomass subsamples positive for microcystin, corresponding to 26.31% and 10.53%, respectively. The predominant microcystin was MC-LR with a maximum of 1.353 μg/L in the extracellular fraction and 0.120 μg/mL in the intracellular fraction. None of the samples exceeded the World Health Organization recommended limit of 24 μg/L for recreational water. In terms of frequency, we observed that there is a higher incidence of microcystins in the ebb and drought of the Tapajós River, especially the community of Parauá with three positive samples of the four collected. The results are in agreement with two other studies previously conducted at the mouth of the Tapajós River, which obtained low values of microcystins. In addition, the values obtained suggest that in lotic systems there is a lower concentration of microcystins than in lentic systems. However, in mighty rivers such as Tapajós cyanobacteria cells can accumulate on the bank of these rivers (in coves), and can produce toxins at dangerous levels. The temperature conditions, abundant lighting, coves and clear waters of the Tapajós River provide a favorable environment for cyanobacteria blooms. The riverside populations of the Tapajós River of this stretch, those that depend more on the river, are more susceptible to contamination by microcystins, because the treatment of surface waters for consumption is practically non-existent, the boiling of water does not degrade cyanotoxins and there is still the risk of consumption of fish that may be contaminated.