BLOOMS OF Microcystis IN THE PATOS LAGOON AND ESTUARY (RIO GRANDE DO SUL, BRAZIL): A TWENTY-YEAR STUDY

Abstract

revista vol 13 nº 2.indd

Cyanobacterial blooms of Microcystis are frequent in the Patos Lagoon and estuary located in Rio Grande do Sul State, Brazil. Although the first scientific record of such occurrence dates to 1987, local villagers claim cyanobacterial blooms have occurred in the area since the begining of the 20th century. Great blooms of M. aeruginosa take place during the summer and fall, as a result of seasonal swells in the estuary and suitable abiotic conditions, as N:P ratios between 10:1 and 16:1 and water temperatures above 20ºC. The reduced water outflow enables the growth of Microcystis populations, and the surplus of sea salt directly affects intracellular and extracellular composition of microcystins. The strain RST9501 was isolated in the summer of 1995 from a cyanobacterial bloom in the area, and the cells proved toxic to mice, microcrustaceans and shrimp. This strain presents four types of microcystins, mainly D-Leu1 MCyst-LR. These toxins also demonstrated to accumulate in marine shellfish from the South Atlantic Sea and in freshwater fish. The toxicity of RST9501 cells was demonstrated by oxidative stress to polychaetes, sea crabs and osmoregulatory alterations in fish. The microcystins isolated from the Lagoa dos Patos lagoon are consumed by bacteria of the genus Burkholderia. Local blooms demonstrated to present toxins related to the quantity of Microcystis cells and caused contact dermatitis to the local human population.