OCCURRENCE OF A TOXIN-PRODUCING CYANOBACTERIA IN THE SINOS RIVER (RIO GRANDE DO SUL STATE, BRAZIL) DURING THE YEARS 2005 AND 2008

Abstract

revista vol 13 nº 2.indd

Cyanobacteria are widely distributed autotrophic prokaryotic microorganisms, whose blooms depend on ideal amounts of nitrogen and phosphorous as well optimal temperature and pH conditions. Cyanobacterial blooms in the Sinos River, Rio Grande do Sul state, Brazil, are triggered by the influx of household and industrial wastes. The present study aimed at monitoring the occurrence of cyanobacteria in the Sinos River and tracking the water quality for human consumption. Water samples were taken from February 2005 to March 2008 at the Alberto Pasqualine Oil Refinery in the municipality of Esteio, Rio Grande do Sul state, totaling 64 samples. The different genera of cyanobacteria found were identified and counted in the laboratory in Sedgewick-Rafter chambers. The genera Planktothrix and Cylindrospermopsis, as well as an unidentified unicelular cyanobacterium were the most frequently found in the samples. Planktothrix sp. achieved 87,009 cells.mL-1 in March 2005, Cylindrospermopsis sp. achieved 65,100 cells.ml-1 in August 2006, and the unidentified unicellular cyanobacterium achieved 20,000 cells.mL-1 in January 2006. Only six samples contained no cyanobacteria. Planktothrix and Cylindrospermopsis are potentially toxic, but cyanotoxins could not be detected in the samples containing more than 20,000 cells.mL-1. Cylindrospermopsis sp, were most numerous during seasonal floods of the Sinos River (over March, June and August) whereas Planktothrix sp. were most numerous during the drought periods (January, February and March). The presence of each predominant cyanobacterium may be correlated with direction of the water flow in Lake Guaíba--Sinos River system.