The Guadiana River flows through the Southwest area of the Iberian Peninsula crossing from the Spanish provinces of Extremadura and Andalucia to the South region of Portugal. Preliminary studies on the presence and distribution of toxic cyanobacteria led to the identification of several cyanobacterial species in the Guadiana River itself as well as in the freshwater reservoirs established along its course.

  Based on the environmental monitoring of natural blooms, and laboratory studies with monocyanobacterial, but not necessarily axenic strains, microcystins production is inferred to occur widely among at least 25 cyanobacterial genera. Thus, microcystins are produced by axenic strains of Anabaena, Microcystis, Nostoc and Oscillatoria.

  Individual blooms and scums which are potentially toxigenic cyanobacteria range widely in their toxicity, and the high probability of an individual sample being acutely toxic argues for the use of a precautionary principle in cyanobacterial bloom-management and water-treatment: a bloom or scum should be assumed to be toxic, and it should be necessary to isolate and identify the toxic strains.

  In this study the growth and microcystins production it is examined in different cyanobacterial strains isolated from Guadiana River. All samples of cyanobacterial blooms were collected from this river along its course, since July 1999 to November 2000.

  Twenty-nine cyanobacterial strains of Microcystis aeruginosa, Aphanizomenon flos-aquae and Anabaena spiroides were isolated and maintained in monoalgal cultures. Medium Z8 was used for batch culture, and organisms were maintained at 20º C with 16/8-h L/D cycle (light intensity 40 µEm2 s-1). All of these cultures were analysed by an enzyme-linked immunosorbent assay (ELISA) in order to detect their toxicity. Only three of the fourteen Microcystis aeruginosa strains were toxic.

  A comparative study of growing and microcystins production in the same culture conditions along the growth rate of these three strains was taken. Cultures were harvested at the mid-exponential phase and concentrated by decantation. The concentrated alga material was preserved frozen at -20º C until lyophilization. The lyophilized cells were extracted with 0.1 M acetic acid-methanol-chloroform. After concentration, the residue was resuspended in 0.1 M acetic acid. In order to confirm the toxin production, ELISA and HPLC analysis of the extracts from these cultures were carried out.

  Results of this study confirm the presence of a variety of strains of only one cyanobacterial species with a different capacity to produce microcystins. This fact can be explained by considering the loss of capacity of toxin production along its culture time, but on the other hand, it could be due to the presence of a single-species bloom which is a mixture of toxic and non-toxic strains.