"Cyanobacteria, also known as blue-green algae, are the most notorious bloom formers in freshwater environments. Their success is due to their ability to adapt their physiological capacities to compete with other phytoplankton species (e.g., for light and nutrients, Padisák 1997), and the fact that they seem less edible for zooplankton and fish than other non-blooming algae (Gilbert 1996; Reynolds 1998). Cyanobacteria are photosynthetic, oxygen-producing and nitrogen-fixing prokaryotic organisms. They are characterized by the presence of the accessory pigment phycobilin. There are three basic morphological groups: 1) unicells, which may be solitary or aggregated in colonies; 2) undifferentiated non-heterocyst filaments, which also may be solitary or aggregated; and 3) filamentous forms with differentiated cells called heterocysts (Paerl et al. 2001). As a result of increasing environmental stress on freshwater ecosystems (e.g., eutrophication), blooms are increasing in number and frequency world-wide. Besides health problems due to their toxicity (such as the Caruaru tragedy in Pernambuco state, Brazil, Azevedo 1996), and environmental problems (e.g. fish kills and food web alterations), such blooms may also give rise to water quality and engineering problems such as clogging of filters in water treatment works, coloration of the water or production of tastes and odours (Paerl et al. 2001)."--p. 10.