Abstract: Blooms of freshwater toxic cyanobacteria are a growing environmental health problem, enhanced by anthropogenic eutrophication and climate change. A variety of techniques were tested for their remediation, from physical methods using artificial mixing or flocculation, to chemical methods employing synthetic and natural compounds, as well as constructed wetlands. In this work, we conducted an evaluation at microcosm scale of the usefulness of the allelochemicals produced by a strain of the filamentous cyanobacteria Phormidium sp. for the bioremediation of proliferations of four strains of toxic freshwater cyanobacteria (Cylindrospermopsis raciborskii, Chrysosporum ovalisporum, Anabaena sp. and Nodularia sp.). Allelochemicals produced by this strain of Phormidium sp. belong to the portoamides compounds family. Their effect was tested in bioassays using cell-free filtrate, the results showing that all the four strains were sensitive. In addition, we performed phosphate-limited long-term competition experiments in continuous cultures, in which Phormidium sp. was co-cultured with each of the toxic cyanobacterial strains. The purpose of these later experiments was to demonstrate that allelopathy and not resource competition was responsible for the ecological exclusion of the toxic cyanobacteria strains, and also to employ higher population abundances to test the effectiveness of the allelochemicals. Before that, we needed to estimate the competitive ability of each species to limit the resource that we employed (phosphate). Phormidium sp. were clearly a better competitor for phosphate than Anabaena sp., worse than Nodularia sp., and very similar to C. raciborskii and C. ovalisporum. Only in the case of Nodularia sp. could we demonstrate that the ecological exclusion of the toxic cyanobacteria was caused by allelopathy. However, the rapid exclusion shown in our experiments suggests that allelopathy was the main cause in all cases. An inter-specific competition model includ ing only competition for phosphate and an allelopathic interaction was able to accurately describe the patterns of population dynamics observed in our experiments

N. Monteiro, J. Cuaresma, V. Vasconcelos, A. Barreiro