-(Effect of nitrate and ammonium on the growth and protein concentration of Microcystis viridis Lemmermann (Cyanobacteria)). Cyanobacteria are a very important group in aquatic systems, particularly in eutrophic waters. Therefore studies about their success in the environment are essential. Many hypotheses have tried to explain the dominance of Cyanobacteria, and several emphasized the importance of various nitrogen sources for the success of the group. In this study, we measured the effect of ammonium and nitrate on the growth and protein concentration of Microcystis viridis (Cyanobacteria). This species is well-known because bloom formation in eutrophic waters. The study was carried out, in experimental batch cultures, using the WC medium with different nitrogen sources: ammonium, nitrate, ammonium + nitrate (50% ammonium + 50% nitrate) and ammonium at different concentrations (to test for possible NH 4 + toxicity). Protein, ammonium and nitrate concentrations were measured at end of each experiment, whereas samples for cell counts were taken daily. Results showed that Microcystis viridis grew faster with ammonium (µ = 0.393 day ) and ammonium + nitrate (µ = 0.325 day -1 ). This pattern is explained by the metabolism of ammonium that presents higher uptake and assimilation rates than nitrate. Maximum cell concentration, however, was higher in the ammonium + nitrate treatment, followed by nitrate treatment. Higher protein concentration were observed in the treatment with nitrate. In the ammonium toxicity test, no difference between the control and NH 4 + at 50% was found. Thus, the ammonium concentrations used in these experiments were not toxic. Our results suggest that Cyanobacteria is able to grow on both nitrogen sources even if ammonium may allow faster growth and bloom development.Key words -ammonium, Cyanobacteria, eutrophication, growth, nitrate, protein concentration RESUMO -(Efeito do nitrato e amônio sobre o crescimento e a concentração de proteínas de Microcystis viridis Lemmermann (Cyanobacteria)). As cianobactérias são um grupo muito importante em sistemas aquáticos, particularmente nos sistemas eutrofizados. Por isso, estudos sobre o sucesso desse grupo são essenciais. Muitas hipóteses têm tentado explicar a dominância de Cyanobacteria e várias enfatizam a importância de fontes nitrogenadas para o sucesso delas. Neste estudo, foi avaliado o efeito dos íons amônio e nitrato sobre o crescimento e a concentração protéica de Microcystis viridis (Cyanobacteria). Esta espécie é bem conhecida por apresentar florações em águas eutróficas. O estudo foi conduzido em culturas do tipo "batch", usando o meio WC com diferentes fontes nitrogenadas: amônio, nitrato, amônio + nitrato (50% amônio + 50% nitrato), e amônio em diferentes concentrações (para testar o possível efeito tóxico de NH 4 + ). Amostras para contagem celular foram retiradas diariamente. As concentrações de proteína, amônio e nitrato foram medidas ao final dos experimentos. Os resultados mostraram que Microcystis viridis cresceu mais r...

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Cylindrospermopsis raciborskii is the focus of many studies due to its toxicity and increasing blooms frequency. The comprehension of the factors that might trigger these blooms is fundamental for the maintenance of good freshwater quality. To better understand the autoecology of C. raciborskii and to identify the factors controlling its dominance, general limnological features were evaluated in Pedalinhos reservoir, Brazil. Samplings were performed monthly between 2011 and 2013. Although C. raciborskii is considered a relatively perennial species in the tropics, it presented an interesting nonunimodal variation, with biovolume varying from 0 to c. 30 mm³ L⁻¹ in short time intervals (< 30 days). These temporal trends allowed the evaluation of the isolated effects of nonclimatic variables. Cylindrospermopsis raciborskii was connected with several variables, but nitrogen was the major determinant of its dynamics. Blooms were observed when dissolved inorganic nitrogen (DIN) was scarce (< 50 μg L⁻¹), mainly in relation to the availability of phosphorus and total-N (DIN : total-P < 3 and DIN : total-N < 0.04). This paper contributes to establish strategies to avoid C. raciborskii blooms, suggesting that a management in P levels would be not sufficient. It is necessary to consider N availability to avoid its dominance in the reservoir.

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