Interações alelopáticas entre cepas de Raphidiopsis raciborskii e Microcystis aeruginosa em diferentes condições de nitrogênio, fósforo e mudanças de temperaturas

Abstract

The combined effects of increased eutrophication and global temperatures on aquatic ecosystems have favored the expansion of blooms and cyanobacterial toxicity. The species Raphidiopsis raciborskii and Microcystis aeruginosa have stood out for their dominance and expansion in aquatic bodies around the world. In tropical ecosystems, Raphidiopsis raciborskii has co-dominated or replaced Microcystis aeruginosa, due to the nutritional conditions of the environment or its ecophysiological characteristics. Given the above, this study aimed to evaluate the effects of changes in temperature and nutritional conditions on allelopathic interactions between R. raciborskii and M. aeruginosa. Mixed culture, cell-free media and co-culture experiments were carried out in partitioned chambers with strains of R. raciborskii and M. aeruginosa. The experiments were carried out with initial biovolume of each strain of 4,0 x 10^6 µM^3 mL^-1 at temperatures 24 ºC and 30 ºC and ASM-1 medium, control condition, and modified ASM 1 medium with high concentrations of nitrate (7 mmol NaNO3), urea (CH4N2O 50 mM), and phosphate (350 µmol of Na2PO3). Cultures were incubated for eight days, samples for measurements of cell number, biomass and cyanotoxin content were collected on alternate days. Three-way analysis of variance (ANOVA) was performed to check the influence of temperature and nutrients on biovolume, concentration of toxins, growth rates and rate of inhibition of strains. High levels of nitrate and phosphate have been shown to induce the growth of strains of cyanobacteria, while urea has not been shown to be ideal for the growth of these organisms. Raphidiopsis raciborskii showed greater growth than M. aeruginosa in monoculture or during competition experiments. The increase in temperature showed an increase in the population growth of R. raciborskii and a reduction in the cell volume of this species, we verified that the temperature intensified the interactions between species, promoting greater competitive advantage to R. raciborskii. The increase in temperature led to an increase in the production of microcystin and a reduction in the content of saxitoxin in monoculture, on the other hand, during the competition the high temperature increased the concentrations of saxitoxins. The results found in this study suggest that toxins can be considered as potential allelochemicals capable of exerting a strong influence on interactions, and the increase in temperature and nutrients (nitrate and phosphate) promoted the dominance of R. raciborskii over M. aeruginosa.