Efeitos da temperatura e nutrientes na interação alelopática da macrófita submersa Egeria densa sobre a cianobactéria potencialmente tóxica Raphidiopsis raciborskii

Abstract

Artificial eutrophication associated with climate change may intensify cyanobacterial blooms in aquatic ecosystems. These microorganisms can cause several impacts to aquatic environments, altering physical, chemical and biological characteristics and producing lethal toxins for human health and other animals. Nitrogen and phosphorus are the main elements that provide the growth of cyanobacteria, being fundamental elements for the synthesis of proteins, aminoacids and nucleic acids, in addition to participating in other metabolic processes. Raphiopsis raciborskii is a species of cyanobacteria that has gained emphasis in studies due to its invasive potential and toxicity, producing neurotoxins (saxitoxin). Therefore, biocontrol methods for cyanobacterial blooms are essential and submerged macrophytes are identified as organisms that control the growth and production of cyanobacterial toxins through allelopathy. The submerged macrophyte Egeria densa is identified as a potential inhibitor of the growth of R. raciborskii. However, the effect of this inhibition at high temperatures and nutritional conditions is still unknown. The aim of this study was to analyze the inhibitory potential of E. Densa on the growth and toxin production of R. raciborskii at different temperatures and nutritional conditions. Interaction treatments of R. raciborskii with biomass (1gPF-1 ) and extracts of E. densa were used at temperatures of 24, 28 and 32ºC in the conditions of excess nitrogen (+N), excess phosphorus (+P) and excesses of nitrogen and phosphorus (+NP). Parameters of chlorophyll-a concentrations, photosynthetic efficiency and saxitoxin concentration were measured. From the chl-a data, growth and inhibition rates were performed. The physical presence and extracts of E. densa were able to inhibit the growth and biomass production of the cyanobacteria R. raciborskii at all temperatures and nutrient concentrations tested. At the temperature of 24ºC, the biomass of E.densa had a greater negative effect on the photosynthetic efficiency of R. raciborskii, while at the temperatures of 28 and 32ºC, the extract reduced the photosynthetic efficiency. Despite the inhibition of R. raciborskii growth, saxitoxin concentrations were high at 28ºC, exceeding 5µg L-1 in treatments exposed to physical presence and 1µgL-1 in exposure to the extract. Although the submerged macrophyte caused the inhibition of R. raciborskii in all temperatures and nutritional conditions, the highest temperatures favored the production and release of saxitoxins by the cyanobacteria.