Efeitos alelopáticos mútuos entre a macrófita aquática Egeria densa Planch. e a cianobactéria formadora de florações Microcystis

Abstract

Aquatic macrophytes are important for the maintenance of clear alternative states of aquatic environments, because they can release allelopathic substances capable of inhibiting the growth of planktonic algae. On the other hand, in turbid environments, resulting from cyanobacterial blooms, the presence of cyanotoxins can negatively affect the macrophytes, causing oxidative stress and reducing their growth. In order to elucidate the mutual allelopathic relationships between bloom-forming cyanobacteria and submerged macrophytes, we tested two hypotheses through laboratory studies: (1) the release of allelochemicals by Egeria densa reduces the growth rates of toxic and non-toxic strains of Microcystis; and (2) toxic strains of Microcystis inhibit the growth and biomass of E. densa, and cause oxidative stress on these plants. Based on this, two articles were written: (1) Why do aquatic macrophytes inhibit cyanobacteria? Allelopathic effects of Egeria densa Planch. on toxic and non-toxic strains of Microcystis; and (2) Can submerged macrophytes inhibit toxic and non-toxic strains of cyanobacteria equally? Allelopathic effects of Egeria densa Planch. on Microcystis spp. In the first article, we verified the influence of the coexistence of E. densa and the application of extracts of this plant (with dissolved allelochemicals) on the biomass and growth rates of toxic strain of M. aeruginosa (MC+) and non-toxic strain of M. panniformis (MC-). In the second, we evaluated the physiological and growth responses of E. densa cultivated with the MC+ and MC- strains, verifying the lipid peroxidation, hydrogen peroxide production, changes in photosynthetic pigment content and activity of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX). The MC+ strain was significantly inhibited when cultivated with E. densa, while MC- exhibited a contrary response, with stimulating growth. When cultivated under the influence of E. densa extract, MC+ and MC- strains were inhibited, but more significantly in MC-. The macrophyte presented a small increment in the total phenolic compounds production when in coexistence with the cyanobacteria, mainly with the toxic strain. The growth and biomass of E. densa were inhibited when in co-culture with the MC+ strain, showing few changes when cultivated with the MC- strain. In addition, the MC+ strain inhibited the emergence of shoots and roots in plants. Both strains showed a reduction in total chlorophyll levels, chlorophyll a and b, as well as an increase in the total carotenoids content. The plants cultivated with MC+ strain presented high lipid peroxidation rates and enzymatic activity, especially for the APX. Our results support the hypothesis that there are mutual allelopathic interactions between Microcystis and E. densa. However, E. densa needs a stimulant to release the allelochemicals, such as the stress caused by microcystins. The tested plant has enzymatic defense mechanisms that allow them to coexist with microcystins during short-time exposure (about eight days), however, prolonged exposure (five weeks) to toxic strains of Microcystis causes a reduction in plant growth.