Efeitos alelopáticos de microcistinas sobre o crescimento de duas linhagens de microalgas verdes (Chlolococcales, Chlorophyta) em condições controladas

Abstract

Some laboratory studies had shown that microcystins exert allelopathic effects on organisms of the aquatic biota. However, the experimental designs rarely test concentrations found in natural environments (above 10 μg.L-1), which makes uncertain the action of microcystins as allelopathic compounds. Our study aimed to evaluate the effects of microcystins in concentrations usually found in natural conditions on the growth of strains of green microalgae (Monoraphidium convolutum and Scenedesmus acuminatus). For this purpose mixed cultures containing cyanobacteria and green algae were analyzed. Cultures of microalgae with the addition of crude extracts of cyanobacteria microcystin-producing and non-microcystin-producing were also performed. Two non-clonal axenic strains of cyanobacteria were used: Microcystis aeruginosa (microcystin-producing – MC+) and Microcystis panniformis (non-producer-producing – MC-). The experiments were conducted under controlled laboratory conditions (Erlenmeyers with a capacity of 1000 mL, with 600 mL of final volume of culture in BG11 medium at pH 7.4, temperature 24 ± 1 ° C, photoperiod 14h: 10h (light:dark), 40 μmol.m-2.s-1 light intensity). It was observed that in mixed cultures, both green microalgae and cyanobacteria had their growth reduced, which was observed when analyzing variations in specific growth rate. The microcystin production by M. aeruginosa (MC +) increased when it was in cultured together with M. convolutum. However when cultured with S. acuminatus the production did not change. The growth of green microalgae has not changed in treatments with addition of crude cell extracts of M. aeruginosa (MC +) containing 5μg.L-1 and 10μg.L-1 of total microcystins. Similarly, for the treatments with addition of crude cell extracts of M. panniformis (MC-) the growth of these green microalgae did not change. Our results do not support of the allelopathic function of microcystins, at least in the growth of these green microalgae.