Avaliação dos efeitos da disponibilidade de luz sobre o crescimento e competição entre cepas de Planktothrix agardhii e Microcystis aeruginosa

Abstract

The bloom-forming cyanobacteria Microcystis aeruginosa and Planktothrix agardhii are common in eutrophic aquatic ecosystems. In such environments, the availability of light is one of the determining factors for the development and structure of the phytoplankton community. The present study aimed to investigate the effects of light on the physiology of strains of M. aeruginosa and P. agardhii, assessing growth, inter and intra-specific variability and light competition. The strains were grown in batch cultures at different light intensities (10, 40, 60, 100 and 500 µmol m-2 s-1). Growth rates and maximum yields of cultures were calculated. The minimum requirement of light for each strain was determined in experiments with monocultures under continuous culture (chemostats) and conditions of light limitation. To evaluate the light competition, experiments were performed with bicultures in chemostats. It was observed intra and interspecific strains variability in the different light intensities tested. At 500 µmol m-2 s-1, both strains of M. aeruginosa had higher biomass than P. agardhii, confirming that P. agardhii has higher sensitivity to light. Despite reducing its maximum yield of culture, P. agardhii grew in light intensities considered high for this species (100 and 500 µmol m-2 s-1). These results demonstrate the ability of P. agardhii occur in environments with large amplitudes of light. All the strains showed similar growth at intensity of 10 µmol m-2 s-1, demonstrating the ability of the two species to grow in low light. In chemostats under conditions of light limitation, the strains of M. aeruginosa reached higher biomass during the equilibrium (steady-state) than P. agadhii, reflecting a higher carrying capacity. However, values of minimum requirement of light were similar between the two species. Under competition, M. aeruginosa exceeded P. agardhii immediately after the start of the experiment. This rapid growth resulted in the dominance of M. aeruginosa for all pairs of strains tested and, in two cases, there was competitive exclusion of P. agardhii. When no exclusion occurred, P. agardhii managed to remain in the system with a low biomass (about 15%). These results help to understand the co-occurrence of these species in the environment and the dominance of M. aeuginosa even in low light conditions.