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Egeria densa release allelochemicals harmful to other species, including cyanobacteria. Under natural conditions, E. densa and Microcystis aeruginosa can be found in various aquatic systems with different light levels. The effect that the coexistence of E. densa has on M. aeruginosa was tested under different photosynthetically active radiation (PAR) intensities. The growth, chlorophyll-a, oxidative stress, and antioxidant activities of M. aeruginosa were quantified after 7 days of coexistence with E. densa. A control experiment without E. densa was conducted for M. aeruginosa under the same conditions to distinguish the effect of the PAR intensity from the allelopathic effect. The allelopathy of E. densa on M. aeruginosa is significantly influenced by the PAR intensity. PAR intensities lower or higher than the favorable range further suppressed M. aeruginosa under coexistence. Research has revealed that the allelopathic effect of E. densa can be utilized to biologically suppress M. aeruginosa under various light conditions. Strong second-order polynomial distributions between the optical density (OD) and cellular H2O2 content of M. aeruginosa were found for both the control and coexistence conditions. In the practical application of E. densa allelopathy, OD can be adopted as a convenient method to obtain an approximate stress status on M. aeruginosa. [ABSTRACT FROM AUTHOR]

In this study, Egeria densa plants were exposed to a low concentration of (OD730 = 0.04) Microcystis aeruginosa for seven days, under different photosynthetically active radiation (PAR) intensities (0, 50, 100, 200, and 300 µmol m−2 s−1). Experiments were conducted under controlled conditions inside incubators. The H2O2 content, antioxidant responses, pigmentation, and chlorophyll fluorescence parameters were measured. The biochemical parameters of E. densa varied with PAR intensity; under the 50 and 100 µmol m−2 s−1 PAR intensities, E. densa performed relatively better than under other intensities. When exposed to M. aeruginosa, the levels of the measured biochemical parameters reduced in E. densa. Under unfavourable light conditions, the optical density of M. aeruginosa was also reduced when E. densa was also present. Even at low concentrations, the presence of M. aeruginosa in the water can have a negative effect on E. densa pigmentation and physiological parameters, though E. densa also has negative effects on M. aeruginosa growth. Further studies are recommended to evaluate the responses of E. densa after prolonged exposure to M. aeruginosa and the effects of E. densa on M. aeruginosa. [ABSTRACT FROM AUTHOR]

Two harmful cyanobacteria species (Phormidium ambiguum and Microcystis aeruginosa) were exposed to diurnal light-intensity variation to investigate their favorable and stressed phases during a single day. The photosynthetically active radiation (PAR) started at 0 µmol·m−2·s−1 (06:00 h), increased by ~25 µmol·m−2·s−1 or ~50 µmol·m−2·s−1 every 30 min, peaking at 300 µmol·m−2·s−1 or 600 µmol·m−2·s−1 (12:00 h), and then decreased to 0 µmol·m−2·s−1 (by 18:00 h). The H2O2 and antioxidant activities were paralleled to light intensity. Higher H2O2 and antioxidant levels (guaiacol peroxidase, catalase (CAT), and superoxidase dismutase) were observed at 600 µmol·m−2·s−1 rather than at 300 µmol·m−2·s−1. Changes in antioxidant levels under each light condition differed between the species. Significant correlations were observed between antioxidant activities and H2O2 contents for both species, except for the CAT activity of P. ambiguum at 300 µmol·m−2·s−1. Under each of the conditions, both species responded proportionately to oxidative stress. Even under maximum light intensities (300 µmol·m−2·s−1 or 600 µmol·m−2·s−1 PAR intensity), neither species was stressed. Studies using extended exposure durations are warranted to better understand the growth performance and long-term physiological responses of both species. [ABSTRACT FROM AUTHOR]