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.
Keywords: harmful cyanobacteria; hydrogen peroxide; linear relationships; photo stress; species characteristics.