Oxidative Stress and Antioxidant Responses of Phormidium ambiguum and Microcystis aeruginosa Under Diurnally Varying Light Conditions

Abstract

Two harmful cyanobacteria species (<i>Phormidium ambiguum</i> and <i>Microcystis aeruginosa</i>) 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 <i>P. ambiguum</i> 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.