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

Abstract

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 H ₂O ₂ and antioxidant activities were paralleled to light intensity. Higher H ₂O ₂ 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 H ₂O ₂ 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.