HYDROGEN PEROXIDE CAN BE A PLAUSIBLE BIOMARKER IN CYANOBACTERIAL RESPONSES TO PHOTOINHIBITION
Mizanur RAHMAN, Takashi ASAEDA, Helayaye Damitha Lakmali ABEYNAYAKA
Saitama University, Japan

Abstract
The effect of combined stresses, photoinhibition and nutrient depletion, on the oxidative stress of cyanobacteria was measured in laboratory experiments and the prediction model for the growth rate was developed. Phormidium ambiguum was exposed to various PAR intensities and phosphorous concentrations with fixed nitrogen concentration. The samples were subjected to stress assays by detecting hydrogen peroxide (H₂O₂) concentration and antioxidant enzyme activities of catalase (CAT), and superoxide dismutase (SOD). Besides these biomarkers, protein contents were also analyzed. H₂O₂ concentration decreased until 30mmol/m²/s of photosynthetically active radiation (PAR), then, increased with further higher PAR intensity. Regarding with phosphorous concentration, generally decreased with increasing phosphorous concentration. SOD activity is proportionate to the H₂O₂ per protein content, and CAT activity linearly increased with H₂O₂ concentration. Almost no H₂O₂ concentration was detected outside cells. These results indicated that H₂O₂ concentration was majorly biologically produced, and thus the accumulated H₂O₂ concentration inside cells is parameterized with H₂O₂ concentration per protein content. Over 30mmol/m²/s of photosynthetically active radiation (PAR), H₂O₂ concentration per protein had a similar increasing trend with PAR intensity, independently of phosphorous concentration. While, with increasing phosphorous concentration, H₂O₂ per protein decreased in the similar pattern regardless of PAR intensity. Protein content decreased with increasing H₂O₂ per protein, gradually until 4nmol/mg of H₂O₂ concentration, which provides the threshold to restrict the growth of cyanobacteria. With these results, an empirical formula to obtain the protein content was developed as a function of PAR intensity and phosphorous concentration.

Keywords: cyanobacteria, hydrogen peroxide, production rate, combined stress, antioxidant activity