Hydrogen peroxide can be a plausible biomarker in cyanobacterial bloom treatment.

The effect of combined stresses, photoinhibition, and nutrient depletion on the oxidative stress of cyanobacteria was measured in laboratory experiments to develop the biomass prediction model. Phormidium ambiguum was exposed to various photosynthetically active radiation (PAR) intensities and phosphorous (P) concentrations with fixed nitrogen concentrations. The samples were subjected to stress assays by detecting the hydrogen peroxide (H2 O2 ) concentration and antioxidant activities of catalase (CAT) and superoxide dismutase (SOD). H2 O2 concentrations decreased to 30 µmol m-2  s-1 of PAR, then increased with higher PAR intensities. Regarding P concentrations, H2 O2 concentrations (nmol L-1 ) generally decreased with increasing P concentrations. SOD and CAT activities were proportionate to the H2 O2 protein-1 . No H2 O2 concentrations detected outside cells indicated the biological production of H2 O2 , and the accumulated H2 O2 concentration inside cells was parameterized with H2 O2 concentration protein-1 . With over 30 µmol m-2  s-1 of PAR, H2 O2 concentration protein-1 had a similar increasing trend with PAR intensity, independently of P concentration. Meanwhile, with increasing P concentration, H2 O2 protein-1 decreased in a similar pattern regardless of PAR intensity. Protein content decreased with gradually increasing H2 O2 up to 4 nmol H2 O2 mg-1 protein, which provides a threshold to restrict the growth of cyanobacteria. With these results, an empirical formula-protein (mg L-1 ) = - 192*Log((H2 O2 /protein)/4.1), where H2 O2 /protein (nmol mg-1 ) = - 0.312*PAR2 /(502  + PAR2 )*((25/PAR)4  + 1)*Log(P/133,100), as a function of total phosphorus concentration, P (µg L-1 )-was developed to obtain the cyanobacteria biomass.