Comparative Evaluation of Nitrogen Metabolism and Antioxidant Enzyme Activities in Selected Cyanobacterial Strains

Cyanobacteria fix nitrogen and produce antioxidant metabolites, making them excellent biological resources for sustainable agriculture. Seven cyanobacterial strains, Anabaena, Westiellopsis, Nostoc, Calothrix, Phormidium, Oscillatoria, and Tolypothrix, were tested for main nitrogen metabolism enzymes (GS, NR, GOGAT, GDH) and antioxidant defense enzymes. Quantifying activities with spectrophotometric tests created a comparative biochemical profile. The results showed continuous strain differences. Phormidium sp. had the highest values for nitrogen assimilation enzymes, including GOGAT (85.9 ± 2.5 nmol NADH min^-1 mg^-1 protein), GDH (43.7 ± 1.7 nmol NAD(P)H min^-1 mg^-1 protein), and antioxidant activity, including SOD (162 ± 4.5 U mg^-1 protein) and APX (15.3 ± 0.7 nmol min^-1 mg^-1). Anabaena sp. exhibited high GS activity (1.98 ± 0.06 U mg^-1 protein) and strong antioxidant responses (SOD 156 ± 4.2 U mg^-1 protein). Tolypothrix sp. had lower metabolic and oxidative defense capabilities, with activity levels of NR (5.8 ± 0.28 µmol NO₂ ^- h^-1 mg^-1 protein) and CAT (17.4 ± 0.7 µmol min^-1 mg^-1 protein) being the lowest nitrogen metabolism and antioxidant enzymes had high positive correlations (e.g., GS-SOD: r = 0.91; NR-CAT: r = 0.88), showing that nitrogen absorption enhances oxidative stress tolerance. Cyanobacterial strains have great biochemical variety, and Phormidium sp., Anabaena sp., and Calothrix sp. are promising prospects for biofertilizer production, plant stress alleviation, and other biotechnological uses.