Differential effect of UV-B radiation on growth, oxidative stress and ascorbate–glutathione cycle in two cyanobacteria under copper toxicity

Effects of low (UV-BL; 0.1 μmol m−2 s−1) and high (UV-BH; 1.0 μmol m−2 s−1) fluence rates of UV-B radiation on growth, oxidative stress and ascorbate–glutathione cycle (AsA–GSH cycle) were investigated in two cyanobacteria viz. Phormidium foveolarum and Nostoc muscorum under copper (2 and 5 μM) toxicity after 24 and 72 h of experiments. Cu at 2 and 5 μM and UV-BH irradiation decreased growth in both the organisms and the effect was more pronounced in N. muscorum. Superoxide radical (SOR) and hydrogen peroxide (H2O2) productions were significantly enhanced by Cu and UV-BH which was accompanied by accelerated lipid peroxidation (malondialdehyde; MDA) and protein oxidation (reactive carbonyl groups; RCG). The components of AsA–GSH cycle, i.e. ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascobate reductase (MDHAR) and dehydroascorbate reductase (DHAR) activities as well as total ascorbate and glutathione contents and their reduced/oxidized ratios were decreased considerably by Cu and UV-BH. Further, combined treatments of Cu and UV-BH exacerbated damaging effects in both the cyanobacteria. Unlike UV-BH, UV-BL irradiation rather than damaging cyanobacteria caused alleviation in Cu-induced toxicity by down-regulating the levels of SOR, H2O2, MDA and RCG due to enhanced activity of APX, GR, MDHAR and DHAR, and contents of ascorbate and glutathione. Results revealed that UV-B radiation at low fluence rate (UV-BL) stimulated protective responses in both the organisms under Cu toxicity while UV-BH irradiation caused damage alone as well as together with Cu, and the components of AsA–GSH cycle play significant role in these responses.