Antioxidant System Against Active Oxygen Species in Cyanobacterium Aphanothece stagnina: Response to Excess Light Under Cadmium Stress

Abstract

The aim of the present study was to investigate the effect of photoinhibitory light induced changes under cadmium stress in electron transport activity, active oxygen species (AOS) generation, oxidative damage, photorespiration, enzymatic and non-enzymatic (low molecular weight) antioxidants in cyanobacterium, Aphanothece stagnina. The photosynthetic pigment contents enhanced progressively with increasing light intensity (20–120 μmol photons m−2 s−1); however, different light intensities together with cadmium (4 μM) decreased the pigment contents. Accumulation of Cd in cyanobacterial cells increased appreciably under photoinhibitory light (1,000 μmol photons m−2 s−1) treatment. Photoinhibitory light inhibited photosynthetic electron transport activity. In the presence of elevated level (50–100 μM) of Cd the inhibition became stronger showing enhanced effect on whole chain activity and photosystem II (PS II) with the least effect on photosystem I. Under photoinhibitory light treatment, the level of oxidants like hydrogen peroxide and superoxide radical increased substantially with simultaneous increase in lipid peroxidation and decrease in membrane intactness. The presence of Cd further enhanced such effects. Under this condition, considerable increase in photorespiration (loss of 14CO2) and content of non-enzymatic antioxidant like ascorbate, and appreciable suppression in the activity of enzymatic antioxidants like peroxidase, superoxide dismutase and catalase were noticed. These results indicate that photoinhibitory light alone and together with Cd strongly arrested electron flow through PS II and significantly inhibited the activity of enzymatic antioxidants. As consequence of this substantial rise in the level of AOS induced oxidative damage in A. stagnina.