Antioxidative role of nitric oxide on copper toxicity to a chlorophycean alga, Chlorella

The response of Chlorella vulgaris to copper exposure was investigated under laboratory batch culture conditions. Increased toxicity of Cu with respect to photosynthetic carbon fixation, O2 evolution, chlorophyll fluorescence, and oxidative burst was observed for N–NH4+-grown cultures. The addition of sodium nitroprusside, a nitric oxide (NO) donor, in combination with Cu to N–NH4+-grown Chlorella not only lowered the inhibition levels of carbon fixation, O2 evolution, and maximum quantum yield of PS II, but also significantly reduced the oxidative burst. The protective action of sodium nitroprusside was, however, arrested in cultures in which sodium nitroprusside was supplemented in combination with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, a specific scavenger of NO in the experimental system. The N–NO3−-grown Chlorella depicted less sensitivity to Cu compared to its N–NH4+-grown counterpart. The N–NO3−-, N–NH4+-, and N–NH4++sodium nitroprusside-grown Chlorella did not show any significant differences with respect to their Cu uptake potential. The role of NO as an antioxidant is discussed.