Interactive effects of UV-B and Cu on photosynthesis, uptake and metabolism of nutrients in a green alga Chlorella vulgaris under simulated ozone column

Abstract

This study demonstrated a general reduction in photosynthesis (carbon fixation, O2-evolution and photochemical electron transport chain), the uptake of NH4+, NO3-, urea and PO43-, and activities of nitrate reductase, urease, acid phosphatase and ATPase following UV-B and copper exposure of Chlorella vulgaris in the absence or presence of 1 and 2 ppm concentrations of a 4-inch-thick ozone layer. Though the effect of stressors used in combination was very detrimental to the above processes, selected concentrations of ozone not only counteracted the UV-B-induced inhibition of the above processes, but also stimulated O2-evolution and the photochemical electron transport chain. Kinetics of nutrient uptake and enzyme activities demonstrated that UV-B causes structural change(s) in the enzymes/carriers responsible for the uptake of NH4+, NO3-, urea and PO43-, as well as their assimilatory enzymes. Except for nitrate reductase, copper was found to compete for the binding sites of all the above enzymes. Synergistic inhibition of photosynthetic activity, nutrient (except NH4+) uptake, and enzyme activities by UV-B + Cu seems to be due to increased Cu uptake as a consequence of altered membrane permeability brought about by the peroxidation of membrane lipids in UV-B-exposed cells.