Alternation in photosynthetic characteristics of Anabaena doliolum following exposure to UVB and Pb

Anabaena doliolum, when exposed to either ultraviolet-B (UVB) radiation or Pb, showed reduced growth rate, carbon fixation, O2-evolution, photosynthetic electron transport activity and ATP pool size. The rate of respiration was found to increase in UVB-treated cells; this increase was more pronounced in the cells exposed to UVB and Pb simultaneously. The UVB-induced inhibition of 2,6-dichlorophenol indophenol (DCPIP) photoreduction and lowering of chlorophyll a fluoroescence could not be reversed by artificial electron donors (diphenyl carbazide, NH2OH and MnCl2). These electron donors, however, substantially reversed the inhibition caused by Pb, thereby suggesting that UVB primarily inhibits the photosystem II (PS II) reaction center and Pb arrests the electron flow at the water splitting site. Nevertheless, the suppressed fluorescence intensity and the reduced emission and excitation peaks of phycobilisomes indicate the involvement of Pb in inhibition of PS II. All combinations of UVB and Pb inhibited the different metabolic processes in a synergistic manner. (Author)