Mercury Induces Alteration of Energy Transfer in Phycobilisome by Selectively Affecting the Pigment Protein, Phycocyanin, in the Cyanobacterium, Spirulina platensis

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Mercury, at a low concentration (3μM) caused an enhancement in the intensity of room temperature fluorescence emitted by phycocyanin and induced a blue shift in the emission peak of Spirulina cells indicating the alterations in the energy transfer within the phycobilisomes. In vitro the isolated intact Spirulina phycobilisomes from control cells exhibited only a reduction in fluorescence yield with low concentration of HgCl_2 without being accompanied by changes in the emission features, whereas the isolated phycobilisomes from mercury treated cells exhibited the alterations in the spectral characteristics at the level of phycocyanin. When isolated phycocyanin and allophycocyanin were exposed to very low concentrations of Hg^<2+> ions, C-phycocyanin exhibited a large decrease in the absorbance in the longer wavelength (615-620 nm) region, but not allophycocyanin. In addition, mercury also caused a monotonous decrease in the C-phycocyanin emission intensity at 646 nm accompanied by a blue shift to 642 nm. These results on isolated C-phycocyanin suggest that selective bleaching of beta-84 chromophore of phycocyanin is induced by mercury. The differential effect of mercury towards C-phycocyanin and allophycocyanin could possibly be due to the difference in the protein conformation of phycocyanin and allophycocyanin.