SPECTROSCOPIC AND BIOCHEMICAL ANALYSES OF UV EFFECTS ON PHYCOBILIPROTEINS OF ANABAENA SP AND NOSTOC CARMIUM

The effects of UV (280-400 nm) irradiation on phycobiliprotein composition have been studied in two N-2-fixing cyanobacteria, Anabaena sp. and Nostoc carmium, isolated from rice paddy fields in India. Phycobiliproteins were isolated and separated by sucrose density gradient centrifugation. After UV exposure the top fraction mainly contained carotenoids (absorption maximum at 485 nm), which first showed an increase in intensity and absorption and then a gradual decrease with increasing UV exposure in Anabaena sp., whereas, in Nostoc carmium this fraction showed a steady increase over the whole exposure time. The bottom fraction of both organisms mainly contained phycocyanin (absorption peak at 620 nm) which showed a steady decline in intensity, as well as absorption. Fluorescence excitation at 620 nm resulted in an emission at 650 nm which underwent a shift towards shorter wavelengths with increasing UV-exposure time, indicating a disassembly of the phycobilisomal complex and of impaired energy transfer from accessory pigments to the reaction centers. SDS PAGE analysis of the fractions revealed a loss of high molecular mass linker proteins and low molecular mass (alpha beta monomers indicating that the phycobiliproteins, which function as accessory pigments for the operation of photosystem II, disassemble during UV irradiation.