Ultraviolet radiation induces both degradation and synthesis of phycobilisomes in Nostoc sp.: a spectroscopic and biochemical approach
A cyanobacterium, Nostoc sp., isolated from an Andean lake located 3980 m above sea level (Cusco, Peru) was used for this study. Phycobilisomes, the light-harvesting complexes of photosystem II in cyanobacteria, were isolated and their components, allophycocyanin, phycocyanin and phycoerythrin, characterized using absorption spectroscopy, chromatography, SDS-PAGE, and 2D-PAGE. When immobilized cells in agarose were exposed to ultraviolet radiation, the photobleaching of phycobilisomes took place with a faster kinetics than that of chlorophyll a and carotenoids. Isolated thylakoid membranes exposed to ultraviolet radiation confirmed the rapid degradation of phycobilisomes. Nostoc cells were irradiated with UV and the phycobilisomes isolated and analyzed. During the first 15 min of ultraviolet radiation, phycobilisomes were degraded followed by an increase in phycobiliproteins after 30 min (P<0.05), which was maintained until 45 min of exposure; then the concentration decreased again. Changes in the composition of the phycobilisome rods were observed; absorption spectroscopy indicates a partial depletion of phycocyanin; 2D gels showed that the isoelectric point of phycocyanin was affected indicating effects of ultraviolet radiation at the amino acid level. The clonogenic capacity of the cells was not significantly affected even after 30 min of exposure (P<0.01).