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DIFFERENTIAL EFFECT OF ULTRAVIOLET‐B RADIATION ON CERTAIN METABOLIC PROCESSES IN A CHROMATICALLY ADAPTING Nostoc

Abstract— The impact of UV‐B radiation on growth, pigmentation and certain physiological processes has been studied in a N2‐fixing chromatically adapting cyanobacterium, Nostoc spongiaeforme. A brownish form (phycoerythrin rich) was found to be more tolerant to UV‐B than the blue‐green (phycocyanin rich) form of N. spongiaeforme. Continuous exposure to UV‐B (5.5 W m‐2) for 90 min caused complete killing of the blue‐green strain whereas the brown strain showed complete loss of survival after 180 min. Pigment content was more strongly inhibited in the blue‐green strain than in the brown. Nitrogenase activity was completely abolished in both strains within 35 min of UV‐B treatment. Restoration of nitrogenase occurred upon transfer to fluorescent or incandescent light after a lag of 5–6 h, suggesting fresh synthesis of nitrogenase. Unlike the above processes, in vivo nitrate reductase activity was stimulated by UV‐B treatment, the degree of enhancement being significantly higher in the blue‐green strain. Like the effect of UV‐B on nitrogenase, 14CO2 uptake was also completely abolished by UV‐B treatment in both strains. Our findings suggest that UV‐B may produce a deleterious effect on several metabolic activities of cyanobacteria, especially in cells lacking phycoerythrin. Strains containing phycoerythrin appear to be more tolerant to UV‐B, probably because of their inherent property of adapting to a variety of light qualities.