Dose dependent variance in UV-C radiation induced effects on carbon and nitrogen metabolism in the cyanobacterium Nostoc muscorum Meg1

With the intention of getting an insight into the differential effect of UV-C radiation on the N 2 -fixing heterocystous cyanobacterium Nostoc muscorum Meg1, various aspects of carbon and nitrogen metabolism was evaluated in the organism. Exposure to different doses of UV-C (6, 12, 18 and 24 mJ/cm 2 ) showed that among various photo-absorbing pigments , phycobiliproteins were most sensitive. Oxygen evolving complex (OEC) activity measured as net oxygen evolution rate decreased by 63% upon 24 mJ/cm 2 exposure. Western blot analysis established that D1 protein of PSII was highly sensitive and its levels decreased even at a radiation dose as low as 6 mJ/cm 2 . In contrast, levels of the Calvin cycle enzyme RuBisCO was increased at 6 and 12 mJ/cm 2 doses but the level decreased drastically (84%) at higher dose (24 mJ/cm 2 ). The nitrogenase enzyme activity decreased at all doses but the ammonia assimilating enzyme glutamine synthetase (GS) activity recorded increase at the lower doses . The reactive oxygen species (ROS) and lipid peroxidation increased upon UV-C exposure. Transmission electron microscopic observation revealed damage to ultrastructure especially the thylakoid membrane organization , aggregation of dissolving phycobilisomes and loss of caboxysomes. Interestingly, sub-lethal radiation (6 and 12 mJ/cm 2 ) dose exposures increased the growth rate in the organism when growth was measured over a period of 11 days after radiation exposure.