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

Abstract

With the intention of getting an insight into the differential effect of UV-C radiation on the N₂-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/cm²) 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/cm² 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/cm². In contrast, levels of the Calvin cycle enzyme RuBisCO was increased at 6 and 12 mJ/cm² doses but the level decreased drastically (84%) at higher dose (24 mJ/cm²). 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/cm²) dose exposures increased the growth rate in the organism when growth was measured over a period of 11 days after radiation exposure.

Keywords: D1 protein; Glutamine synthetase; Nitrogenase; PSII; RuBisCO; UV-C.