Impacts of ultraviolet-B irradiation on nitrogen-fixing cyanobacteria of rice paddy fields

Abstract

Summary The impact of ultraviolet-B (UV-B; 280–315 nm) irradiation on ribulose 1,5-bisphosphate carboxylase (RuBISCO) activity, membrane permeability, NaH14CO3 uptake and isolated phycocyanin has been studied in five N2-fixing cyanobacterial strains, viz., Anabaena sp., Nostoc sp., Nostoc carmium, Nostoc commune and Scytonema sp., isolated from Indian rice paddy fields. UV-B exposure for as little as 30 min resulted in a considerable decrease in the RuBISCO activity, which further decreased with increasing exposure time. Similarly, NaH14CO3 uptake was also severely affected by UV-B irradiation in all cyanobacterial strains studied so far. Isolated phycocyanin (λmax 620 nm) of Anabaena sp. showed a drastic decline in absorbance with increasing exposure time. Fluorescence excitation at 620 nm resulted in an emission at 640 nm that decreased as well and showed a gradual shift towards shorter wavelengths with increasing UV-B exposure time, indicating an impaired energy transfer from the accessory pigment, phycocyanin, to the photosynthetic reaction centers. SDS-PAGE analyses of the isolated phycocyanin revealed a loss in the αβ monomers (20kDa), the rod-core linker polypeptides (45kDa) and the core-membrane linker polypeptides (70kDa), denoting a disintegration of the aggregation state of phycocyanin following UV-B exposure and thus the uncoupling of the energy transfer pathway. Cyanobacterial membrane disruption occurs during UV-B irradiation, leading to changes in membrane permeability and release of l4C labelled compounds.