Impact of UV Radiation on Rice-Field Cyanobacteria: Role of Photoprotective Compounds

Abstract

1. AbstractMembers of the cyanobacteria are cosmopolitan in distribution, forming a prominent component of microbial populations in aquatic as well as terrestrial ecosystems. They play a central role in successional processes, global photosynthetic biomass production and nutrient cycling. In addition, N2-fixing cyanobacteria are often the dominant microflora in wetland soils, especially in rice paddy fields, where they significantly contribute to fertility as a natural biofertilizer. Recent studies have shown a continuous depletion of the stratospheric ozone layer, as a result of anthropogenically released atmospheric pollutants such as chlorofluorocarbons (CFCs) and the consequent increase in solar UV-B radiation reaching the Earth’s surface. Considering the vital role of cyanobacteria in crop production, the fluence rates of UV-B radiation impinging on the natural habitats are of major concern since, being photoautotrophic organisms, cyanobacteria depend on solar radiation as their primary source of energy. UV-B radiation causes reduction in growth, survival, protein content, heterocyst frequency and fixation of carbon and nitrogen, bleaching of pigments, disassembly of phycobilisomal complexes, DNA damage and alteration in membrane permeability in cyanobacteria. However, a number of cyanobacteria have developed photoprotective mechanisms to counteract the damaging effects of UV-B which includes synthesis of water soluble colourless mycosporine-like amino acids (MAA) and the lipid soluble yellow-brown coloured sheath pigment, scytonemin. Cyanobacteria, such as Anabaena sp., Nostoc commune, Scytonema sp. and Lyngbya sp. were isolated from rice paddy fields and other habitats in India and screened for the presence of photoprotective compounds. Spectroscopic and biochemical analyses revealed the presence of only shinorine, a bisubstituted MAA containing both a glycine and a serine group with an absorption maximum at 334 nm in all cyanobacteria except Lyngbya sp. There was a circadian induction in the synthesis of this compound by UV-B. A polychromatic action spectrum for the induction of MAAs in Anabaena sp. and Nostoc commune also shows the induction to be UV-B-dependent and peaking at around 290 nm. Another photoprotective compound, scytonemin, with an absorption maximum at 386 nm (which also absorbs significantly at 300, 278, 252 and 212 nm) was detected in all cyanobacteria except Anabaena sp. In addition, two unidentified, water-soluble, yellowish (induced by high white light) and brownish (induced by UV-B) compounds with an absorption maximum at 315 nm were recorded only in Scytonema sp. In conclusion, cyanobacteria having photoprotective mechanisms may be potent candidates as biofertilizers for crop plants.