Environmental Engineering Reference
In-Depth Information
IMPACT OF UV RADIATION ON RICE-FIELD CYANOBACTERIA: ROLE
OF PHOTOPROTECTIVE COMPOUNDS
RAJESHWAR P. SINHA AND DONAT-P. HÄDER
Friedrich Alexander Universität, Institut für Botanik und
Pharmazeutische Biologie, Staudtstr. 5, 91058 Erlangen, Germany
1. Abstract
Members 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, N 2 -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
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