Cyanobacteria are cosmopolitan in distribution, forming a prominent
component of microbial populations in both aquatic as well as
terrestrial ecosystems. They are the dominant microflora in
rice-fields, where they significantly contribute to fertility as a
natural biofertilizer. The fluence rates of UV-B radiation impinging
on the natural habitats is of major concern since being
photoautotrophic organisms, cyanobacteria depend on solar radiation as
the primary source of energy. UV-B radiation causes reduction in
growth, survival, pigment and protein content, heterocyst frequency
and fixation of carbon and nitrogen in cyanobacteria. However, a
number of cyanobacteria synthesize water soluble colorless
mycosporine-like amino acids (MAAs) and the lipid soluble yellow-brown
colored sheath pigment, scytonemin to counteract the damaging effects
of UV-B. Anabaena sp., Nostoc commune, Scytonema
sp. and Lyngbya sp. were isolated from rice-fields in India and
screened for the presence of photoprotective compounds. Spectroscopic
and biochemical analyses revealed the presence of shinorine, a
bisubstituted MAA containing both glycine and 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. Polychromatic action spectra for the induction
of MAAs in Anabaena sp. and Nostoc commune show the
induction to be UV-B dependent peaking at 290 nm. Another
photoprotective compound, scytonemin, with an absorption maximum at
386 nm (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 and brownish compounds with
an absorption maximum at 315 nm were recorded only in Scytonema
sp. The spectral characteristics of these compounds neither matches
with MAAs nor with scytonemin and seems to be a novel compound acting
as an effective UV screens. In conclusion, a particular cyanobacterium
having photoprotective compounds may be a potent candidate as
biofertilizers for crop plants.
Key words: Action spectrum, Cyanobacteria, Mycosporine-like amino acids (MAAs), Rice-field, Scytonemin, Ultraviolet-B