Environmental Engineering Reference
In-Depth Information
conclusion, cyanobacteria having photoprotective mechanisms may be potent candidates
as biofertilizers for crop plants.
2. Introduction
The stratospheric ozone layer shields the Earth from the biologically most
hazardous short-wavelength solar radiation. There is mounting evidence that the solar
flux of UV-B radiation has increased at the Earth's surface due to the depletion of the
ozone layer by anthropogenically released atmospheric pollutants, such as
chlorofluorocarbons (CFCs), chlorocarbons (CCs) and organo-bromides (OBs) 1-2 .
Ozone depletion has been reported in the Antarctic, where ozone levels commonly have
declined by more than 70 % in late winter and early spring during the last few decades,
as well as in the Arctic and subarctic regions 3 . This decline in ozone level is commonly
attributed to a unique combination of extreme cold temperatures and stratospheric
circulation (the polar vortex) which results in conditions that are favourable for the
CFC-ozone reactions. Polar stratospheric clouds play important roles in the formation of
the springtime Antarctic “ozone hole” by activating chlorine and denitrifying the
stratosphere. Recent TOMS (Total Ozone Mapping Spectrometer) data indicate an
Antarctic 'ozone hole' that is three times larger than the entire land mass of the United
States. The 'hole' had expanded to a record size of approximately 28.3 million square
kilometres in the year 2000 4 . Moreover, ozone depletion has been predicted to continue
throughout this century 5-6 . Recent reports indicate that widespread severe denitrification
could enhance future Arctic ozone loss by up to 30 % 6 .
Light is one of the most important factors determining the growth of
photosynthetic organisms in their natural habitats. As biologically effective doses of
UV-B radiation can penetrate deep into ecologically significant depths in natural waters,
the observed increases in surface UV-B radiation may adversely affect the productivity
of aquatic organisms 7 . UV-B has the potential to cause wide ranging effects, including
alteration in the structure of proteins, DNA and other biologically relevant molecules,
chronic depression of key physiological processes and acute physiological stress leading
to either reduction in growth and cell division rates or death of the organism. Studies
have shown a wide variation in tolerance to UV-B among species and taxonomic
groups 8-10 .
Although almost all aquatic organisms including cyanobacteria are more or less
susceptible to UV-B, they are not defenceless. As a first line of defence, organisms can
limit the damage of DNA and other chromophoric molecules by using photoprotective
mechanisms 11-12 . Photoprotective compounds such as scytonemin and mycosporine-like
amino acids (MAAs) play an important role in screening UV-B radiation in
cyanobacteria 11-13 . An additional defence against UV-B radiation includes the induction
of carotenoids 14 and the production of enzymes such as superoxide dismutases, catalases
and peroxidases and scavengers such as vitamins C, B and E or cysteine and glutathione
which can quench or scavenge UV-induced excited states and reactive oxygen
species 15-16 . Certain organisms show diurnal rhythms and thereby protect themselves
during the period of high UV-B irradiances. In the case that photoprotective
mechanisms are insufficient to prevent damage, organisms have developed certain
repair mechanisms for reversing UV-B-induced damage. DNA repair by the enzyme
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