Environmental Engineering Reference
In-Depth Information
photolyase in the presence of light (photoreactivation) and nucleotide excision repair
(NER) are the two major pathways to remove UV-induced DNA lesions from the
genome, thereby preventing mutagenesis and cell death
17-18
. This chapter deals with the
effects of UV-B radiation on rice-field cyanobacteria and the defence mechanisms
operative against the negative effects of UV-B in these organisms.
3. Impacts of UV-B radiation on rice-field cyanobacteria
Cyanobacteria are phylogenetically a primitive group of gram-negative
prokaryotes possessing higher plant-type oxygenic photosynthesis. Fossil evidence dates
their appearance to the Precambrian era (between 2.8 and 3.5 x 10
9
years ago). At that
time they might have been the main primary producers of organic matter and the first
organisms to release oxygen into the then oxygen-free atmosphere. The advent of
oxygenic photosynthesis on Earth probably have increased global biological
productivity by a factor of 100 - 1000 and thereby profoundly affected both
geochemical and biological evolution
19
. Thus, cyanobacteria were responsible for a
major global evolutionary transformation leading to the development of aerobic
metabolism and the subsequent rise of higher plant and animal forms
20-21
. Cyanobacteria
are thought to have survived from a wide spectrum of environmental stresses such as
extreme temperatures, drought, salinity, nitrogen starvation, photooxidation,
anaerobiosis, osmotic and ultraviolet radiation stress
10,22
.
Cyanobacteria have a cosmopolitan distribution ranging from hot springs to
Arctic and Antarctic regions showing high variability in adapting to diverse
environmental factors. They not only colonize oceans, lakes, rivers and various soils
(especially rice paddy fields where cyanobacteria contribute significantly to fertility as
natural biofertilizer), but, also make symbiotic associations with several organisms
10,23
.
Members of cyanobacteria also possess a central position in the nutrient cycling largely
due to their inherent capacity to fix atmospheric N
2
with the help of the enzyme
nitrogenase, directly into ammonium (NH
4
+
), a form through which nitrogen enters into
the food chain
10
. Free-living as well as symbiotic cyanobacteria, particularly
Azolla-
Anabaena
symbiosis, have been recognized as one of the most promising biofertilizer
systems for wetland soils
10,24-25
. In nature the process of nitrogen fixation is of utmost
importance because it counterbalances losses of nitrogen from the environment by
denitrification
26
.
Considering the vital role of cyanobacteria as a biofertilizer in rice and other crop
production, the fluence rate of UV-B radiation impinging on their natural habitats, is of
major concern since UV radiation has been reported not only to impair motility and
photoorientation
27
but also to affect growth, survival, pigmentation, heterocyst
differentiation, enzymes of nitrogen metabolism and total protein profiles
10,28-31
. In the
following we discuss the effects of UV-B on some of the important physiological and
biochemical processes in cyanobacteria.
Effects on growth and survival
The growth and survival of several cyanobacteria have been reported to be
severely affected following UV irradiation for different durations. Growth and survival
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