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Ultraviolet radiation is a well known damaging factor of plant photosynthesis. Here we studied the mechanism of damage induced by the UV-B and UV-A spectral regions to the light energy converting Photosystem II (PSII) complex, which is the origin of electron flow for the whole photosynthetic process. Our results show that the primary UV damage occurs at the catalytic Mn cluster of water oxidation, which is most likely sensitized by the UV absorption of Mn(III) and Mn(IV) ions ligated by organic residues. The presence of visible light enhances the photodamage of PSII, but has no synergistic interaction with UV radiation. UV-induced damage of PSII can be repaired via synthesis of the D1 and D2 reaction center protein subunits. This process is facilitated by low intensity visible light, which thereby can protect against UV-induced damage. However, the photodamage induced by visible light at high intensity (above 1000 µEms) cancels the protective effect. The protein repair of PSII is also retarded by the lack of DNA repair as shown in a photolyase deficient cyanobacterial mutant.

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-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 sp., sp. and 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 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 sp. and 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 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 sp. In conclusion, cyanobacteria having photoprotective mechanisms may be potent candidates as biofertilizers for crop plants.

The effects of UV-B radiation on photosynthesis and photosynthetic productivity of higher plants are reviewed. When plants were exposed to large UV-B doses in a glasshouse in order to study the mechanistic basis of UV-B-induced inhibition of photosynthesis, direct effects on stomata and on Calvin cycle enzymes (i.e. large decreases in ribulose-1,5-bisphosphate carboxylase/oxygenase and in sedoheptulose-1,7-bisphosphate) without any significant effect on photosystem II were observed. When plants were continuously grown and developed under UV-B in a glasshouse, exposure to UV-B only decreased adaxial stomatal conductance (g) of the leaves and consequently increased the stomatal limitation of CO uptake. Furthermore, field studies using realistic UV-B levels (i.e. a predicted 30% increase in this radiation) demonstrate a lack of UV-B effects on photosynthesis or on plant biomass. It is concluded that any predicted future increase in UV-B irradiation is unlikely to have a significant impact on the photosynthetic characteristics and productivity of higher plants.

This paper reviews some of the results described in the literature on the effect of UV-B radiation on ciliated protozoa, concentrating in particular on the changes induced in motility and photomotility, which are both important in determining the capability of these organisms to survive in their environment. It will be shortly described what ciliates are and why they are an important component of ecological systems. A summary will follow of the early works, where the effects of UV radiation on ciliates were investigated. Finally, it will be described in some more detail the results of studies on a marine ciliate, , and two fresh-water ciliates, and .

To assess the risks to human health and ecosystems from an enhanced UV-B radiation, accurate and reliable UV monitoring systems are required that weights the spectral irradiance according to the biological responses under consideration. Biological dosimetry meets these requirements by directly weighting the incident UV components of sunlight in relation to the biological effectiveness of the different wavelengths and to potential interactions between them. Bacteria, viruses and biomolecules have been developed as biological dosimeters. Their responses to environmental UV radiation, indicated as inactivation, mutagenesis or photochemical injury, reflect the UV sensitivity of DNA.

For assessing the applicability of a biological UV dosimeter, photobiological as well as radiometric criteria have to be met. If radiometrically properly characterized, there is a broad scope of applications of biological UV dosimeters, which include the determination of (i) long-term trends of biologically effective solar radiation; (ii) the contribution of the UV-B range to the BED; (iii) the sensitivity of the biologically effective solar irradiance to ozone; (iii) vertical attenuation coefficient of biologically effective solar irradiance in natural waters; (iv) UV tolerance and protection mechanisms; (v) the individual UV exposure of specific professional groups.

The use of BSWF in assessing the ozone reduction issue is clearly highly important. Even small differences among BSWF can have large consequences when employed in various predictions of RAF values, latitudinal gradients and in experimental design involving lamp systems. It is unlikely that the same BSWF will be appropriate for all responses to UV radiation. Thus, realistic testing of these BSWF under polychromatic radiation, especially under field conditions, is critical.

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-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 sp., sp. and 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 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 sp. and 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 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 sp. In conclusion, cyanobacteria having photoprotective mechanisms may be potent candidates as biofertilizers for crop plants.

A network of three channel dosimeters has been installed in Europe and other continents to continuously and automatically monitor solar radiation in the UV-B (280 – 315 nm), UV-A (315 – 400 nm) and PAR (photosynthetic active radiation, 400 – 700 nm) wavelength ranges to follow long-term and short-term changes in the light climate. The instruments are housed in rugged cases to withstand extreme environments from the polar circle to the tropics. The entrance optic is based on an integrating sphere and the wavelength selection is done by appropriate filter and photodiode combinations. Software packages have been developed to poll the data, display them graphically and store them. Quality control is warranted by careful and frequent calibration of the instruments as well as national and international intercalibrations. The data are sent to a central server in Pisa from where they can be downloaded on the Internet free of charge. The network has been in operation for the last five years and has constantly been growing in numbers. The data are being used to extract important information on changing light climate conditions and the development of stratospheric ozone.

Alterations of the structural genes coding for DNA repair enzymes can have severe effects on the organisms, whether a simple bacterium, or a higher plant or person. Modern biochemical and molecular biological approaches allow determination of the molecular origin of radiation sensitivity, including those resulting from deficiencies in DNA repair.

Ultraviolet radiation is a well known damaging factor of plant photosynthesis. Here we studied the mechanism of damage induced by the UV-B and UV-A spectral regions to the light energy converting Photosystem II (PSII) complex, which is the origin of electron flow for the whole photosynthetic process. Our results show that the primary UV damage occurs at the catalytic Mn cluster of water oxidation, which is most likely sensitized by the UV absorption of Mn(III) and Mn(IV) ions ligated by organic residues. The presence of visible light enhances the photodamage of PSII, but has no synergistic interaction with UV radiation. UV-induced damage of PSII can be repaired via synthesis of the D1 and D2 reaction center protein subunits. This process is facilitated by low intensity visible light, which thereby can protect against UV-induced damage. However, the photodamage induced by visible light at high intensity (above 1000 µEms) cancels the protective effect. The protein repair of PSII is also retarded by the lack of DNA repair as shown in a photolyase deficient cyanobacterial mutant.