Ultraviolet-B exposure induces photo-oxidative damage and subsequent repair strategies in a desert cyanobacterium Microcoleus vaginatus Gom.

Biological soil crusts are important in reversing desertification. Ultraviolet radiation, however, may be detrimental for the development of soil crusts. The cyanobacterium Microcoleus vaginatus can be a dominant species occurring in desert soil crusts all over the world. To investigate the physico-chemical consequences of ultraviolet-B radiation on M. vaginatus, eight parameters including the contents of chlorophyll a, reactive oxygen species, malondialdehyde and proline, as well as the activities of photosynthesis, superoxide dismutase (EC 1.15.1.1), peroxiclase (EC 1.11.1.7) and catalase (EC 1.11.1.6) were determined. As shown by the results of determinations, ultraviolet-B radiation caused decreases both in contents of chlorophyll a and in ratios of variable fluorescence over maximum fluorescence that indicate the growth and photosynthesis of M. vaginatus, besides, increases both in levels of reactive oxygen species and in contents of malondialdehyde and proline, while intensified activities of superoxide dismutase, peroxiclase and catalase reflecting the abilities of enzymatic preventive substances to oxidative stress of the treated cells. Therefore, ultraviolet-B radiation affects the growth of M. vaginatus and leads to oxidative stress in cells. Under ultraviolet-B radiation, the treated cells can improve their antioxidant abilities to alleviate oxidative injury. The change trends of reactive oxygen species, superoxide dismutase, peroxiclase and catalase are synchronous. These results suggest that a balance between the antioxidant system and the reactive oxygen species content may be one part of a complex stress response pathway in which multiple environmental factors including ultraviolet-B radiation affect the Survival of M. vaginatus. (C) 2009 Elsevier Masson SAS. All rights reserved.

Light histories influence the impacts of solar ultraviolet radiation on photosynthesis and growth in a marine diatom, Skeletonema costatum

Phytoplanktonic species acclimated to high light are known to show less photoinhibition. However, little has been documented on how cells grown under indoor conditions for decades without exposure to UV radiation (UVR, 280-400 nm) would respond differently to solar UVR compared to those in situ grown under natural solar radiation. Here, we have shown the comparative photosynthetic and growth responses to solar UVR in an indoor-(IS) and a naturally grown (WS) Skeletonema costatum type. In short-term experiment (<1 day), phi(PSII) and photosynthetic carbon fixation rate were more inhibited by UVR in the IS than in the WS cells. The rate of UVR-induced damages of PSII was faster and their repair was significantly slower in IS than in WS. Even under changing solar radiation simulated for vertical mixing, solar UVR-induced higher inhibition of photosynthetic rate in IS than in WS cells. During long-term (10 days) exposures to solar radiation, the specific growth rate was much lower in IS than WS at the beginning, then increased 3 days later to reach an equivalent level as that of WS. UVR-induced inhibition of photosynthetic carbon fixation in the IS was identical with that of WS at the end of the long-term exposure. The photosynthetic acclimation was not accompanied with increased contents of UV-absorbing compounds, indicating that repair processes for UVR-induced damages must have been accelerated or upgraded. (C) 2008 Elsevier B.V. All rights reserved.

Combined effects of ultraviolet radiation and temperature on morphology, photosynthesis, and DNA of Arthrospira (Spirulina) platensis (Cyanophyta)

Natural levels of solar UVR were shown to break and alter the spiral structure of Arthrospira (Spirulina) platensis (Nordst.) Gomont during winter. However, this phenomenon was not observed during summer at temperatures of similar to 30 degrees C. Since little has been documented on the interactive effects of solar UV radiation (UVR; 280-400 nm) and temperature on cyanobacteria, the morphology, photosynthesis, and DNA damage of A. platensis were examined using two radiation treatments (PAR [400-700 nm] and PAB [PAR + UV-A + UV-B: 280-700]), three temperatures (15, 22, and 30 degrees C), and three biomass concentrations (100, 160, and 240 mg dwt [dry weight] . L-1). UVR caused a breakage of the spiral structure at 15 degrees C and 22 degrees C, but not at 30 degrees C. High PAR levels also induced a significant breakage at 15 degrees C and 22 degrees C, but only at low biomass densities, and to lesser extent when compared with the PAB treatment. A. platensis was able to alter its spiral structure by increasing helix tightness at the highest temperature tested. The photochemical efficiency was depressed to undetectable levels at 15 degrees C but was relatively high at 30 degrees C even under the treatment with UVR in 8 h. At 30 degrees C, UVR led to 93%-97% less DNA damage when compared with 15 degrees C after 8 h of exposure. UV-absorbing compounds were determined as negligible at all light and temperature combinations. The possible mechanisms for the temperature-dependent effects of UVR on this organism are discussed in this paper.

Effects of solar ultraviolet radiation on photosynthesis of the marine red tide alga Heterosigma akashiwo (Raphidophyceae)

In order to assess the short- and long-term impacts of UV radiation (LTVR, 280-400 nm) on the red tide alga, Heterosigma akashiwo, we exposed the cells to three different solar radiation treatments (PAB: 280-700 rim, PA: 320-700 nm, R 400-700 nm) under both solar and artificial radiation. A significant decrease in the effective quantum yield () during high irradiance periods (i.e., local noon) was observed, but the cells partially recovered during the evening hours. Exposure to high irradiances for 15, 30, and 60 min under a solar simulator followed by the recovery (8 h) under dark, 9 and 100 mu mol photons m(-2) s(-1) of PAR, highlighted the importance of the irradiance level during the recovery period. Regardless the radiation treatments, the highest recovery (both in rate and total Y) was found at a PAR irradiance of 9 mu mol photons m(-2) s(-1), while the lowest was observed at 100 mu mol photons m(-2) s(-1). In all experiments, PAR was responsible for most of the observed inhibition; nevertheless, the cells exposed only to PAR had the highest recovery in any condition, as compared to the other radiation treatments. In long-term experiments (10 days) using semi-continuous cultures, there was a significant increase of UV-absorbing compounds (UVabc) per cell from 1.2 to > 4 x 10(-6) mu g UVabc cell(-1) during the first 3-5 days of exposure to solar radiation. The highest concentration of UVabc was found in samples exposed in the PAB as compared to PA and P treatments. Growth rates (mu) mimic the behavior of UV-absorbing compounds, and during the first 5 days mu increased from < 0.2 to ca. 0.8, and stayed relatively constant at this value during the rest of the experiment. The inhibition of the Y decreased with increasing acclimation of cells. All our data indicates that H. akashiwo is a sensitive species, but was able acclimate relatively fast (3-5 days) synthesizing UV-absorbing compounds and thus reducing any impact either on photosystem 11 or on growth. (c) 2006 Published by Elsevier B.V.

Effects of solar ultraviolet radiation on the photochemical efficiency, photosynthetic pigments and biomass production of Spirulina platensis

Effects of solar ultraviolet radiation (UVR) on Spirulina platensis were studied by investigating its photochemical efficiency, photosynthetic pigments and biomass production while exposed to full spectrum solar radiation or depleted of UVR for understanding how and to what extent UVR influences its photosynthetic physiology and production. It was found that UVR brought about an extra inhibition of photochemical efficiency by 26%-30%. The greatest inhibition of photochemical efficiency in S. platensis was observed at noontime, and then recovered to some extent in late afternoon no matter which treatment they were exposed to. The contents of chlorophyll a, phycocyanin and carotenoids increased during initial stage of the exposure, but decreased with elongated exposure. UVR decreased the biomass yield by about 6%. It indicated that filtering out UVR of solar radiation would raise the productivity of S. platensis, which is an important factor that should be considered in the production.

Hole concentration test of p-type GaN by analyzing the spectral response of p-n(+) structure GaN ultraviolet photodetector

A new method to test the hole concentration of p-type GaN is proposed, which is carried out by analyzing the spectral response of p-n(+) structure GaN ultraviolet photodetector. It is shown that the spectral response of the photodetector changes considerably with reversed bias. It is found that the difference between photodetector's quantum efficiency at two wavelengths, i.e. 250 and 361 nm, varies remarkably with increasing reversed bias. According to the simulation calculation, the most characteristic change occurs at a reversed voltage under which the p-GaN layer starts to be completely depleted. Based on this effect the carrier concentration of p-GaN can be derived.

Role of Ga vacancies in enhancing the leakage current of GaN Schottky barrier ultraviolet photodetectors

The leakage current of GaN Schottky barrier ultraviolet photodetectors is investigated. It is found that the photodetectors adopting undoped GaN instead of lightly Si-doped GaN as an active layer show a much lower leakage current even when they have a higher dislocation density. It is also found that the density of Ga vacancies in undoped GaN is much lower than in Si-doped GaN. The Ga vacancies may enhance tunneling and reduce effective Schottky barrier height, leading to an increase of leakage current. It suggests that when undoped GaN is used as the active layer, it is necessary to reduce the leakage current of GaN Schottky barrier ultraviolet photodetector.

Characterization of ZnMgO hexagonal-nanotowers/films on m-plane sapphire synthesized by metal organic chemical vapour deposition

ZnMgO hexagonal-nanotowers/films grown on m-plane sapphire substrates were successfully synthesized using a vertical low-pressure metal organic chemical vapour deposition system. The structural and optical properties of the as-obtained products were characterized using various techniques. They were grown along the non-polar [1 0 (1) over bar 0] direction and possessed wurtzite structure. The ZnMgO hexagonal-nanotowers were about 200 nm in diameter at the bottom and 120 nm in length. Photoluminescence and Raman spectra show that the products have good crystal quality with few oxygen vacancies. With Mg incorporation, multiple-phonon scattering becomes weak and broad, and the intensities of all observed vibrational modes decrease. The ultraviolet near band edge emission shows a clear blueshift (as much as 100 meV) and broadening compared with that of pure ZnO products.

A GaN photodetector integrated structure for wavelength characterization of ultraviolet light

An interesting GaN photodetector structure, which can be used for characterizing the wavelength of incident ultraviolet light, is proposed. It is composed of two back-to-back integrated diodes, i.e. p-n and p-i-n GaN ultraviolet photodiodes with different spectral response. The wavelength of monochromatic ultraviolet light could be identified by measuring the photocurrent ratio value through a simple electronic circuit.

A simple route of morphology control and structural and optical properties of ZnO grown by metal-organic chemical vapour deposition

Employing the metal-organic chemical vapour deposition (MOCVD) technique, we prepare ZnO samples with different morphologies from the film to nanorods through conveniently changing the bubbled diethylzinc flux (BDF) and the carrier gas flux of oxygen (OCGF). The scanning electron microscope images indicate that small BDF and OCGF induce two-dimensional growth while the large ones avail quasi-one-dimensional growth. X-ray diffraction (XRD) and Raman scattering analyses show that all of the morphology-dependent ZnO samples are of high crystal quality with a c-axis orientation. From the precise shifts of the 2 theta. locations of ZnO (002) face in the XRD patterns and the E-2(high) locations in the Raman spectra, we deduce that the compressive stress forms in the ZnO samples and is strengthened with the increasing BDF and OCGF. Photoluminescence spectroscopy results show all the samples have a sharp ultraviolet luminescent band without any defects-related emission. Upon the experiments a possible growth mechanism is proposed.

Effect of p-GaN layer thickness on the performance of p-i-n structure GaN ultraviolet photodetectors

We investigated the influence of thickness of p-GaN layer on the performance of p-i-n structure GaN ultraviolet photodetector. Through the simulation calculation, it was found that both the quantum efficiency and dark current of device decrease when employing thicker p-GaN layer, while both the quantum efficiency and dark current increase with decreasing thickness of p-GaN layer. It is suggested that the Schottky contact junction between the metal and p-GaN may be responsible for the incompatible effect. We has to make a suitable choice of the thickness of p-GaN in the device design according to the application requirement.

High-power quantum-dot superluminescent LED with broadband drive current insensitive emission spectra using a tapered active region

High-power and broadband quantum-dot (QD) superluminescent light-emitting diodes are realized by using a combination of self-assembled QDs with a high density, large inhomogeneous broadening, a tapered angled pump region, and etched V groove structure. This broad-area device exhibits greater than 70-nm 3-dB bandwidth and drive current insensitive emission spectra with 100-mW output power under continuous-wave operation. For pulsed operation, greater than 200-mW output power is obtained.

Development of solar-blind AlGaN 128x128 ultraviolet focal plane arrays

This paper reports the development of solar-blind aluminum gallium nitride (AlGaN) 128x128 UV Focal Plane Arrays (FPAs). The back-illuminated hybrid FPA architecture consists of an 128x128 back-illuminated AlGaN PIN detector array that is bump-mounted to a matching 128x128 silicon CMOS readout integrated circuit (ROIC) chip. The 128x128 p-i-n photodiode arrays with cuton and cutoff wavelengths of 233 and 258 nm, with a sharp reduction in response to UVB (280-320 nm) light. Several examples of solar-blind images are provided. This solar-blind band FPA has much better application prospect.

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector is proposed. In comparision with conventional i-CaN/n(+)-GaN structure, an additional thin p-GaN cap layer is introduced on the i-GaN(n(-)-GaN) in the new structure. The simulation results showed that the additional layer makes the dark current to decrease in the photodetector due to the increase of the Schottky barrier height. The effects of thickness and carrier concentration of p-GaN layer on the dark current of the photodetector were also studied. It is suggested that the dark current of the new structure device could be better reduced by employing p-GaN with higher carrier concentration as the cap layer.