Effects of solar UV radiation on morphology and photosynthesis of filamentous cyanobacterium Arthrospira platensis

To study the impact of solar UV radiation (UVR) (280 to 400 nm) on the filamentous cyanobacterium Arthrospira (Spirulina) platensis, we examined the morphological changes and photosynthetic performance using an indoor-grown strain (which had not been exposed to sunlight for decades) and an outdoor-grown strain (which had been grown under sunlight for decades) while they were cultured with three solar radiation treatments: PAB (photosynthetically active radiation [PAR] plus UVR; 280 to 700 nm), PA (PAR plus UV-A; 320 to 700 nm), and P (PAR only; 400 to 700 nm). Solar UVR broke the spiral filaments of A. platensis exposed to full solar radiation in short-term low-cell-density cultures. This breakage was observed after 2 h for the indoor strain but after 4 to 6 h for the outdoor strain. Filament breakage also occurred in the cultures exposed to PAR alone; however, the extent of breakage was less than that observed for filaments exposed to full solar radiation. The spiral filaments broke and compressed when high-cell-density cultures were exposed to full solar radiation during long-term experiments. When UV-B was screened off, the filaments initially broke, but they elongated and became loosely arranged later (i.e., there were fewer spirals per unit of filament length). When UVR was filtered out, the spiral structure hardly broke or became looser. Photosynthetic 0, evolution in the presence of UVR was significantly suppressed in the indoor strain compared to the outdoor strain. UVR-induced inhibition increased with exposure time, and it was significantly lower in the outdoor strain. The concentration of UV-absorbing compounds was low in both strains, and there was no significant change in the amount regardless of the radiation treatment, suggesting that these compounds were not effectively used as protection against solar UVR. Self-shading, on the other hand, produced by compression of the spirals over adaptive time scales, seems to play an important role in protecting this species against deleterious UVR. Our findings suggest that the increase in UV-B irradiance due to ozone depletion not only might affect photosynthesis but also might alter the morphological development of filamentous cyanobacteria during acclimation or over adaptive time scales.

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.

Solar UV radiation drives CO2 fixation in marine phytoplankton: A double-edged sword

Photosynthesis by phytoplankton cells in aquatic environments contributes to more than 40% of the global primary production (Behrenfeld et al., 2006). Within the euphotic zone (down to 1% of surface photosynthetically active radiation [PAR]), cells are exposed not only to PAR (400-700 nm) but also to UV radiation (UVR; 280-400 nm) that can penetrate to considerable depths (Hargreaves, 2003). In contrast to PAR, which is energizing to photosynthesis, UVR is usually regarded as a stressor (Hader, 2003) and suggested to affect CO2-concentrating mechanisms in phytoplankton (Beardall et al., 2002). Solar UVR is known to reduce photosynthetic rates (Steemann Nielsen, 1964; Helbling et al., 2003), and damage cellular components such as D1 proteins (Sass et al., 1997) and DNA molecules (Buma et al., 2003). It can also decrease the growth (Villafane et al., 2003) and alter the rate of nutrient uptake (Fauchot et al., 2000) and the fatty acid composition (Goes et al., 1994) of phytoplankton. Recently, it has been found that natural levels of UVR can alter the morphology of the cyanobacterium Arthrospira (Spirulina) platensis (Wu et al., 2005b). On the other hand, positive effects of UVR, especially of UV- A (315-400 nm), have also been reported. UV- A enhances carbon fixation of phytoplankton under reduced (Nilawati et al., 1997; Barbieri et al., 2002) or fast-fluctuating (Helbling et al., 2003) solar irradiance and allows photorepair of UV- B-induced DNA damage (Buma et al., 2003). Furthermore, the presence of UV-A resulted in higher biomass production of A. platensis as compared to that under PAR alone (Wu et al., 2005a). Energy of UVR absorbed by the diatom Pseudo-nitzschia multiseries was found to cause fluorescence (Orellana et al., 2004). In addition, fluorescent pigments in corals and their algal symbiont are known to absorb UVR and play positive roles for the symbiotic photosynthesis and photoprotection (Schlichter et al., 1986; Salih et al., 2000). However, despite the positive effects that solar UVR may have on aquatic photosynthetic organisms, there is no direct evidence to what extent and howUVR per se is utilized by phytoplankton. In addition, estimations of aquatic biological production have been carried out in incubations considering only PAR (i. e. using UV-opaque vials made of glass or polycarbonate; Donk et al., 2001) without UVR being considered (Hein and Sand-Jensen, 1997; Schippers and Lurling, 2004). Here, we have found that UVR can act as an additional source of energy for photosynthesis in tropical marine phytoplankton, though it occasionally causes photoinhibition at high PAR levels. While UVR is usually thought of as damaging, our results indicate that UVR can enhance primary production of phytoplankton. Therefore, oceanic carbon fixation estimates may be underestimated by a large percentage if UVR is not taken into account.

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.

Calculation of the hourly and daily radiation incident on three step tracking planes

Mathematical formulas for estimating the hourly and daily radiation incident on planes of azimuth three step tracking and hour angle three step tracking have been derived in this paper. Based on the hourly solar radiation data of an average day in each month at Er-Lian-Hao-Te city, the hourly and monthly radiation received by planes of these two kinds of tracking have been calculated. The results show that in this district, one axis azimuth three step tracking and hour angle three step tracking could, respectively, obtain 66.5% and 63.3% higher radiation than that on the horizontal surface all year. Moreover, a two axis azimuth three step tracking plane could receive 72% more radiation than the horizontal surface. (C) 2002 Elsevier Science Ltd. All rights reserved.

Analysis of leakage current in GaAs micro-solar cell arrays

The output characteristics of micro-solar cell arrays are analyzed on the basis of a modified model in which the shunt resistance between cell lines results in current leakage. The modification mainly consists of adding a shunt resistor network to the traditional model. The obtained results agree well with the reported experimental results. The calculation results demonstrate that leakage current in substrate affects seriously the performance of GaAs micro- solar cell arrays. The performance of arrays can be improved by reducing the number of cells per line. In addition, at a certain level of integration, an appropriate space occupancy rate of the single cell is recommended for ensuring high open circuit voltages, and it is more appropriate to set the rates at 80%-90% through the calculation.

Radiation-use efficiency and the harvest index of winter wheat at different nitrogen levels and their relationships to canopy spectral reflectance

Radiation-use efficiency (RUE, g/MJ) and the harvest index (HI, unitless) are two helpful characteristics in interpreting crop response to environmental and climatic changes. They are also increasingly important for accurate crop yield simulation, but they are affected by various environmental factors. In this study, the RUE and HI of winter wheat and their relationships to canopy spectral reflectance were investigated based on the massive field measurements of five nitrogen (N) treatments. Crop production can be separated into light interception and RUE. The results indicated that during a long period of slow growth from emergence to regreening, the effect of N on crop production mainly showed up in an increased light interception by the canopy. During the period of rapid growth from regreening to maturity, it was present in both light interception and RUE. The temporal variations of RUEAPAR (aboveground biomass produced per unit of photosynthetically active radiation absorbed by the canopy) during the period from regreening to maturity had different patterns corresponding to the N deficiency, N adequacy and N-excess conditions. Moreover, significant relationships were found between the RUEAPAR and the accumulative normalised difference vegetation index (NDVI) in the integrated season (R-2 = 0.68), between the HI and the accumulative NDVI after anthesis (R-2 = 0.89), and between the RUEgrain (ratio of grain yield to the total amount of photosynthetically active radiation absorbed by the canopy) and the accumulative NDVI of the whole season (R-2 = 0.89) and that after anthesis (R-2 = 0.94). It suggested that canopy spectral reflectance has the potential to reveal the spatial information of the RUEAPAR, HI and RUEgrain. It is hoped that this information will be useful in improving the accuracy of crop yield simulation in large areas.

Diurnal and seasonal variations of UV radiation on the northern edge of the Qinghai-Tibetan Plateau

We monitored UVA, UVB, and solar radiation from August 2001 to 2003 on the northern Qinghai-Tibetan Plateau to characterize the diurnal and seasonal variations of UV radiation on the world's highest plateau. Daily UVB radiation and the ratio of UVB to total solar radiation increased significantly when the atmospheric ozone concentration decreased as estimated by the total ozone mapping spectrometer (TOMS), as well as when cloud coverage decreased. The UVB/UVA ratio also showed a significant increase when the TOMS ozone concentration decreased in the morning. The seasonal variation pattern of UVB, however, was closely correlated with solar elevation but was little affected by the seasonal pattern of the atmospheric ozone amount. Compared to observations from the central plateau, the magnitude of the UVB increase attributed to ozone depletion was smaller at the northern edge. The study suggests that the temporal variation of ground UV radiation is determined by both solar elevation and the ozone amount, but the spatial difference on the plateau is likely to be ascribed mainly to the spatial variation of the ozone amount. (c) 2007 Published by Elsevier B.V.

Depression of net ecosystem CO2 exchange in semi-arid Leymus chinensis steppe and alpine shrub

Uptake and release of carbon in grassland ecosystems is very critical to the global carbon balance and carbon storage. In this study, the dynamics of net ecosystem CO2 exchange (FNEE) of two grassland ecosystems were observed continuously using the eddy covariance technique during the growing season of 2003. One is the alpine shrub on the Tibet Plateau, and the other is the sem-arid Leymus chinensis steppe in Inner Mongolia of China. It was found that the FNEE of both ecosystems was significantly depressed under high solar radiation. Comprehensive analysis indicates that the depression of FNEE in the L. chinensis steppe was the results of decreased plant photosynthesis and increased ecosystem respiration (R-eco) under high temperature. Soil water stress in addition to the high atmospheric demand under the strong radiation was the primary factor limiting the stomatal conductance. In contrast, the depression of FNEE in the alpine shrub was closely related to the effects of temperature on both photosynthesis and ecosystem respiration, coupled with the reduction of plant photosynthesis due to partial stomatal closure under high temperature at mid-day. The R,c of the alpine shrub was sensitive to soil temperature during high turbulence (u* > 0.2 m s(-1)) but its FNEE decreased markedly when the temperature was higher than the optimal value of about 12 degrees C. Such low optimal temperature contrasted the optimal value (about 20 degrees C) for the steppe, and was likely due to the acclimation of most alpine plants to the long-term low temperature on the Tibet Plateau. We inferred that water stress was the primary factor causing depression of the FNEE in the semi-arid steppe ecosystem, while relative high temperature under strong solar radiation was the main reason for the decrease of FNEE in the alpine shrub. This study implies that different grassland ecosystems may respond differently to climate change in the future. (c) 2006 Elsevier B.V All rights reserved.

Leaf orientation, incident sunlight, and photosynthesis in the alpine species Suassurea superba and Gentiana straminea on the Qinghai-Tibet Plateau

The extremely high level of solar radiation on the Qinghai-Tibet Plateau may induce photoinhibition and thus limit leaf carbon gain. To assess the effect of high light, we examined gas exchange and chlorophyll fluorescence for two species differing in light interception: the prostrate Saussurea superba and the erect-leaved Gentiana straminea. In controlled conditions with favorable water and temperature, neither species showed apparent photoinhibition in gas exchange measurements. In natural environment, however, their photosynthetic rate decreased remarkably at high light. Photosynthesis depression was aggravated under high leaf temperature or soil water stress. Relative stomatal limitation was much higher in S. superba than in G. straminea and it remarkably increased in the later species at midday when soil was dry. F-v/F-m as an indicator for photoinhibition was generally higher in S. superba than in the other species. F-v/F-m decreased significantly under high light at midday in both species, even when soil moisture was high. F-0 linearly elevated with the increment of leaf temperature in G. straminea, but remained almost constant in S. superba. Electron transport rate (ETR) increased with photosynthetically active photon flux density (PPFD) in S. superba, but declined when PPFD was high than about 1000 mumol m(-2) s(-1) in G. straminea. Compared to favorable environment, the estimated daily leaf carbon gain at PPFD above 800 mumol m(-2) s(-1) was reduced by 32% in S. superba and by 17% in G. straminea when soil was moist, and by 43% and 53%, respectively, when soil was dry. Our results suggest that the high radiation induces photoinhibition and significantly limits photosynthetic carbon gain, and the limitation may further increase at higher temperature and in dry soil.