Biological weighting functions as a tool for evaluating two ways to measure UVB radiation inhibition on photosynthesis

To estimate the inhibitory effect of the changing UVB radiation (UVBR, 280-315nm) on earth's ecosystems, an understanding of its wavelength dependency is needed. The tool used for these estimations is the biological weighting function (BWF), whereby the inhibition of different wavelengths is calculated. BWFs were determined for three algae species from different classes, Phaeodactylum tricornutum (Bacillariophyceae), Dunaliella tertiolecta (Chlorophyceae) and Rhodomonas sp. (Cryptophyceae), using polychromatic irradiation, where the UVBR spectra were varied with cut-off filters. For each alga, BWFs were determined for two photosynthetic parameters; the quantum yield measured as fluorescence from Photo System II in a pulse-amplitude-modulation (PAM) fluorometer, and the fixation of (14)C-labelled carbon dioxide. The BWFs were calculated with the Rundel method, using the radiation data between 270 and 360nm with 1nm resolution. The results show that the UVBR damages were generally higher when using the carbon fixation measurements than when measuring with the PAM technique. When using PAM, P. tricornutum in particular had a sensitivity intermediate between the sensitive Rhodomonas sp. and the more tolerant D. tertiolecta, but was as sensitive as, or even more sensitive, than Rhodomonas sp. when using carbon fixation. D. tertiolecta was shown to be less sensitive when using both techniques and the inhibition of its photosynthesis was almost as high when using PAM as when using carbon fixation. We concluded that, although the PAM technique has advantages such as being cleaner and easier to use, it is unable to substitute the carbon fixation measurements. Not only are the algae less sensitive when measured with PAM than they are when measured as carbon fixation, the relationship between the effects on the algae measured with the two techniques also differs. As fixation of carbon dioxide integrates a larger part of the photosynthetic machinery, it should be favoured as a measure of photosynthesis.     

Effects of nitrogen source and UV radiation on the growth, chlorophyll fluorescence and fatty acid composition of Phaeodactylum tricornutum and Chaetoceros muelleri (Bacillariophyceae)

Cultures of the marine diatoms Phaeodactylum tricornutum and Chaetoceros muelleri were grown in f/2 medium supplied with either nitrate (N-Nt), ammonium (N-Am) or urea (N-Ur) as the nitrogen (N) source at the same final N concentration (0.88 mM). Exponential growth phase cultures of the two diatoms were exposed to four different light regimes for 2 days: (UVAR) PAR (60 micromol quanta m-2 s-1) plus 8.22 W m-2 (unweighted) UVAR; (high UVBR) PAR (60 micromol quanta m-2 s-1) plus 1.04 W m-2 (unweighted) UVBR plus 13.73 W m-2 (unweighted) UVAR; (low UVBR) PAR (60 micromol quanta m-2 s-1) plus 0.19 W m-2 (unweighted) UVBR plus 2.76 W m-2 (unweighted) UVAR and (PAR) PAR (60 micromol quanta m-2 s-1) alone (control). No significant effects of N source on the growth rates of the two diatoms were detected. The maximum effective quantum yield of PSII, PhiPSIIe-max, and the initial slope of the light curve, alpha, of P. tricornutum and C. muelleri were all inhibited, whereas Ik was somewhat increased, as a consequence of 2 days of exposure to all the UVR treatments. Multiple factor ANOVA revealed that all the major fatty acids, both in P. tricornutum and C. muelleri, were influenced more strongly by N source than by UVR. The composition of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) in P. tricornutum and C. muelleri exhibited almost the same pattern of variation with N source and UVR. The maximum value of SFA was found in the N-Am treatment, that of MUFA in the N-Nt treatment and for PUFA in the N-Ur treatment irrespective of the UV radiation. On the other hand, the impact of UVR resulted in an increase of PUFA and a reduction of SFA both in P. tricornutum and C. muelleri under all N sources.     

Patterns of DNA damage and photoinhibition in temperate South-Atlantic picophytoplankton exposed to solar ultraviolet radiation.

Natural marine phytoplankton assemblages from Bahía Bustamante (Chubut, Argentina, 45 degrees S, 66.5 degrees W), mainly consisting of cells in the picoplankton size range (0.2-2 microm), were exposed to various UVBR (280-315 nm) and UVAR (315-400 nm) regimes in order to follow wavelength-dependent patterns of cyclobutane pyrimidine dimer (CPD) induction and repair. Simultaneously, UVR induced photosynthetic inhibition was studied in radiocarbon incorporation experiments. Biological weighting functions (BWFs) for photoinhibition and for CPD induction, the latter measured in bare calf thymus DNA, differed in the UVAR region: carbon incorporation was reduced markedly due to UVAR, whereas no measurable UVAR effect was found on CPD formation. In contrast, BWFs for inhibition of photosynthesis and CPD accumulation were fairly similar in the UVBR region, especially above 300 nm. Incubation of phytoplankton under full solar radiation caused rapid CPD accumulation over the day, giving maximum damage levels exceeding 500 CPD MB(-1) at the end of the afternoon. A clear daily pattern of CPD accumulation was found, in keeping with the DNA effective dose measured by a DNA dosimeter. In contrast, UVBR induced photosynthetic inhibition was not dose related and remained nearly constant during the day. Screening of UVBR or UVR did not cause significant CPD removal, indicating that photoreactivation either by PAR or UVAR was of minor importance in these organisms. High CPD levels were found in situ early in the morning, which remained unaffected notwithstanding treatments favoring photorepair. These results imply that a proportion of cells had been killed by UVBR exposure prior to the treatments. Our data suggest that the limited potential for photoreactivation in picophytoplankton assemblages from the southern Atlantic Ocean causes high CPD accumulation as a result of UVBR exposure.     

Delayed fluorescence as a measure of nutrient limitation in Dunaliella tertiolecta

The applicability of the delayed fluorescence (DF) for the purpose of distinguishing the cells growing in different nutrient conditions was researched on the marine unicellular algae Dunaliella tertiolecta Butcher (Chlorophyta). The DF intensity (DFI), as a measure of living algal biomass, was compared with other biomass measures--the cell concentration, chlorophyll a and fluorescence. The photosynthetic activity index (PhAI), a non-dimensional physiological index of photosynthesis calculated from a combination of DFI and F(0) was introduced. The nitrogen deprivation was indicated by more than 50% drop of PhAI. DF decay kinetics was measured with two different illuminations (<600 nm and >650 nm). The measured curves were divided and the resulting peak utilized for the differentiation among nutrient conditions. DF decay kinetics of D. tertiolecta differed among the cells growing in various nutrient conditions, indicating changes in the photosynthesis physiology.     

Spectral Modeling of UV Inhibition of In Situ Antarctic Primary Production Using A Field-Derived Biological Weighting Function

Abstract— Our major aim is to illustrate an approach for hindcasting or forecasting UV radiation (UVR, 280–400 nm) effects on in situ rates of aquatic primary production when field measurements do not include estimates of UVR effects. A composite of spectral field measurements is employed to model UVR-dependent rates of photosynthesis in diatomdominated waters in a coastal region of the Southern Ocean. Assumptions, caveats and limitations of the modeling are discussed. Calculations begin with the 1991 Palmer Long Term Ecological Research (LTER) primary production and optical databases, from which daily integrated rates of carbon fixation in the absence of UVR are calculated as a function of depth for a 140 km transect line sampled between dawn and dusk of a single day (14 November 1991). The UVR measurements from the nearby NSF/OPP Polar Network at Palmer Station are used to determine ozone (O₃) concentration on the day of the transect, which is then employed in Madronich's (In UV-B Radiation and Ozone Depletion (Edited by M. Tevini), pp. 17–68. Lewis, Boca Raton, FL, 1993) spectral code to model daytime variations in surface spectral irradiances under clear sky conditions. These data are corrected for cloudiness and then combined with estimates of in-water UVR spectral attenuation coefficients, derived from Icecolors 90 data, to estimate in situ light exposure for phytoplankton collected at different depths and locations. An absolute chlorophyllspecific biological weighting function (BWF), determined under natural solar light fields for Antarctic diatom communities and shown to be reproducible while differing from a laboratory diatom BWF and other in situ BWF determined for other phytoplankton assemblages, is combined with estimates of in situ UVR exposure to derive in situ estimates of chlorophyllspecific losses of carbon fixation due to UVR inhibittion. By repeating calculations for every sampling site along the transect, we derive a spatial map of estimated UVR effects on primary production across the region. We repeat calculations for different O, concentrations expected during the austral spring over Antarctica and illustrate the O, dependency of UVB (280–320 nm) inhibition effects in near surface waters. We estimate ambient UVR reduced carbon fixation rates up to 65% in surface waters, depending upon location, down to undetectable levels at 36 m. Reducing stratospheric O₃ concentrations by 50% further inhibits near surface primary production by 8% and integrated primary production by 5%. Primary production was forced to subsurface maxima across the entire transect line in the presence of UVR.     

PHOTOCONTROL OF HYPOCOTYL ELONGATION IN LIGHT-GROWN Cucumis sativus L. PHOTOSYNTHETIC REQUIREMENT FOR A FLUENCE RATE DEPENDENT PHYTOCHROME RESPONSE

We investigated the role of photosynthesis in the photocontrol of extension growth of the hypocotyl of light-grown Cucumis sativus L. Previous work [Gaba and Black (1985b) Plant Physiol. 79 , 1011] demonstrated that the inhibition of cucumber hypocotyl elongation is a fluence rate dependent response in red light. However, the relative contributions of phytochrome and photosynthesis to the photon flux dependent inhibition response were not clear. Here we have shown that photoperception by the foliar cotyledons as well as the hypocotyl itself are responsible for fluence rate dependence in red light. The inhibitor of photosynthesis diuron [3-(3,4-dichlorophenyl)-1, 1-dimethylurea] reduced both the magnitude of inhibition and the fluence rate dependency in red light, indicating an involvement of photosynthesis. Furthermore, the growth of non-pigmented seedlings (treated with the herbicide norflurazon) was less inhibited by red light, with no fluence rate dependency. In particular, inhibition due to cotyledon photoperception was completely lost in non-pigmented (norflurazon-treated) plants, and much reduced by diuron treatment. Hypocotyl-perceived red light inhibition was only slightly reduced by treatment with norflurazon and diuron. Photosynthesis was compared directly to photo-inhibition of growth: the photon flux response curve of oxygen evolution of green Cucumis cotyledons was distinctly different from that of hypocotyl inhibition. In conclusion, photosynthesis is an essential requirement for the cotyledon-perceived inhibition, but the response itself is not due to photosynthesis.     

Ultraviolet (280-400 nm)-induced DNA damage in the eggs and larvae of Calanus finmarchicus G. (Copepoda) and Atlantic cod (Gadus morhua)

In previous work, we evaluated the effects of ultraviolet (UV = 280-400 nm) radiation on the early life stages of a planktonic Calanoid copepod (Calanus finmarchicus Gunnerus) and of Atlantic cod (Gadus morhua). Both are key species in North Atlantic food webs. To further describe the potential impacts of UV exposure on the early life stages of these two species, we measured the wavelength-specific DNA damage (cyclobutane pyrimidine dimer [CPD] formation per megabase of DNA) induced under controlled experimental exposure to UV radiation. UV-induced DNA damage in C. finmarchicus and cod eggs was highest in the UV-B exposure treatments. Under the same spectral exposures, CPD loads in C. finmarchicus eggs were higher than those in cod eggs, and for both C. finmarchicus and cod embryos, CPD loads were generally lower in eggs than in larvae. Biological weighting functions (BWF) and exposure response curves that explain most of the variability in CPD production were derived from these data. Comparison of the BWF revealed significant differences in sensitivity to UV-B: C. finmarchicus is more sensitive than cod, and larvae are more sensitive than eggs. This is consistent with the raw CPD values. Shapes of the BWF were similar to each other and to a quantitative action spectrum for damage to T7 bacteriophage DNA that is unshielded by cellular material. The strong similarities in the shapes of the weighting functions are not consistent with photoprotection by UV-absorbing compounds, which would generate features in BWF corresponding to absorption bands. The BWF reported in this study were applied to assess the mortality that would result from accumulation of a given CPD load: for both C. finmarchicus and cod eggs, an increased load of 10 CPD Mb(-1) of DNA due to UV exposure would result in approximately 10% mortality.     

Ultraviolet B-photoprotection efficiency of mesocosm-enclosed natural phytoplankton communities from different latitudes: Rimouski (Canada) and Ubatuba (Brazil)

Photoprotection against UV-B radiation (UVBR; 280-320 nm) was examined in natural phytoplankton communities from two coastal environments at different latitudes: temperate Rimouski (Canada) and tropical Ubatuba (Brazil). Mesocosm experiments were performed at these sites to examine the response of phytoplankton to increases in UVBR that corresponded to local depletions of 30% and 60% in atmospheric ozone levels (low and high UVBR treatments, respectively). A fluorescence method using a pulse amplitude modulation fluorometer (Xe-PAM, Walz, Germany) with selective UV filters was used to estimate photoprotection, and these results were compared with an index of mycosporine-like amino acid (MAA) concentrations determined using spectrophotometry of methanol extracts. The present study provided the first evidence, to our knowledge, of the suitability of this in vivo fluorescence method for the estimation of UV photoprotection efficiency in natural phytoplankton. No significant differences were found for most of the variables analyzed between the light treatments used at both sites, but differences were found between sites throughout the duration of the experiments. Vertical mixing, used to maintain cells in suspension, likely alleviated serious UVBR-induced damage during both experiments by reducing the length of time of exposure to the highest UVBR irradiances at the surface. In Rimouski, this was the main factor minimizing the effects of treatment, because optical properties of the coastal seawater rapidly attenuated UVBR throughout the water column of the ca 2 m deep mesocosms. In this location, synthesis of MAAs and photoprotective pigments likely contributed to the observed phototolerance of phytoplankton and, hence, to their growth; however, in a comparison of the UVBR treatments, these variables showed no differences. In Ubatuba, where nutrient concentrations were significantly lower than those in Rimouski, light attenuation was less than that in Rimouski and UVBR reached the bottom of the mesocosms. UVBR penetration and the forced vertical mixing of the cells, without the possibility of vertical migration below this photostress zone, resulted in photo-inhibition, because confinement in the mesocosms forced cells to remain constantly exposed to high levels of irradiance during the daytime. Hence, additional effects of UVBR were masked in this experiment, because cells were damaged too much and phytoplankton populations were rapidly declining. There was also an overall preservation of MAAs, in contrast with chlorophyll (Chl) degradation, in spite of the fact that this UV screening was not sufficient to counteract photo-inhibition, which suggests an important role for these molecules, either in the overall photoprotection strategy or in other physiological processes. Altogether, local water characteristics, namely attenuation, mixing, and nutrients concentration, can strongly modulate the photoprotection strategies used by natural phytoplankton populations in coastal environments.     

Antioxidative Responses of Two Marine Microalgae During Acclimation to Static and Fluctuating Natural UV Radiation

Photoacclimation properties were investigated in two marine microalgae exposed to four ambient irradiance conditions: static photosynthetically active radiation (PAR: 400–700 nm), static PAR + UVR (280–700 nm), dynamic PAR and dynamic PAR + UVR. High light acclimated cultures of Thalassiosira weissflogii and Dunaliella tertiolecta were exposed outdoors for a maximum of 7 days. Dynamic irradiance was established by computer controlled vertical movement of 2 L bottles in a water filled basin. Immediate (<24 h), short-term (1–3 days) and long-term (4–7 days) photoacclimation was followed for antioxidants (superoxide dismutase, ascorbate peroxidase and glutathione cycling), growth and pigment pools. Changes in UVR sensitivity during photoacclimation were monitored by measuring UVR-induced inhibition of carbon assimilation under standardized UV conditions using an indoor solar simulator. Both species showed immediate antioxidant responses due to their transfer to the outdoor conditions. Furthermore, upon outdoor exposure, carbon assimilation and growth rates were reduced in both species compared with initial conditions; however, these effects were most pronounced in D. tertiolecta . Outdoor UV exposure did not alter antioxidant levels when compared with PAR-only controls in both species. In contrast, growth was significantly affected in the static UVR cultures, concurrent with significantly enhanced UVR resistance. We conclude that antioxidants play a minor role in the reinforcement of natural UVR resistance in T. weissflogii and D. tertiolecta .     

Interaction of UV radiation and inorganic carbon supply in the inhibition of photosynthesis: spectral and temporal responses of two marine picoplankters

The effect of ultraviolet radiation (UVR) on inhibition of photosynthesis was studied in two species of marine picoplankton with different carbon concentration mechanisms: Nannochloropsis gaditana Lubian possesses a bicarbonate uptake system and Nannochloris atomus Butcher a CO2 active transport system. Biological weighting functions (BWFs) for inhibition of photosynthesis by UVR and photosynthesis vs irradiance (PI) curves for photosynthetically active radiation (PAR) were estimated for both species grown with an enriched CO2 supply (high dissolved inorganic carbon [DIC]: 1% CO2 in air) and in atmospheric CO2 levels (low DIC: 0.03% CO2). The response to UVR and PAR exposures was different in each species depending on the DIC treatment. Under PAR exposure, rates of maximum photosynthesis were similar between treatments in N. gaditana. However, the cultures growing in high DIC had lower sensitivity to UVR than the low DIC cultures. In contrast, N. atomus had higher rates of photosynthesis under PAR exposure with high DIC, but the BWFs were not significantly different between treatments. The results suggest that one or more processes in N. gaditana associated with HCO3- transport are target(s) for UV photodamage because there was relatively less UV inhibition of the high DIC-grown cultures in which inorganic carbon fixation is supplied by passive CO2 diffusion. Time courses of photochemical efficiency in PAR, during UV exposure and during subsequent recovery in PAR, were determined using a pulse amplitude modulated fluorometer. The results were consistent with the BWFs. In all time courses, a steady state was obtained after an initial decrease, consistent with a dynamic balance between damage and repair as found for other phytoplankton. However, the relationship of response to exposure showed a steep decline in activity that is consistent with a constant rate of repair. A novel feature of a model developed from a constant repair rate is an explicit threshold for photosynthetic response to UV.     

Biological weighting function of the UV-B-induced impairment of phototaxis in the freshwater ciliate Ophryoglena flava

We determined the biological weighting function (BWF) of the effect of UV radiation on phototaxis of the freshwater, histophagous ciliate Ophryoglena flava. Dose-effect curves were measured by exposing the cells to 12 different irradiation regimens obtained with two different levels of UV-B radiation and by using six filters with cutoff wavelengths ranging from 280 to 335 nm. The results show that there are significant damages to phototaxis at the doses used and that the effect increases when the cutoff is shifted toward short wavelengths. The data were used to calculate the BWF of phototaxis impairment by applying a nonlinear fit procedure. The BWF thus obtained decays exponentially with increasing wavelength in agreement with similar findings reported in the literature for other systems.     

Ultraviolet-B radiation effects on the structure and function of lower trophic levels of the marine planktonic food web

The impact of UV-B radiation (UVBR; 280-320 nm) on lower levels of a natural plankton assemblage (bacteria, phytoplankton and microzooplankton) from the St. Lawrence Estuary was studied during 9 days using several immersed outdoor mesocosms. Two exposure treatments were used in triplicate mesocosms: natural UVBR (N treatment, considered as the control treatment) and lamp-enhanced UVBR (H treatment, simulating 60% depletion of the ozone layer). A phytoplankton bloom developed after day 3, but no significant differences were found between treatments during the entire experiment for phytoplankton biomass (chlorophyll a and cell carbon) nor for phytoplankton cell abundances from flow cytometry and optical microscopy of three phytoplankton size classes (picoplankton, nanoplankton and microplankton). In contrast, bacterial abundances showed significantly higher values in the H treatment, attributed to a decrease in predation pressure due to a dramatic reduction in ciliate biomass (approximately 70-80%) in the H treatment relative to the N treatment. The most abundant ciliate species were Strombidinium sp., Prorodon ovum and Tintinnopsis sp.; all showed significantly lower abundances under the H treatment. P. ovum was the less-affected species (50% reduction in the H treatment compared with that of the N control), contrasting with approximately 90% for the other ones. Total specific phytoplanktonic and bacterial production were not affected by enhanced UVBR. However, both the ratio of primary to bacterial biomass and production decreased markedly under the H treatment. In contrast, the ratio of phytoplankton to bacterial plus ciliate carbon biomass showed an opposite trend than the previous results, with higher values in the H treatment at the end of the experiment. These results are explained by the changes in the ciliate biomass and suggest that UVBR can alter the structure of the lower levels of the planktonic community by selectively affecting key species. On the other hand, linearity between particulate organic carbon (POC) and estimated planktonic carbon was lost during the postbloom period in both treatments. On the basis of previous studies, our results can be attributed to the aggregation of carbon released by cells to the water column in the form of transparent exopolymer particles (TEPs) under nutrient limiting conditions. Unexpectedly, POC during such a period was higher in the H treatment than in controls. We hypothesize a decrease in the ingestion of TEPs by ciliates, in coincidence with increased DOC release by phytoplankton cells under enhanced UVBR. The consequences of such results for the carbon cycle in the ocean are discussed.     

Comparison by PAM fluorometry of photosynthetic activity of nine marine phytoplankton grown under identical conditions

The photosynthetic activity of marine phytoplankton from five algal classes (Phaeodactylum tricornutum, Skeletonema costatum, Thalassiosira oceanica, Thalassiosira weissflogii, Dunaliella tertiolecta, Mantoniella squamata, Emiliania huxleyi, Pavlova lutheri and Heterosigma akashiwo) was investigated under identical growth conditions to determine interspecies differences. Primary photochemistry and electron transport capacity of individual species were examined by pulse amplitude-modulated (PAM) fluorescence. Although few differences were found in maximal photosystem II (PSII) photochemical efficiency between various species, large differences were noticed in their PSII-photosystem I (PSI) electron transport activity. We found that species such as T. oceanica and M. squamata have much lower photochemical activity than H. akashiwo. It appeared that processes involved in electron transport activity were more susceptible to change during algal evolution compared with the primary photochemical act close to PSII. Large variations in the nonphotochemical energy dissipation event among species were also observed. Light energy required to saturate photosynthesis was very different between species. We have shown that M. squamata and H. akashiwo required higher light energy (>1300 micromol m(-2) s(-1)) to saturate photosynthesis compared with S. costatum and E. huxleyi (ca 280 micromol m(-2) s(-1)). These differences were interpreted to be the result of variations in the size of light-harvesting complexes associated with PSII. These disparities in photosynthetic activity might modulate algal community structure in the natural environment where light energy is highly variable. Our results suggest that for an accurate evaluation of primary productivity from fluorescence measurements, it is essential to know the species composition of the algal community and the individual photosynthetic capacity related to the major phytoplankton species present in the natural phytoplankton assemblage.     

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.     

Photosynthetic Performance of the Red Alga Pyropia haitanensis During Emersion,With Special Reference to Effects of Solar UV Radiation,Dehydration and Elevated CO2 Concentration

Macroalgae distributed in intertidal zones experience a series of environmental changes, such as periodical desiccation associated with tidal cycles, increasing CO₂ concentration and solar UVB (280–315 nm) irradiance in the context of climate change. We investigated how the economic red macroalga, Pyropia haitanensis, perform its photosynthesis under elevated atmospheric CO₂ concentration and in the presence of solar UV radiation (280–400 nm) during emersion. Our results showed that the elevated CO₂ (800 ppmv) significantly increased the photosynthetic carbon fixation rate of P. haitanensis by about 100% when the alga was dehydrated. Solar UV radiation had insignificant effects on the net photosynthesis without desiccation stress and under low levels of sunlight, but significantly inhibited it with increased levels of desiccation and sunlight intensity, to the highest extent at the highest levels of water loss and solar radiation. Presence of UV radiation and the elevated CO₂ acted synergistically to cause higher inhibition of the photosynthetic carbon fixation, which exacerbated at higher levels of desiccation and sunlight. While P. haitanensis can benefit from increasing atmospheric CO₂ concentration during emersion under low and moderate levels of solar radiation, combined effects of elevated CO₂ and UV radiation acted synergistically to reduce its photosynthesis under high solar radiation levels during noon periods.     

Effects of surfactant and boron doping on the BWF feature in the Raman spectrum of single-wall carbon nanotube aqueous dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.     

Ultraviolet radiation has no effect on respiratory oxygen consumption or enhanced post-illumination respiration in three species of microalgae

A 30-min exposure to UV-B radiation (1.1 Wm(-2), unweighted) from a xenon arc lamp caused pronounced inhibition (33-78%) of net photosynthetic oxygen production in three species of microalgae, Phaeodactylum tricornutum Bohlin, Dunaliella tertiolecta Butcher and Wolozynskia sp., however, no statistical differences (t-test, alpha=0.05) in dark-respiration rates were found between the control group and the UV-treated group, for any of the species tested. These results indicate: (i) that the respiratory processes responsible for oxygen consumption do not sustain any appreciable impairment registered in the first half-hour after ultraviolet radiation (UVR) exposure; and (ii) any change in respiration that may occur in response to increased repair demands is not detected in this period. Dark-respiration rates were observed to be significantly higher in all species tested (17-29%; t-test, alpha=0.05) following illumination with photosynthetically active radiation, compared to dark-respiration before illumination. This increase, interpreted as enhanced post-illumination respiration (EPIR), was observed in all three species. The magnitude of this increase was not affected by prior exposure to UVR.     

Leaves of Citrus aurantifolia exhibit a different sensibility to solar UV-B radiation according to development stage in relation to photosynthetic pigments and UV-B absorbing compounds production

Plants of Citrus aurantifolia grown in a greenhouse without solar UV radiation (UVR) were transferred outdoors to evaluate the effect of solar UV-B radiation (UVBR, 280–315 nm) in prior-developed leaves, constituted by apical bud and those fully expanded before being taken outdoors, and post-developed leaves, formed by those expanded outdoors. Results demonstrated that over a 40 d outdoor period leaf chlorophyll content and distribution pattern were different with and without solar UVBR. Chlorophyll a, chlorophyll b and total chlorophyll contents in both treatments were higher in prior-developed leaves than in post-developed ones. However, highest values were observed in prior-developed leaves under solar UVBR, whereas in post-developed leaves an opposite trend was observed. Carotenoids content in prior-developed leaves was higher with solar UVBR, whereas in post-developed leaves there were no significant differences in both with and without solar UVBR. In addition, prior-developed leaves under solar UVBR accumulated flavonoids, but not anthocyanins. Growth parameters (e.g. DW, DW/FW ratio, LMA, plant height, length and width of foliar lamina) did not show significant differences between plants grown with and without solar UVBR. Thus, our results demonstrated that C. aurantifolia leaves exhibited a different sensibility to solar UVBR according to development stage in relation to photosynthetic pigments and UV-B absorbing compounds production. In addition, the solar UVBR was not necessary as inductor of photosynthetic protection mechanisms in a short-time growth period. On the other hand, our results also demonstrated that solar UVBR acted as an effective feeding deterrent against citrus leafminer.     

Use of UV-A Energy for Photosynthesis in the Red Macroalga Gracilaria lemaneiformis

UV radiation is known to inhibit photosynthetically active radiation (PAR)-driven photosynthesis; however, moderate levels of UV-A have been shown to enhance photosynthesis and growth rates of some algae. Here, we have shown that UV-A alone could drive photosynthetic utilization of bicarbonate in the red alga Gracilaria lemaneiformis as evidenced in either O₂ evolution or carbon fixation as well as pH drift. Addition of UV-B inhibited the apparent photosynthetic efficiency, raised the photosynthetic compensation point and photosynthesis-saturating irradiance level, but did not significantly affect the maximal rate of photosynthetic O₂ evolution. The electron transport inhibitor, DCMU, inhibited the photosynthesis completely, reflecting that energy of UV-A was transferred in the same way as that of PAR. Inorganic carbon acquisition for photosynthesis under UV alone was inhibited by the inhibitors of carbonic anhydrase. The results provided the evidence that G. lemaneiformis can use UV-A efficiently to drive photosynthesis based on the utilization of bicarbonate, which could contribute significantly to the enhanced photosynthesis in the presence of UV-A observed under reduced levels of solar radiation.     

Modeling the effects of ultraviolet radiation on estuarine phytoplankton production: impact of variations in exposure and sensitivity to inhibition.

Spectral ultraviolet (UV) irradiance, water column attenuation and biological weighting functions for inhibition of phytoplankton photosynthesis have been measured for the Rhode River, a subestuary of the Chesapeake Bay. Together, these measurements can be used to estimate UV effects on water column production, but each factor shows a significant range of variability even just considering summer time conditions. A sensitivity analysis of UV inhibition is described which assesses the effect of this variation for different combinations of 28 irradiance spectra, 8 biological weighting functions (BWFs) and 16 water column irradiance profiles. Over all combinations, production averaged about 84% relative to potential production in the absence of UV effects. For a few combinations, relative production was as low as 67%, or as high as 97%, but for most combinations the range was 75-95%. Variations in the sensitivity of the phytoplankton assemblage, i.e. the BWF, and optical properties, represented by a transparency ratio of biologically effective UV to photosynthetically available radiation (PAR), had large effects on water column production. A simple relationship for UV inhibition of water column production is developed based on inhibition at the surface and the ratio of UV and PAR transparency.