Influence of solar ultraviolet radiation on photosynthesis and motility of marine phytoplankton

Abstract: The effects of solar ultraviolet radiation (UV) on carbon uptake, oxygen evolution and motility of marine phytoplankton were investigated in coastal waters at Kristineberg Marine Research Station on the west coast of Sweden (58° 30'N, 11° 30'E). The mean irradiances at noon above the water surface during the investigation period were: photosynthetic active radiation (PAR, 400–700 nm) 1670 μmol m⁻² s⁻¹; ultraviolet-A radiation (UV-A, 320–400 nm) 35.9 W m⁻² and ultraviolet-B radiation (UV-B, 280–320 nm) 1.7 W m⁻². UV-B radiation was much more attenuated with depth in the water column than were PAR and UV-A radiation. UV-B radiation could not be detected at depths greater than 100–150 cm. Inhibition of carbon uptake by UV-A and UV-B in natural phytoplankton populations was greatest at 50 cm depth and the effects of UV-B were greater than those of UV-A. At depths greater than 50 cm there was almost no effect of ultraviolet radiation on carbon uptake. PAR, UV-A and UV-B decreased oxygen evolution by the dinoflagellate Prorocentrum minimum . Inhibition of oxygen evolution was greater after 4 h than 2 h but it was not possible to distinguish the negative effects of the different light regimes. The motility of P. minimum was not affected by PAR, UV-A and UV-B. The importance of exposure of phytoplankton to different light regimes before being exposed to natural solar radiation is discussed.     

Short-term interactive effects of ultraviolet radiation,carbon dioxide and nutrient enrichment on phytoplankton in a shallow coastal lagoon

The main goal of this study was to evaluate short-term interactions between increased CO₂, UVR and inorganic macronutrients (N, P and Si) on summer phytoplankton assemblages in the Ria Formosa coastal lagoon (SW Iberia), subjected to intense anthropogenic pressures and highly vulnerable to climate change. A multifactorial experiment using 20 different nutrient-enriched microcosms exposed to different spectral and CO₂ conditions was designed. Before and after a 24-h in situ incubation, phytoplankton abundance and composition were analysed. Impacts and interactive effects of high CO₂, UVR and nutrients varied among different functional groups. Increased UVR had negative effects on diatoms and cyanobacteria and positive effects on cryptophytes, whereas increased CO₂ inhibited cyanobacteria but increased cryptophyte growth. A positive synergistic interaction between CO₂ and UVR was observed for diatoms; high CO₂ counteracted the negative effects of UVR under ambient nutrient concentrations. Nutrient enrichments suppressed the negative effects of high CO₂ and UVR on cyanobacteria and diatoms, respectively. Beneficial effects of CO₂ were observed for diatoms and cryptophytes under combined additions of nitrate and ammonium, suggesting that growth may be limited by DIC availability when the primary limitation by nitrogen is alleviated. Beneficial effects of high CO₂ and UVR in diatoms were also induced or intensified by ammonium additions.     

Acclimatization of Antarctic natural phytoplankton assemblages when exposed to solar ultraviolet radiation

The effects of solar ultraviolet radiation (UVR) on photosyntheticrates of natural assemblages of Antarctic phytoplankton weredetermined in both 1 day and 2 week incubations during australspring at Palmer Station, Antarctica. During the first day ofthe long-term incubations, photosynthetic rates were enhancedby     

Effects of ultraviolet radiation and nutrients on the structure-function of phytoplankton in a high mountain lake

The combined effect of high solar ultraviolet radiation (UVR) and nutrient supply in a phytoplankton community of a high mountain lake is analyzed in a in situ experiment for 6 days with 2 × 2 factorial design. Interactive UVR × nutrient effects on structural and functional variables (algal biomass, chlorophyll a (chl a), primary production (PP), maximal electron transport rate (ETR(max)), and alkaline phosphatase activity (APA)), as well as stoichiometric ones (sestonic N per cell and N:P ratio) were found. Under non-nutrient enriched conditions, no deleterious effects of UVR on structural variables, PP, photosynthetic efficiency and ETR(max) were observed, whereas only particulate and total APA were affected by UVR. However, percentage excreted organic carbon (%EOC), dissolved APA and sestonic C and P per cell increased under UVR, leading to a decrease in algal C:P and N:P ratios. After nutrient enrichment, chl a, total algal biomass and PP were negatively affected by UVR whereas %EOC, ETR(max) and internal C, P and N content increased. We suggest that the mechanism of algal acclimation to UVR in this high UVR flux ecosystem seems to be related to the increase of internal algal P-content mediated by physiological mechanisms to save P and by a stimulatory UVR effect on dissolved extracellular APA. The mechanism involved in the unmasking effect of UVR after nutrient-enrichment may be the result of a greater sensitivity to UVR-induced cell damage, making the negative UVR effects more evident.     

Time-series uptake of carbon into photosynthetic products of Benguela phytoplankton populations

The uptake of carbon into primary and major products of photosynthesisin natural populations has been determined in 4 h and 24 h time-seriesexperiments at 50% of the incident radiation. During activegrowth the communities assimilated the largestproportion ofcarbon into poiysaccharides at all time intervals and the amountof label incorporated into primary products was approximatelyequal to or exceeded that in the proteins and lipids. When therewas no growth the synthesis of protein was the dominant metabolicprocess. The community in experiment 6 appeared to be in anactive phase, however, in one experiment the uptake of carboninto protein was faster than with no growth and during the darkperiod actively growing cells maintained protein productionby utilizing carbon stored in the polysaccharides only; stationarygrowth cells required both primary products and polysacchaiidcsfor protein synthesis at night.     

Annual patterns of ultraviolet radiation effects on temperate marine phytoplankton off Patagonia, Argentina

We carried out experiments to evaluate the effects of solarultraviolet radiation (UVR; 280–400 nm) upon primary productionof different natural phytoplankton assemblages (i.e. characteristicof a seasonal cycle) from Patagonia (Argentina) from January2001 to January 2002. The short-term impact of UVR (i.e. measuredas radiocarbon incorporation) was assessed by exposing samplesto solar radiation under six radiation treatments: uncoveredquartz tubes and tubes covered with different cut-off Schottfilters (WG295, WG305, WG320, WG360), and Plexiglas UF-3 (cut-offat 400 nm), so that samples received radiation at five differentintervals within the UVR in addition to photosynthetically activeradiation (PAR), and only PAR, respectively. Phytoplankton compositionand abundance allowed us to differentiate pre-bloom, bloom andpost-bloom periods, with pre- and post-bloom samples characterizedby small cells (e.g. flagellates <10 µm), whereas thebloom was dominated by large diatoms (50 µm). Absolutevalues of photosynthesis inhibition were lower during the bloom,but biological weighting functions (i.e. inhibition per unitenergy) indicated that this assemblage was more sensitive toUVR (especially in the UV-B region, 280–320 nm) than thoseof the pre- and post-bloom periods. UV-A radiation (320–400nm) accounted for most of the reduction in carbon incorporation(>60%), especially during the pre- and post-bloom periods.Most of the observed variability was inter-seasonal, althoughsmall intra-seasonal fluctuations were also observed. Our resultsindicate that the taxonomic composition and cellular size areespecially important when addressing UVR effects upon theseassemblages. However, other factors such as mixing can alsocontribute to the variability in responses to UVR.     

Diel changes of uptake of inorganic carbon and nitrogen by phytoplankton, and the relationship between inorganic carbon and nitrogen uptake in Lake Nakanuma, Japan

Diel changes of uptake of inorganic carbon and nitrogen wereexamined in a small freshwater lake, Lake Nakanuma, Japan, bythe ¹³C and ¹⁵N method. Experiments were earned out in spring,summer and autumn in 1984. Carbon and nitrogen uptake in thelight incubation showed maxima around noon at the three seasons.Carbon uptake ceased at night, but ammonium uptake was fairlylarge at night. In the dark incubation carbon uptake did notoccur. Ammonium uptake showed a maximum at dusk in the darkexperiments. Diel changes of nitrate uptake were less clearthan those of ammonium uptake. These results indicate that nitrogenuptake partly depended on the carbon uptake. Then, we triedto explain the diel changes of nitrogen uptake, assuming thatthe nitrogen uptake partly depends on stored carbohydrate. Thediel changes may be elucidated by the sum of three terms: oneis the term of decay of stored carbohydrate, the second is theterm which indicates cumulative increase of stored carbohydrateand the third is the term which directly depends on light.     

Diel variations in inorganic carbon and nitrogen uptake by phytoplankton in an arctic lake

Time—depth variations in inorganic carbon and nitrogenuptake by phytoplankton in Toolik Lake were examined over 36h using isotope tracer techniques. Rates of dissolved inorganiccarbon (DIC = CO₂ + + ) and maximum uptake were phased with the did high light/low light regime characteristic of the briefarctic summer with the amplitude of oscillation greater forDIC than for . Ammonium uptake was continuous at uptake-saturating concentrations. No conclusive evidencewas found for a diel periodicity in nitrogenous nutrient levelsor uptake of and ambient concentrations. A pronounced light—temperature effecton dissolved inorganic nitrogen (DIN = + ) uptake was evident at depth when rates of uptake were maximum. Depth-integrated daily C/N uptake ratios(mol) estimated as the mean of four consecutive 6 h incubationsranged from 1.8–6.4 under conditions of substrate saturationand from 6.0–16.1 at ambient levels of DIN. The efficacyof 24 h incubations to estimate accurately day-rates of DICand substrate-saturated DIN uptake was assessed by comparingestimates obtained from 24 h exposures to those approximatedby summing results from serial short-term incubations. Experimentsof 24 h duration accurately predicted day-rates of maximum uptake but underestimated daily DIC uptake by 13 7% ( SD). Day-length incubations introduced serious errors in the estimation of day-rates of maximum uptake, effecting an underestimation of 29 5%( SD). ¹Institute of Marine Science Contribution No. 538.     

The effect of long wave ultraviolet radiation (UV-A) on the photosynthetic activity of natural population of freshwater phytoplankton

The effect of ultraviolet radiation on diel changes and depth profiles of phytoplankton photosynthesis was studied in four temperate freshwater lakes. Photosynthetic oxygen production was determined by incubating lake water in light and dark bottles under various weather conditions. Half the light bottles were wrapped with sheets of vinyl chloride film to exclude light with wavelengths shorter than 400 nm. The inhibition of photosynthesis due to UV-A (320–400 nm) was observed during most of the daytime and was very strong around noon on both sunny and cloudy days. On sunny days, when the surface waters of the highly eutrophic Lake Suwa and Senzoku Pond were dominated by denseMicrocystis populations, cumulative daily production at the surface, estimated from the incubation of bottles from which UV-A was excluded by the vinyl film, were about double the rates obtained from glass bottles in which UV-A was present. The UV-A inhibition was detected from the surface toca 20 cm depth in hypereutrophic lakes and at depths greater than 50 cm in mesotrophic lakes. Analysis of the photosynthesis-irradiance (P-I) relationship obtained in the present study shows β, a parameter that describes photo-inhibition, is higher in the presence of UV-A than in its absence. This indicates that UV-A is the major cause of photo-inhibition of phytoplankton photosynthesis.     

Sources and measurement of ultraviolet radiation

Ultraviolet (UV) radiation is part of the electromagnetic spectrum. The biological effects of UV radiation vary enormously with wavelength and for this reason the UV spectrum is further subdivided into three regions: UVA, UVB, and UVC. Quantities of UV radiation are expressed using radiometric terminology. A particularly important term in clinical photobiology is the standard erythema dose (SED), which is a measure of the erythemal effectiveness of a UV exposure. UV radiation is produced either by heating a body to an incandescent temperature, as is the case with solar UV, or by passing an electric current through a gas, usually vaporized mercury. The latter process is the mechanism whereby UV radiation is produced artificially. Both the quality (spectrum) and quantity (intensity) of terrestrial UV radiation vary with factors including the elevation of the sun above the horizon and absorption and scattering by molecules in the atmosphere, notably ozone, and by clouds. For many experimental studies in photobiology it is simply not practicable to use natural sunlight and so artificial sources of UV radiation designed to simulate the UV component of sunlight are employed; these are based on either optically filtered xenon arc lamps or fluorescent lamps. The complete way to characterize an UV source is by spectroradiometry, although for most practical purposes a detector optically filtered to respond to a limited portion of the UV spectrum normally suffices.     

Differential sensitivity to natural ultraviolet radiation among phytoplankton species in Arctic lakes (Spitsbergen,Norway)

Incubation experiments demonstrated a differential sensitivity to natural UV-radiation among the dominant phytoplankton species from three Arctic lakes, situated near Ny-Ålesund, Spitsbergen (79° N). The growth of small chlorophytes, diatoms and picocyanobacteria from two oligotrophic lakes was inhibited primarily by the shorter wavelength UV components, while the growth of the larger colony-forming species (cyanobacteria, Planktothrix sp., Woronichinia sp. and the chrysophyte, Uroglena americana Calkins) apparently was stimulated. These colonies (not easily eaten by daphnids) dominated at the end of the experiment in those treatments where the short wavelength UV components were not excluded. For the two oligotrophic localities, 70 and 61%, respectively, of total phytoplankton biovolume were edible in the treatments excluding short wavelength UV, compared to only 13 and 19%, respectively, in the treatments including such radiation. For the third, more productive and less transparent lake, the percentage of edible species in the treatments with and without short wavelength UV radiation did not differ (ca. 75% for both treatments).     

Effects of nutrients and dissolved organic matter on the response of phytoplankton to ultraviolet radiation: experimental comparison in spring versus summer

The effects of nutrients and dissolved organic matter (DOM) on the response of phytoplankton community structure to ultraviolet radiation (UVR) was studied using natural phytoplankton assemblages from Lake Giles (Northeastern Pennsylvania), a temperate, oligotrophic, highly UVR-transparent lake. Microcosm experiments were conducted in 1-l bags in the spring and summer. A factorial design was used, with two UVR treatments (ambient and reduced), two nutrient treatments (control with no nutrients added, and nitrogen and phosphorus addition together), and two DOM treatments (control of 1 mg l⁻¹ and doubled). In April, UVR affected the overall phytoplankton community structure, causing a shift in the dominant species. Significant interactive effects of UVR × nutrients and UVR × DOM were found on total phytoplankton biovolumes. In July, all taxa responded positively to the N + P addition, and were affected differentially by the UVR treatments. The initial communities varied in April and July, but Synura sp. and Chroomonas sp. were present in both seasons. Synura sp. responded positively to the addition of DOM in April and the reduction of UVR in July. Chroomonas sp. responded positively to the reduction of UVR in April and the addition of nutrients in July. The differential sensitivity of these two species suggests that changing environmental factors between spring and summer promoted differences in the relative importance of UVR in changing phytoplankton community structure. Handling editor: Luigi Naselli-Flores     

Vertical mixing within the epilimnion modulates UVR-induced photoinhibition in tropical freshwater phytoplankton from southern China

1. The importance of vertical mixing in modulating the impact of UVR on phytoplankton photosynthesis was assessed in a tropical, shallow lake in southern China from late winter to mid‐spring of 2005. 2. Daily cycles of fluorescence measurements (i.e. photosynthetic quantum yield, Y) were performed on both ‘static’ and in situ samples. Static samples were of surface water incubated at the surface of the lake under three radiation treatments – PAB (PAR + UVR, 280–700 nm), PA (PAR + UV‐A, 320–700 nm) and P (PAR, 400–700 nm). In situ samples were collected every hour at three different depths – 0, 0.5 and 1 m. 3. The general daily pattern was of a significant decrease in Y from early morning towards noon, with partial recovery in the afternoon. Samples incubated under static conditions always had lower Y than those under in situ conditions at the same time of the day. 4. Under stratified conditions, no overall impact of UVR impact could be detected in situ when compared with the static samples. Further rapid vertical mixing not only counteracted the impact of UVR but also stimulated photosynthetic efficiency. 5. Based on these measurements of fluorescence, the mixing speed of cells moving within the epilimnion was estimated to range between 0.53 and 6.5 cm min⁻¹. 6. These data show that mixing is very important in modulating the photosynthetic response of phytoplankton exposed to natural radiation and, hence, strongly conditions the overall impact of UVR on aquatic ecosystems.     

Mutualistic relationships between phytoplankton and bacteria caused by carbon excretion from phytoplankton

For the competition system of phytoplankton and bacteria through inorganic phosphorus, our mathematical model showed that mutualistic relationships between them could occur due to production and consumption of extracellular organic carbon by phytoplankton and bacteria. In our model, phytoplankton are limited in their growth by light and phosphorus, and bacteria are limited in their growth by phosphorus and carbon released from phytoplankton. We adopted permanence as a criterion of the coexistence in mathematical analysis, and led necessary conditions of permanence in the model. Under these coexistence conditions, we estimated the strength of total effects of interactions between phytoplankton and bacteria at the steady state by press perturbation method. The results of this estimation indicated the mutualistic interactions between phytoplankton and bacteria. This suggests that mutualistic situation could occur due to the introduction of carbon flow from phytoplankton to bacteria, even if phytoplankton and bacteria compete with each other through common resource, inorganic phosphorus.     

Oxygen uptake during mineralization of photosynthesized carbon from phytoplankton of the Barra Bonita reservoir: a mesocosm study.

This study aimed to discuss and describe the oxygen consumption during aerobic mineralization of organic products (cells and excretion products) from five unialgal cultures: Cryptomonas sp., Microcystis aeruginosa, Anabaena spiroides, Thalassiosira sp. and Aulacoseira granulata. These species were isolated from Barra Bonita reservoir (22 degrees 29' S and 48 degrees 34' W) and cultivated in the laboratory. From each culture, two decomposition chambers were prepared; each chamber contained about 130 mg.L(-1) of carbon from water samples of the reservoir. The chambers were aerated and incubated in the dark at 20.0 degrees C. The concentration of dissolved oxygen, pH values and electrical conductivity of the solutions were determined during a period of 10 days. The results indicated increases in oxygen consumption for all the solutions studied and also for electrical conductivity. The pH values presented a decreasing tendency throughout the experiment. Oxygen consumption varied from 43 (Aulacoseira granulata chamber) to 345 mg O2 g(-1) C (Anabaena spiroides chamber). Decrease in pH values was probably due to increase in CO2 concentration from microbial respiration. Increase in electrical conductivity might be due to the liberation of ions during decomposition. The results demonstrate the potentiality of the studied genera in influencing oxygen availability followed by a die-off event. It also indicates the possibility of changing of the electrical conductivity and pH values in the water column due the aerobic algae mineralization.     

Estimating ultraviolet radiation at the earth's surface

Surface ultraviolet (UV) irradiance depends not only on stratospheric ozone amounts, but also varies with time and date, latitude, cloud amount and aerosol load. Any assessment of the effect of stratospheric ozone depletion on surface UV irradiance must take into consideration all of the above parameters. Measurements in the UV-B region may be accomplished using filter and detector combinations which mimic a biological response curve. However there are uncertainties such as in determining the exact filter response and in the cosine error of the detector. The UV-A region lacks a strong ozone absorption band and approaches which relate measured UV-A irradiance to measured global irradiance show promise. Theoretical models have been derived which calculate spectral UV irradiance in cloudless and cloudy conditions. Results show that cloud transmissivities increase as wavelength increases; however, there is a strong dependence on cloud type. In the absence of surface observations of clouds, satellite data may be used to map UV-A and UV-B irradiance in a region, and this approach is illustrated using two specific examples.     

Modelling the interactions between ammonium and nitrate uptake in marine phytoplankton

An empirically based mathematical model is presented which can simulate the major features of the interactions between ammonium and nitrate transport and assimilation in phytoplankton. The model (ammonium-nitrate interaction model), which is configured to simulate a generic microalga rather than a specified species, is constructed on simplified biochemical bases. A major requirement for parametrization is that the N:C ratio of the algae must be known and that transport and internal pool sizes need to be expressed per unit of cell C. The model uses the size of an internal pool of an early organic product of N assimilation (glutamine) to regulate rapid responses in ammonium-nitrate interactions. The synthesis of enzymes for the reduction of nitrate through to ammonium is induced by the size of the internal nitrate pool and repressed by the size of the glutamine pool. The assimilation of intracellular ammonium (into glutamine) is considered to be a constitutive process subjected to regulation by the size of the glutamine pool. Longer term responses have been linked to the nutrient history of the cell using the N:C cell quota. N assimilation in darkness is made a function of the amount of surplus C present and thus only occurs at low values of N:C. The model can simulate both qualitative and quantitative temporal shifts in the ammonium-nitrate interaction, while inclusion of a derivation of the standard quota model enables a concurrent simulation of cell growth and changes in nutrient status. <br>     

Membranes as targets of ultraviolet radiation

In non-photosynthetic cells, evidence for UV (ultraviolet radiation) damage to membranes comes from electron microscopy, chemical analysis and observations of transport processes. Specific perturbations in transport across membranes occur quickly after a relatively low fluence of UV. As an example, irradiation of suspension-cultured rose cells with 500-2000 J m^?2 (at 254 nm) causes an appearance of K⁺ in the extracellular medium at the rate of 5 × 10^?10μmol cell^?1 min^?1 for 30 to 60 min and more slowly thereafter. The early, rapid phase of appearance of K⁺ reflects both an increase in efflux and a decrease in influx. The appearance of K⁺ is matched by an appearance of HCO^?₃ in the medium. The HCO^?₃ comes from respiratory CO₂, which hydrates and dissociates in the cytoplasm, leading to a decrease in cytoplasmic pH. Overall, these results not only demonstrate UV damage to membrane function, but also suggest several ways by which UV may alter the general metabolic state of the cell. A demonstration of direct effects of UV on membrane components requires a purified system. At lower fluence, < 1800 J m^?2 (254 nm), the ATPase of membrane vesicle preparations is inactivated in a two-phase process that suggests the presence of enzymes with different UV sensitivities. The existence of two non-mitochondrial enzymes in rose cell vesicles has been confirmed by solubilizing the vesicle proteins with 1% cholic acid and separating the components on G-150 Sephadex. One component of relatively high molecular weight is especially sensitive. The fact that it is still sensitive when it is dissolved in cholic acid strongly suggests that its sensitivity is intrinsic and does not depend on sensitization by other membrane components. The action spectrum for the inactivation of the ATPase has a major peak at 290 nm and extends into the UV-C and UV-A regions. The physiological effects of UV-stimulated membrane changes are uncertain. There is little evidence that the UV damage to membranes is responsible for cell death. A UV-induced loss of K⁺ from guard cells may result in lower stomatal conductance. UV-stimulated membrane changes may play a role in the UV-induced synthesis of anthocyanins.