Effects of solar radiation on bacterial and fungal density on aquatic plant detritus

1. The effects of solar radiation on bacterial and fungal growth on aquatic macrophyte detritus were studied in a microcosm experiment. Senescent leaves of Phragmites australis were incubated for 63 days in shallow water in the shade under photosynthetically active radiation (PAR) together with ultraviolet radiation, or under filters removing either ultraviolet B (UVB) or both UVB and ultraviolet A (UVA). 2. Bacterial abundance and bacterial ³H-leucine incorporation in the water were measured, together with α- and β-D-glucosidase activity. In addition, bacterial abundance and fungal biomass associated with the litter were measured. 3. The results indicate that both PAR and UVA affect the micro-organisms involved in the decomposition of leaf litter. The α/β-D-glucosidase activity ratio was less than one in irradiated and more than one in shaded microcosms, suggesting a change in the substrate dissolved organic matter composition towards more β- than α-glycosidic linkages as a result of solar radiation. 4. Microcosms receiving UVB displayed a significantly higher β-D-glucosidase activity than shaded microcosms, and those exposed to PAR or PAR + UVA, demonstrating the potential importance of UVB radiation. 5. The free-living bacteria tended to be dominated by filamentous forms in microcosms subject to solar radiation, especially PAR, and attached microbial communities showed a greater tendency to be dominated by bacteria in irradiated microcosms than in shaded microcosms.     

Effects of Solar Radiation on the Patagonian Rhodophyte Corallina officinatis (L.)

Experiments were conducted in Patagonian waters (Argentina) to assess the impact of solar radiation (PAR, 400-700 nm, and UVR, 280-400 nm) upon two strains of the red alga Corallina officinalis Linnaeus, characteristic of the mid and lower intertidal zone, during March 2000. Fluorescence parameters were determined using a pulse amplitude modulated (PAM) fluorometer. The two strains had different initial optimal quantum yields but similar strong decreases in the quantum yield when the algae were exposed to short-term solar radiation and similar recovery characteristics in dim light. The quantum yield had the lowest values at noon, but it increased in the afternoon / evening hours, when irradiances were lower. PAR (irradiance at noon about 500 W m(-2)) was responsible for most of the decrease in the yield ( approximately 50%) on clear days, with UVR accounting for a significant increment. However, on cloudy days the UVR component caused an even more pronounced decrease. In their natural environment, specimens in the shade had a higher effective quantum yield than in sun-lit areas. Fluence rate response curves indicated that thalli from the mid intertidal had a pronounced nonphotochemical quenching at intermediate and higher irradiances; however, this was not observed in the thalli from the lower intertidal. Fast induction and relaxation kinetics showed obvious differences between the two strains, but also demonstrated a rapid adaptation of the species to the changing light conditions as well as a fast decrease of PS II fluorescence upon exposure to solar radiation. All photosynthetic pigments were bleached during exposure to solar radiation over a full day. Strong absorption in the UV-A range, most likely due to mycosporine like amino acids, was determined in both strains. The study of the differential sensitivity to solar radiation and recovery capacity of these Corallina strains, as well as the presence of protective compounds, suggests that a combination of mechanisms allow these algae to adapt to the relatively high radiation levels and fast changes in irradiance found in the Patagonian area at this time of the year.     

Differential responses of tetrasporophytes and gametophytes of Mazzaella laminarioides (Gigartinales,Rhodophyta) under solar UV radiation

The effects of solar UV radiation on mycosporine‐like amino acids (MAAs), growth, photosynthetic pigments (Chl a, phycobiliproteins), soluble proteins (SP), and C and N content of Mazzaella laminarioides tetrasporophytes and gametophytes were investigated. Apical segments of tetrasporophytes and gametophytes were exposed to solar radiation under three treatments (PAR [P], PAR+UVA [PA], and PAR+UVA+UVB [PAB]) during 18 d in spring 2009, Punta Arenas, Chile. Samples were taken after 2, 6, 12, and 18 d of solar radiation exposure. Most of the parameters assessed on M. laminarioides were significantly influenced by the radiation treatment, and both gametophytes and tetrasporophytes seemed to respond differently when exposed to high UV radiation. The two main effects promoted by UV radiation were: (i) higher synthesis of MAAs in gametophytes than tetrasporophytes at 2 d, and (ii) a decrease in phycoerythrin, phycocyanin, and SPs, but an increase in MAA content in tetrasporophytes at 6 and 12 d of culture. Despite some changes that were observed in biochemical parameters in both tetrasporophytes and gametophytes of M. laminarioides when exposed to UVB radiation, these changes did not promote deleterious effects that might interfere with the growth in the long term (18 d). The tolerance and resistance of M. laminarioides to higher UV irradiance were expected, as this intertidal species is exposed to variation in solar radiation, especially during low tide.     

IMPACTS OF SOLAR UV RADIATION ON THE PHOTOSYNTHESIS,GROWTH, AND UV‐ABSORBING COMPOUNDS IN GRACILARIA LEMANEIFORMIS (RHODOPHYTA) GROWN AT DIFFERENT NITRATE CONCENTRATIONS1

Solar ultraviolet radiation (UVR, 280–400 nm) is known to affect macroalgal physiology negatively, while nutrient availability may affect UV‐absorbing compounds (UVACs) and sensitivity to UVR. However, little is known about the interactive effects of UVR and nitrate availability on macroalgal growth and photosynthesis. We investigated the growth and photosynthesis of the red alga Gracilaria lemaneiformis (Bory) Grev. at different levels of nitrate (natural or enriched nitrate levels of 41 or 300 and 600 μM) under different solar radiation treatments with or without UVR. Nitrate‐enrichment enhanced the growth, resulted in higher concentrations of UVACs, and led to negligible photoinhibition of photosynthesis even at noon in the presence of UVR. Net photosynthesis during the noon period was severely inhibited by both ultraviolet‐A radiation (UVA) and ultraviolet‐B radiation (UVB) in the thalli grown in seawater without enriched nitrate. The absorptivity of UVACs changed in response to changes in the PAR dose when the thalli were shifted back and forth from solar radiation to indoor low light, and exposure to UVR significantly induced the synthesis of UVACs. The thalli exposed to PAR alone exhibited higher growth rates than those that received PAR + UVA or PAR + UVA + UVB at the ambient or enriched nitrate concentrations. UVR inhibited growth approximately five times as much as it inhibited photosynthesis within a range of 60–120 μg UVACs · g^?1 (fwt) when the thalli were grown under nitrate‐enriched conditions. Such differential inhibition implies that other metabolic processes are more sensitive to solar UVR than photosynthesis.     

Light response curves for Gelidium sesquipedale from different depths, determined by two methods: O2 evolution and chlorophyll fluorescence

Photosynthesis-light response curves of Gelidium sesquipedale from the west coast of Portugal (Cape Espichel) were determined at four different depths, 3, 10, 15 and 22 m. Data acquisition using chlorophyll a fluorescence methodology and oxygen electrode measurements were compared. Response curves were determined over an increasing range of irradiance values (I), from darkness to 900 μmol photon m-2 s-1 PAR. In general, light response curves obtained for G. sesquipedale showed a similar pattern whether determined by the chlorophyll fluorescence method or by oxygen evolution. The photosynthetic capacity of G. sesquipedale decreased with depth, as expected, revealing a ‘sun’ and ‘shade’ acclimation pattern, between shallow and deeper waters. This revised version was published online in June 2006 with corrections to the Cover Date.     

Research note: Irradiance of photosynthetically active radiation determines ultraviolet-susceptibility of photosynthesis in Ulva lactuca L.(Chlorophyta)

The respective ratio of photosynthetically active to ultraviolet radiation is of crucial importance to results obtained in ultraviolet (UV)‐research on photoautotrophic organisms. Specimens of the green macroalga Ulva lactuca L. were exposed to a constant irradiance of UV‐radiation at increasing irradiances of photosynthetically active radiation (PAR). The effects of experimental irradiance and spectral composition on photoinhibition of photosynthesis and its recovery were monitored by chlorophyll fluorescence measurements and the activity of the xanthophyll cycle was assessed by high performance liquid chromatography‐(HPLC) based pigment analysis. Results indicate a UV‐induced delay in recovery from PAR‐induced photoinhibition and a deceleration of violaxanthin conversion within the xanthophyll cycle due to the presence of UV‐radiation. Also the concentration of the protective pigment lutein increased considerably and could be indicative of the existence of an additional light‐protective mechanism, as, for example, the lutein‐epoxid cycle in Ulva. In total, results clearly show that the extent of UV‐induced inhibition of photosynthesis to be found in UV‐exposure experiments is highly dependent on the irradiance of background photosynthetically active radiation: with increasing irradiance of PAR the UV‐effects were diminished. Exemplified by the green algae Ulva lactuca this study demonstrates the crucial importance of the ratios of PAR:UV applied in UV‐research, particularly when conducting laboratory experiments in an ecological context.     

NUTRIENT LIMITATION AND HIGH IRRADIANCE ACCLIMATION REDUCE PAR AND UV‐INDUCED VIABILITY LOSS IN THE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE)1

The effects of high PAR (400–700 nm), UVA (315–400 nm), and UVB (280–315 nm) radiation on viability and photosynthesis were investigated for Chaetoceros brevis Schütt. This Antarctic marine diatom was cultivated under low, medium, and high irradiance and nitrate, phosphate, silicate, and iron limitation before exposure to a simulated surface irradiance (SSI) treatment, with and without UVB radiation. Light‐harvesting and protective pigment composition and PSII parameters were determined before SSI exposure, whereas viability was measured by flow cytometry in combination with a viability stain after the treatment. Recovery of PSII efficiency was measured after 20 h in dim light in a separate experiment. In addition, low and high irradiance acclimated cells were exposed outdoors for 4 h to assess the effects of natural PAR, UVA, and UVB on viability. Low irradiance acclimated cells were particularly sensitive to photo induced viability loss, whereas no viability loss was found after acclimation to high irradiance. Furthermore, nutrient limitation reduced sensitivity to photo induced viability loss, relative to nutrient replete conditions. No additional viability loss was found after UVB exposure. Sunlight exposed cells showed no additional UVB effect on viability, whereas UVA and PAR significantly reduced the viability of low irradiance acclimated cells. Recovery of PSII function was nearly complete in cultures that survived the light treatments. Increased resistance to high irradiance coincided with an increased ratio between protective‐ and light‐harvesting pigments before the SSI treatment, demonstrating the importance of nonphotochemical quenching by diatoxanthin for survival of near‐surface irradiance. We conclude that a sudden transfer to high irradiance can be fatal for low irradiance acclimated C. brevis.     

UVB radiation may ameliorate photoinhibition in specific shallow-water tropical marine macrophytes

Anthropogenic stratospheric ozone depletion causes an increase in UVB radiation impinging on the earth's surface, which is a threat to plants not adapted to higher UVB irradiances. Investigations were carried out among tropical marine macrophytes, Turbinaria turbinata, Sargassum polyceratium var. ovatum, Padina sanctae-crucis, Lobophora variegate, Dictyota spec., Halimeda discoidea, Udotea flabellum, Thalassia testudinum and Syringodium filiforme collected from 0.3 to 26 m depths at the Belizean barrier reef, where ultraviolet radiation (UV)-irradiances are naturally high. Photoinhibition was induced under full solar, UV (UVA + UVB), and UVA only-depleted radiation conditions. Photosynthetic activity during high radiation stress and during recovery in reduced solar radiation was determined in vivo by measuring fluorescence changes using a PAM fluorometer device. Generally, UV caused an additional decrease of photosynthetic performance during high light stress which varies according to species, depth of growth and UV penetration at the site of collection; an observation in concordance with the conventional harmful UV-radiation effects on phototrophs. When solar radiation was reduced by 50%, significant photosynthetic recovery was observed. However, some shallow water species which are adapted to high UV were observed to recover less under treatment with depleted solar UVB radiation. Our result supports earlier reports that UVB causes not only negative effects on photosynthesis, but may also facilitate or induce recovery processes in aquatic macrophytes acclimated to high solar radiation which grow at the upper shoreline. Among the eulittoral macroalgae, e.g. Dictyota spec., P. sanctae-crucis, and H. discoidea and the seagrass T. testudinum, initiation of photosynthetic recovery processes in the presence of low irradiance of short UV-wavelengths may present an ecophysiological advantage compared to macrophytes which initiate photosynthetic recovery process during low light or in the absence of UV.     

Cyclic AMP levels in several macroalgae and their relation to light quantity and quality

Total cAMP levels were measured in the macroalgae Dictyota dichotoma, Gelidium sesquipedale and Ulva rigida under different light conditions in order to study its regulation either by phytochrome or photosynthesis. Incubation in red or far-red light did not promote a phytochrome-like response; instead, it showed a synergistic effect upon cAMP accumulation. cAMP levels seemed to depend on the amount of energy applied. The correlation between photosynthetic oxygen evolution and cAMP variations at sub-saturating white light irradiance pointed to photosynthetic electron transport as involved in the regulation of cAMP accumulation at least in G. sesquipedale and U. rigida. Inhibitors of thylakoidal and mitochondrial electron transport chains reduced cAMP levels in 70 to 99%. We conclude that cAMP accumulation could be regulated by photosynthetic activity rather than phytochrome in the macroalgae studied.     

Exposure to ultraviolet radiation delays photosynthetic recovery in Arctic kelp zoospores

Seasonal reproduction in some Arctic Laminariales coincides with increased UV-B radiation due to stratospheric ozone depletion and relatively high water temperatures during polar spring. To find out the capacity to cope with different spectral irradiance, the kinetics of photosynthetic recovery was investigated in zoospores of four Arctic species of the order Laminariales, the kelps Saccorhiza dermatodea, Alaria esculenta, Laminaria digitata, and Laminaria saccharina. The physiology of light harvesting, changes in photosynthetic efficiency and kinetics of photosynthetic recovery were measured by in vivo fluorescence changes of Photosystem II (PSII). Saturation irradiance of freshly released spores showed minimal I _k values (photon fluence rate where initial slope intersects horizontal asymptote of the curve) values ranging from 13 to 18 μmol photons m⁻² s⁻¹ among species collected at different depths, confirming that spores are low-light adapted. Exposure to different radiation spectra consisting of photosynthetically active radiation (PAR; 400–700 nm), PAR+UV-A radiation (UV-A; 320–400 nm), and PAR+ UV-A+UV-B radiation (UV-B; 280–320 nm) showed that the cumulative effects of increasing PAR fluence and the additional effect of UV-A and UV-B radiations on photoinhibition of photosynthesis are species specific. After long exposures, Laminaria saccharina was more sensitive to the different light treatments than the other three species investigated. Kinetics of recovery in zoospores showed a fast phase in S. dermatodea, which indicates a reduction of the photoprotective process while a slow phase in L. saccharina indicates recovery from severe photodamage. This first attempt to study photoinhibition and kinetics of recovery in zoospores showed that zoospores are the stage in the life history of seaweeds most susceptible to light stress and that ultraviolet radiation (UVR) effectively delays photosynthetic recovery. The viability of spores is important on the recruitment of the gametophytic and sporophytic life stages. The impact of UVR on the zoospores is related to the vertical depth distribution of the large sporophytes in the field.     

Differential responses to UVR by bacterioplankton and phytoplankton from the surface and the base of the mixed layer

SUMMARY 1. We tested the influence of ultraviolet radiation (UVR) and shallow stratification on phytoplankton and bacterioplankton from the surface and the base of the mixed layer in two boreal lakes in north-western Ontario, Canada.
2. We measured phytoplankton biomass and production, bacterioplankton production and plankton respiration after transplantation under three solar radiation treatments: ambient radiation (Photosynthetically active radiation (PAR) + ultraviolet-A (UVA) + ultraviolet-B (UVB)), minus UVB (PAR + UVA) and PAR only. We repeated this experiment on three occasions in each lake during the summer.
3. Solar stress (measured as reduced growth and photoinhibition) was generally only found in the 'base phytoplankton' (i.e. originating from the base of the mixed layer). No inhibition of photosynthesis by UVB exposure was found in near-surface phytoplankton. On the other hand, production of near-surface bacterioplankton was reduced following a 4-h UVR exposure but had increased after a 48-h exposure to both UVA and UVB compared with the PAR only treatment.
4. Negative effects of UVR on phytoplankton and bacterioplankton were not ubiquitous. We emphasise the importance of conducting experiments repeatedly, particularly those which test the effects of UVR on different community assemblages from different lakes.     

UV‐A/BLUE LIGHT–INDUCED REACTIVATION OF SPORE GERMINATION IN UV‐B IRRADIATED ULVA PERTUSA (CHLOROPHYTA)1

Recent reduction in the ozone shield due to manufactured chlorofluorocarbons raised considerable interest in the ecological and physiological consequences of UV‐B radiation (λ=280–315 nm) in macroalgae. However, early life stages of macroalgae have received little attention in regard to their UV‐B sensitivity and UV‐B defensive mechanisms. Germination of UV‐B irradiated spores of the intertidal green alga Ulva pertusa Kjellman was significantly lower than in unexposed controls, and the degree of reduction correlated with the UV doses. After exposure to moderate levels of UV‐B irradiation, subsequent exposure to visible light caused differential germination in an irradiance‐ and wavelength‐dependent manner. Significantly higher germination was found at higher photon irradiances and in blue light compared with white and red light. The action spectrum for photoreactivation of germination in UV‐B irradiated U. pertusa spores shows a major peak at 435 nm with a smaller but significant peak at 385 nm. When exposed to December sunlight, the germination percentage of U. pertusa spores exposed to 1 h of solar radiation reached 100% regardless of the irradiation treatment conditions. After a 2‐h exposure to sunlight, however, there was complete inhibition of germination in PAR+UV‐A+UV‐B in contrast to 100% germination in PAR or PAR+UV‐A. In addition to mat‐forming characteristics that would act as a selective UV‐B filter for settled spores under the parental canopy, light‐driven repair of germination after UV‐B exposure could explain successful continuation of U. pertusa spore germination in intertidal settings possibly affected by intense solar UV‐B radiation.     

Sensitivity of Laminariales zoospores from Helgoland (North Sea) to ultraviolet and photosynthetically active radiation: implications for depth distribution and seasonal reproduction

Depth distribution of kelp species in Helgoland (North Sea) is characterized by occurrence of Laminaria digitata in the upper sublittoral, whereas L. saccharina and L. hyperborea dominate the mid and lower sublittoral region. Laminaria digitata is fertile in summer whereas both other species are fertile in autumn/winter. To determine the light sensitivity of the propagules, zoospores of L. digitata, L. saccharina and L. hyperborea were exposed in the laboratory to different exposure times of photosynthetically active radiation (PAR; 400–700 nm), PAR + UVA radiation (UVAR; 320–400 nm) and PAR + UVAR + UVB radiation (UVBR; 280–320 nm). Optimum quantum yield of PSII and DNA damage were measured after exposure. Subsequently, recovery of photosynthetic efficiency and DNA damage repair, as well as germination rate were measured after 2 and 3 d cultivation in dim white light. Photosynthetic efficiency of all species was photoinhibited already at 20 µmol photons m⁻² s⁻¹ PAR, whereas UV radiation (UVR) had a significant additional effect on photoinhibition. Recovery of the PSII function was observed in all species but not in spores exposed to irradiation longer than 4 h of PAR + UVA + UVB and 8 h of PAR + UVA. The amount of UVB-induced DNA damage measured as cyclobutane–pyrimidine dimers (CPDs) increased with exposure time and highest damage was detected in the spores of lower subtidal L. hyperborea relative to the other two species. Significant removal of CPDs indicating repair of DNA damage was observed in all species after 2 d in low white light especially in the spores of upper subtidal L. digitata. Therefore, efficient DNA damage repair and recovery of PSII damage contributed to the germination success but not in spores exposed to 16 h of UVBR. UV absorption of zoospore suspension in L. digitata is based both on the absorption by the zoospores itself as well as by exudates in the medium. In contrast, the absorption of the zoospore suspension in L. saccharina and L. hyperborea is based predominantly on the absorption by the exudates in the medium. This study indicates that UVR sensitivity of zoospores is related to the seasonal zoospore production as well as the vertical distribution pattern of the large sporophytes.     

BRYOZOANS INCREASE AVAILABLE CO2 FOR PHOTOSYNTHESIS IN GELIDIUM SESQUIPEDALE (RHODOPHYCEAE)

A possible benefit of the presence of the epiphytic bryozoan Electra pilosa (L.) for the red macroalga Gelidium sesquipedale (Clem.) Thuret et Bornet is described. Absorption spectra and photosynthetic parameters of O₂ evolution vs. irradiance curves were determined for both epiphytized and nonepiphytized thalli. The absorptance of G. sesquipedale thalli for PAR was not modified by the presence of the epiphyte. Gross photosynthetic rates at saturating light were approximately doubled in epiphytized thalli. Photosynthesis by G. sesquipedale was enhanced when CO₂ concentration was increased in the medium by a decrease in pH. On the other hand, an increase in pH from 8.1 to 8.7 produced a significant reduction of the O₂ evolution rates indicating that G. sesquipedale has a very low capacity to use HCO₃⁻. The decrease in photosynthesis at high pH was higher in nonepiphytized thalli than in epiphytized ones, suggesting that the amount of available CO₂ is higher in the presence of E. pilosa. This positive effect was attributed to the CO₂ released by respiration of the epiphyte.     

Ultraviolet radiation,ozone depletion,and marine photosynthesis

Concerns about stratospheric ozone depletion have stimulated interest in the effects of UVB radiation (280–320 nm) on marine phytoplankton. Research has shown that phytoplankton photosynthesis can be severely inhibited by surface irradiance and that much of the effect is due to UV radiation. Quantitative generalization of these results requires a biological weighting function (BWF) to quantify UV exposure appropriately. Different methods have been employed to infer the general shape of the BWF for photoinhibition in natural phytoplankton, and recently, detailed BWFs have been determined for phytoplankton cultures and natural samples. Results show that although UVB photons are more damaging than UVA (320–400 nm), the greater fluxes of UVA in the ocean cause more UV inhibition. Models can be used to analyze the sensitivity of water column productivity to UVB and ozone depletion. Assumptions about linearity and time-dependence strongly influence the extrapolation of results. Laboratory measurements suggest that UV inhibition can reach a steady-state consistent with a balance between damage and recovery processes, leading to a non-linear relationship between weighted fluence rate and inhibition. More testing for natural phytoplankton is required, however. The relationship between photoinhibition of photosynthesis and decreases in growth rate is poorly understood, so long-term effects of ozone depletion are hard to predict. However, the wide variety of sensitivities between species suggests that some changes in species composition are likely. Predicted effects of ozone depletion on marine photosynthesis cannot be equated to changes in carbon flux between the atmosphere and ocean. Nonetheless, properly designed studies on the effects of UVB can help identify which physiological and ecological processes are most likely to dominate the responses of marine ecosystems to ozone depletion.Abbreviations BWF biological weighting function - BWF/P-I photosynthesis versus photosynthetically available irradiance as influenced by biologically-weighted UV - Chl chlorophyll a - DOM dissolved organic matter - E _PAR irradiance in energy units (PAR) - E _s saturation parameter for PAR in the BWF/P-I model - E _inh ^* biologically-weighted dimensionless fluence rate for photoinhibition of photosynthesis by UV and PAR - biological weighting coefficient - % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGafqyTduMbae% baaaa!37AC!\[\bar \varepsilon \]_PAR biological weighting coefficient for damage to photosynthesis by E _PAR - k() diffuse attenuation coefficient for wavelength - MAAs mycosporine-like amino acids - PAR photosynthetically available radiation - P ^B rate of photosynthesis normalized to Chl - P _s ^B maximum attainable rate of photosynthesis in the absence of photoinhibition - UVA ultraviolet A (320–400 nm) - UVB ultraviolet B (280–320 nm)     

Seasonal patterns in the sunlight sensitivity of bacterioplankton from Mediterranean surface coastal waters

The sensitivity of coastal marine bacterioplankton to natural photosynthetically active radiation (PAR, 400-700?nm) and ultraviolet radiation (UVR, 280-400?nm) was evaluated in five experiments over a seasonal cycle in the Blanes Bay, NW Mediterranean Sea. Exposure to natural solar radiation generally inhibited bulk bacterial activities or damaged membrane integrity when irradiances were high (i.e. spring and summer experiments) and, in general, UVB (280-320?nm) accounted for most of the inhibition. When assessing activity ((3) H-leucine uptake) at the single-cell level by microautoradiography and rRNA gene probing, seasonally varying responses and sensitivities were found among bacterial groups. While autumn and winter irradiances seemed too low to cause changes in activity, variable effects were found in spring and summer. SAR11 was consistently inhibited by UVR and PAR exposure, whereas Gammaproteobacteria and Bacteroidetes showed higher resistance. Roseobacter, Synechococcus and the NOR5 clade were occasionally photostimulated in their activity, mainly because of PAR. Our results indicate that a component of seasonality exists in the bacterial responses to solar radiation, which vary not only depending on the irradiance and the spectral characteristics, but also on the previous light history and the taxonomic composition of the community.     

Effects of ultraviolet radiation on the transmission process of an intertidal trematode parasite

The transmission of parasites takes place under exposure to a range of fluctuating environmental factors, one being the changing levels of solar ultraviolet radiation (UVR). Here, we investigated the effects of ecologically relevant levels of UVR on the transmission of the intertidal trematode Maritrema novaezealandensis from its first intermediate snail host (Zeacumantus subcarinatus) to its second intermediate amphipod host (Paracalliope novizealandiae). We assessed the output of parasite transmission stages (cercariae) from infected snail hosts, the survival and infectivity of cercariae, the susceptibility of amphipod hosts to infection (laboratory experiments) and the survival of infected and uninfected amphipod hosts (outdoor experiment) when exposed to photo-synthetically active radiation only (PAR, 400-700 nm; no UV), PAR+UVA (320-700 nm) or PAR+UVA+UVB (280-700 nm). Survival of cercariae and susceptibility of amphipods to infection were the only two steps significantly affected by UVR. Survival of cercariae decreased strongly in a dose-dependent manner, while susceptibility of amphipods increased after exposure to UVR for a prolonged period. Exposure to UVR thus negatively affects both the parasite and its amphipod host, and should therefore be considered an influential component in parasite transmission and host-parasite interactions in intertidal ecosystems.     

Contribution of the UV component of natural sunlight to photoinhibition of photosynthesis in six species of subtidal brown and red seaweeds

Field‐collected specimens of three species of Laminaria and three species of subtidal red algae (Delesseria sanguinea, Plocamium cartilagineum and Phyllophora pseudoceranoides) were exposed to natural summer sunlight on Helgoland (southern North Sea) for up to 4 h at 15 °C. Dark‐adapted variable fluorescence (F_v : F_m) was measured immediately after these treatments, and following 6, 24 and 48 h of recovery in moderate irradiances of white light. The response of plants to the full spectrum of natural sunlight was compared with that to PAR alone, UV‐A + visible, UV‐A + UV‐B, or UV‐A alone. The F_v : F_m values of all species were reduced to minimal values after 4 h in all of these treatments, but those of the more resistant species (Laminaria spp. and P. pseudoceranoides) were higher after shorter exposures to UV radiation alone than to PAR with or without UV. The recovery of F_v : F_m in all species was also more rapid in the two treatments that contained UV radiation alone than in those that included PAR. These results suggest that it is the high irradiances of PAR in natural sunlight which are responsible for the photoinhibition of photosynthesis of subtidal seaweeds and that the current ambient irradiances of UV radiation (either UV‐B or UV‐A) in northern temperate latitudes would not contribute significantly to this photoinhibition.     

Increase in photosynthetic efficiency as a strategy of planktonic organisms exploiting deep lake layers

1. The photosynthetic efficiencies of the mixotrophic ciliate Ophrydium naumanni and the autotrophic dinoflagellate Gymnodinium paradoxum were investigated using laboratory and field experiments in Lake Moreno Oeste (41°5′S and 71°33′W, 758 m a.s.l.), in the Nahuel Huapi System (North Patagonia, Argentina). 2. The effect of different underwater light intensities on net primary production (NPP) was assessed during one summer. Additionally, laboratory experiments were carried out to obtain photosynthesis‐irradiance response curves for each species. 3. Ophrydium naumanni and G. paradoxum dominated the metalimnetic (30 m depth) deep chlorophyll maximum (DCM) in the lake. 4. Despite these deep higher abundances, the cell‐specific production of both species was higher at 10 m than at 30 m (DCM) depth. In addition, at 5 m depth, NPP was reduced by PAR + UV‐A radiation. 5. Both species exhibited a positive NPP at very low irradiance but the mixotrophic ciliate was more efficient in exploiting the DCM irradiance level both in situ and at comparable light intensities in laboratory experiments. Light acclimatised O. naumanni showed a higher NPP at lower irradiances and photoinhibition at medium and high irradiances. 6. Under the strong wind‐driven turbulence commonly found in Patagonian lakes, organisms cannot select their position in the epilimnetic water column and will be dragged to potentially harmful UV radiation levels. Thus, metalimnetic DCM colonisation by these two species represents a tradeoff between higher survival and lower cell‐specific NPP.     

SENSITIVITY OF ANTARCTIC UROSPORA PENICILLIFORMIS (ULOTRICHALES,CHLOROPHYTA) TO ULTRAVIOLET RADIATION IS LIFE‐STAGE DEPENDENT1

The sensitivity of different life stages of the eulittoral green alga Urospora penicilliformis (Roth) Aresch. to ultraviolet radiation (UVR) was examined in the laboratory. Gametophytic filaments and propagules (zoospores and gametes) released from filaments were separately exposed to different fluence of radiation treatments consisting of PAR (P = 400–700 nm), PAR + ultraviolet A (UVA) (PA, UVA = 320–400 nm), and PAR + UVA + ultraviolet B (UVB) (PAB, UVB = 280–320 nm). Photophysiological indices (ETR_max, E_k, and α) derived from rapid light curves were measured in controls, while photosynthetic efficiency and amount of DNA lesions in terms of cyclobutane pyrimidine dimers (CPDs) were measured after exposure to radiation treatments and after recovery in low PAR; pigments of propagules were quantified after exposure treatment only. The photosynthetic conversion efficiency (α) and photosynthetic capacity (rETR_max) were higher in gametophytes compared with the propagules. The propagules were slightly more sensitive to UVB‐induced DNA damage; however, both life stages of the eulittoral inhabiting turf alga were not severely affected by the negative impacts of UVR. Exposure to a maximum of 8 h UVR caused mild effects on the photochemical efficiency of PSII and induced minimal DNA lesions in both the gametophytes and propagules. Pigment concentrations were not significantly different between PAR‐exposed and PAR + UVR–exposed propagules. Our data showed that U. penicilliformis from the Antarctic is rather insensitive to the applied UVR. This amphi‐equatorial species possesses different protective mechanisms that can cope with high UVR in cold‐temperate waters of both hemispheres and in polar regions under conditions of increasing UVR as a consequence of further reduction of stratospheric ozone.