Combined effects of light and nitrate supplies on the growth,photosynthesis and ultraviolet‐absorbing compounds in marine macroalga Gracilaria lemaneiformis (Rhodophyta), with special reference to the effects of solar ultraviolet radiation

It is well known that light and nutrients are essential to plants; however, there are few investigations in which these have been studied in combination on macroalgae, especially when solar ultraviolet radiation (UVR) is concerned. We cultured the red alga Gracilaria lemaneiformis (Bory) at different nitrate concentrations and light levels with or without UVR for 24 days. The results showed that nitrate supply markedly enhanced the growth and photosynthesis, increased the absorptivity of UV‐absorbing compounds (UVACs), and decreased photoinhibition in the presence of UVR. The thalli that received photosynthetically active radiation (PAR) treatment exhibited higher growth rates than those that received PAR + UVR at ambient or enhanced nitrate concentrations. However, under PAR + UVR treatment, the absorptivity of UVACs was higher than that of PAR and fluctuated with light levels. UVR was found to reduce the maximal net photosynthetic rate, apparent photosynthetic efficiency and light‐saturating irradiance while increasing the dark respiration rate, and inducing higher inhibition of growth and photosynthesis under high light versus under low light. Ultraviolet B significantly induced the synthesis of UVACs but led to higher inhibition on growth and photosynthesis than ultraviolet A.     

Combined effects of ocean acidification and solar UV radiation on photosynthesis,growth, pigmentation and calcification of the coralline alga Corallina sessilis (Rhodophyta)

Previous studies have shown that increasing atmospheric CO₂ concentrations affect calcification in some planktonic and macroalgal calcifiers due to the changed carbonate chemistry of seawater. However, little is known regarding how calcifying algae respond to solar UV radiation (UVR, UVA+UVB, 280–400 nm). UVR may act synergistically, antagonistically or independently with ocean acidification (high CO₂/low pH of seawater) to affect their calcification processes. We cultured the articulated coralline alga Corallina sessilis Yendo at 380 ppmv (low) and 1000 ppmv (high) CO₂ levels while exposing the alga to solar radiation treatments with or without UVR. The presence of UVR inhibited the growth, photosynthetic O₂ evolution and calcification rates by13%, 6% and 3% in the low and by 47%, 20% and 8% in the high CO₂ concentrations, respectively, reflecting a synergistic effect of CO₂ enrichment with UVR. UVR induced significant decline of pH in the CO₂‐enriched cultures. The contents of key photosynthetic pigments, chlorophyll a and phycobiliproteins decreased, while UV‐absorptivity increased under the high pCO₂/low pH condition. Nevertheless, UV‐induced inhibition of photosynthesis increased when the ratio of particulate inorganic carbon/particulate organic carbon decreased under the influence of CO₂‐acidified seawater, suggesting that the calcified layer played a UV‐protective role. Both UVA and UVB negatively impacted photosynthesis and calcification, but the inhibition caused by UVB was about 2.5–2.6 times that caused by UVA. The results imply that coralline algae suffer from more damage caused by UVB as they calcify less and less with progressing ocean acidification.     

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.     

THE CONCHOCELIS OF PORPHYRA HAITANENSIS (RHODOPHYTA) IS PROTECTED FROM HARMFUL UV RADIATION BY THE COVERING CALCAREOUS MATRIX1

Previous study has shown that Porphyra conchocelis is sensitive to high levels of PAR (400–700 nm) as well as ultraviolet radiation (UVR: 280–400 nm), resulting in high inhibition of photosynthesis. However, little is known about whether the inner covering layer of the shell, in which the conchocelis lives, may provide protection against solar UVR. Our study indicates that the covering calcareous matrix is about 0.06 mm thick, transmitting 63, 47, and 28% of PAR, ultraviolet radiation A (UVA: 315–400 nm), and ultraviolet radiation B (UVB: 280–315 nm), respectively. We used a shading layer that simulated the above transmissions, and the effective quantum yield of PSII and photosynthetic carbon fixation in the conchocelis increased to greater extents in the presence of UVA or UVB. Attenuation of UVA by 19% and UVB by 37% due to the shading layer increased the PSII yield by 44%–77% and photosynthetic carbon fixation by about 60%. Our study clearly shows that the photosynthetic machinery of Porphyra haitanensis T. J. Chang et B. F. Zheng conchocelis was efficiently protected from harmful UVR by the covering calcareous matrix.     

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.     

RESPONSE OF LAMINARIA SACCHARINA (PHAEOPHYTA) GROWTH AND PHOTOSYNTHESIS TO SIMULTANEOUS ULTRAVIOLET RADIATION AND NITROGEN LIMITATION1

The brown macroalga Laminaria saccharina (L.) J. V. Lamour. was grown in large outdoor tanks at 50% ambient solar radiation for 3–4 weeks in July and August of 2000, 2001, and 2002, in either ambient or nitrogen (N)–enriched seawater and in either ambient light [PAR + ultraviolet radiation (UVR)] or ambient light minus UVR. Growth, N‐content, photosynthetic pigments, and RUBISCO content increased in N‐enriched seawater, indicating N‐limitation. UVR inhibited growth, but this inhibition was ameliorated by N‐enrichment. The response of growth to UVR could not be explained by changes in respiration and photosynthesis. Gross light‐saturated photosynthesis (P_max) remained unaffected by UVR but was significantly higher under N‐enrichment, as was dark respiration (R_d). UVR had no effect on pigments or N content. However, RUBISCO contents were low in the presence of UVR, reflecting the overall change in soluble cellular protein. Overall, our data indicate that the response to UVR in L. saccharina depends on other environmental factors, such as N, and these effects need to be considered when evaluating the response of macroalgae to increased UVR.     

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.     

BIOLOGICAL WEIGHTING FUNCTIONS FOR UV INHIBITION OF PHOTOSYNTHESIS IN THE KELP LAMINARIA HYPERBOREA (PHAEOPHYCEAE)1

Different wavelengths of sunlight either drive or inhibit macroalgal production. Ultraviolet radiation (UVR) effectively disrupts photosynthesis, but since UVR is rapidly absorbed in coastal waters, macroalgal photoinhibition and tolerance to UVR depend on the depth of attachment and acclimation state of the individual. The inhibition response to UVR is quantified with a biological weighting function (BWF), a spectrum of empirically derived weights that link irradiance at a specific wavelength to overall biological effect. We determined BWFs for shallow (0 m, mean low water [MLW]) and deep (10 m) Laminaria hyperborea (Gunnerus) Foslie collected off the island of Finnøy, Norway. For each replicate sporophyte, we concurrently measured both O₂ evolution and ¹³C uptake in 48 different light treatments, which varied in UV spectral composition and irradiance. The relative shape of the kelp BWF was most similar to that of a land plant, and the absolute spectral weightings and sensitivity were typically less than phytoplankton, particularly in the ultraviolet radiation A (UVA) region. Differences in BWFs between O₂ and ¹³C photosynthesis and between shallow (high light) and deep (low light) kelp were also most significant in the UVA. Because of its greater contribution to total incident irradiance, UVA was more important to daily loss of production in kelp than ultraviolet radiation B (UVB). Photosynthetic quotient (PQ) also decreased with increased UVR stress, and the magnitude of PQ decline was greater in deepwater kelp. Significantly, BWFs assist in the comparison of biological responses to experimental light sources versus in situ sunlight and are critical to quantifying kelp production in a changing irradiance environment.     

SPOROGENESIS UNDER ULTRAVIOLET RADIATION IN LAMINARIA DIGITATA (PHAEOPHYCEAE) REVEALS PROTECTION OF PHOTOSENSITIVE MEIOSPORES WITHIN SORAL TISSUE: PHYSIOLOGICAL AND ANATOMICAL EVIDENCE1

To study the effect of different radiation conditions on sporogenesis of Laminaria digitata (Huds.) J. V. Lamour., excised disks were induced to form sporangia under PAR (P), PAR + ultraviolet‐A (UVA) (PA), and PAR + UVA + ultraviolet‐B (UVB) (PAB) conditions in the laboratory. Vitality of meiospores, released from sori induced under different radiation conditions in the laboratory and from sori of wild sporophytes acclimated to in situ solar radiation in the presence and absence of ultraviolet radiation (UVR), was measured in terms of their germination capacity. Sorus induction in disks of laboratory‐grown sporophytes was not hampered under light supplemented with UVR, and sorus area was not significantly different among P, PA, and PAB. Vitality and germination rate of meiospores released from sori induced under different radiation treatments was comparable. Likewise, screening of UVR of the natural solar radiation did not promote higher germination rates of meiospores released from wild sporophytes. Germination rates were, however, higher in meiospores released from laboratory‐induced sori compared to sori of wild sporophytes. Higher DNA damage (formation of cyclobutane pyrimidine dimers, CPDs) was observed in laboratory‐grown nonsorus compared to sorus tissue, while CPDs were nondetectable in both sorus and nonsorus tissue of wild sporophytes. To explain the apparent protection of developing meiospores and the unexpected UV resistance of soral tissue, concurrent anatomical investigations of sporogenic tissue were performed. We observed the previously unreported existence of two types of sterile paraphysis cells. One type of paraphysis cells, the most frequent type, contained several red‐fluorescing plastids. The other type, less frequently occurring, was completely filled with substances emitting blue fluorescence under violet excitation, presumably brown algal phenolic compounds (phlorotannins). Cells of this type were irregularly scattered within the sorus and did not contain red‐fluorescing plastids. Meiospore‐containing sporangia were positioned embedded between both types of paraphysis cells. In vegetative tissue, blue autofluorescence was observed only in injured parts of the blade. Results of our study suggest that the sorus structure with phlorotannins localized in the specialized paraphysis cells may be able to screen harmful UVR and protect UV‐sensitive meiospores inside the sporangia.     

Effects of temperature and ultraviolet radiation on microbial foodweb structure: potential responses to global change

1. A series of growth experiments were conducted with natural plankton communities from a lake and river in northern Quebec, to evaluate the response of microbial foodweb structure to changes in ambient temperature and solar ultraviolet radiation (UVR). 2. Bioassays were incubated for 6 days at two temperatures (10 and 20 °C) and three near-surface irradiance conditions [photosynthetically active radiation (PAR) + UVA + UVB, PAR + UVA, and PAR only). 3. The concentration of total bacteria showed no net response to temperature, but the percentage of actively respiring bacteria, as measured with the cellular redox probe 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), was up to 57% higher at 20 °C relative to 10 °C. Chlorophyll a concentrations in the < 2 μm size fraction also reacted strongly to temperature, with a net increase of up to 61% over the temperature range of 10–20 °C. 4. The UVR effects were small or undetectable for most of the measured variables; however, the percentage of actively respiring bacteria was significantly inhibited in the presence of UVR at 20 °C, decreasing by 29–48% on day 6 in the lake experiments and by 59% on day 2 in one river experiment. 5. The results show differential sensitivity to temperature among organisms of microbial communities in subarctic freshwaters, and a resilience by the majority of micro-organisms to their present UVR conditions. Microbial foodwebs in northern freshwaters appear to be relatively unresponsive to short-term (days) changes in UVR. However, the observed responses to temperature suggest that climate change could influence community structure, with warmer temperatures favouring picoplanktonic phototrophs and heterotrophs, and a shift in nanoplankton species composition and size structure.     

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.     

MAA Synthesis and Accumulation in Polar Macroalgae are Controlled by Abiotic Factors

Mycosporine‐like amino acids (MAAs) are regarded as powerful sunscreens protecting the algae against harmful UV radiation. The MAA protection efficiency was tested in algal samples by measuring the optimum quantum yield of photosynthesis using photosystem II fluorescence. It could be demonstrated that the recovery of photosynthesis after exposure to enhanced UV radiation is faster in individuals with high MAA content. MAAs can be synthesized in several polar macroalgae in response to different radiation conditions. Although MAA induction patterns are very species‐specific, some similarities can be found. Field studies indicate that plants from different growth habitats providing distinct radiation climate can be grouped into three physiological categories depending on their MAA content. The first group (I) includes mainly deep‐water species, typically lacking MAAs. The second group (II), algal species found in a broad range of water depths (eu‐ and sublittoral), which are able to flexibly synthesize and accumulate MAAs. The third group (III) includes supra‐ and eulittoral taxa, which always contain high MAA concentrations. In laboratory studies, we showed that taxa of group II and III responded in three different ways based on MAA accumulation when exposed to different radiation conditions (PAR, PAR + UVA, PAR + UVA + UVB). Either they: (a) exhibit highest total MAA concentration under the full artificial spectrum; (b) increase their MAA concentration after exposure to PAR and PAR + UVA or (c) MAA concentration declines after exposure to the full spectrum. Our studies have indicated that when coupled with UVR, exposure to temperature fluctuations ranging from 0 to 10 °C also affect MAA biosynthesis.     

Ultraviolet radiation and consumer effects on a field-grown intertidal macroalgal assemblage in Antarctica

Ultraviolet radiation (UVR) research on marine macroalgae has hithero focussed on physiological effects at the organism level, while little is known on the impact of UV radiation on macroalgal assemblages and even less on interactive effects with other community drivers, e.g. consumers. Field experiments on macrobenthos are scarce, particularly in the Antarctic region. Therefore, the effects of UVR and consumers (mainly limpets were excluded) on early successional stages of a hard bottom macroalgal community on King George Island, Antarctica, were studied. In a two‐factorial design experimental units [(1) ambient radiation, 280–700 nm; (2) ambient minus UVB, 320–700 nm and (3) ambient minus UVR, 400–700 nm vs. consumer–no consumer] were installed between November 2004 and March 2005 (n= 4 plus controls). Dry mass, species richness, diversity and composition of macroalgal assemblages developing on ceramic tiles were followed. Consumers significantly suppressed green algal recruits and total algal biomass but increased macroalgal richness and diversity. Both UVA and UVB radiation negatively affected macroalgal succession. UVR decreased the density of Monostroma hariotii germlings in the first 10 weeks of the experiment, whereas the density of red algal recruits was significantly depressed by UVR at the end of the study. After 106 days macroalgal diversity was significantly higher in UV depleted than in UV‐exposed assemblages. Furthermore, species richness was significantly lower in the UV treatments and species composition differed significantly between the UV‐depleted and the UV‐exposed treatment. Marine macroalgae are very important primary producers in coastal ecosystems, serving as food for herbivores and as habitat for many organisms. Both, UVR and consumers significantly shape macroalgal succession in the Antarctic intertidal. Consumers, particularly limpets can mediate negative effects of ambient UVR on richness and diversity till a certain level. UVB radiation in general and an increase of this short wavelength due to stratospheric ozone depletion in particular may have the potential to affect the zonation, composition and diversity of Antarctic intertidal seaweeds altering trophic interactions in this system.     

Effects of UVB Radiation on the Initial Stages of Growth of Gigartina Skottsbergii, Sarcothalia Crispata and Mazzaella Laminarioides (Gigartinales, Rhodophyta)

The effects of UVB radiation on the growth of macroalgal thalli were evaluated using tetrasporophytic fronds of the Rhodophytes Gigartina skottsbergii, Sarcothalia crispata and Mazzaella laminarioides. The tetrasporophytic fronds were collected from nature and the tetrasporophyte sporelings grown in a temperature regulated chamber at 8 ± 2 ^∘C with a 12L:12D (Light: Dark) photoperiod, Photosynthetically Active Radiation (PAR) of 55 μmol photons m⁻² s⁻¹ and seawater enriched with 20 mL L⁻¹ of Provasoli medium. We exposed the thalli of these macroalgae to PAR (55 μmol photons m⁻² s⁻¹) and three treatments using a combination of PAR with three different levels of UVB radiation (0.10, 0.15 and 0.23 W m⁻² for G. skottsbergii and S. crispata and 0.02, 0.05 and 0.10 W m⁻² for M. laminarioides) during a period of 71 days. Growth of thalli was quantified by measuring their length using digitized photographs of samples.Important differences were detected in the growth of individuals cultured under the effects of UVB radiation, when compared to the control (i.e. plants exposed to PAR only). In the case of G. skottsbergii and S. crispata higher levels of UVB radiation resulted in slower growth of thalli. In nearly all measurements for the first two species, UVB radiation levels of 0.1 W m⁻² induced differences in thallus growth, while for M. laminarioides levels of UVB radiation of 0.1 W m⁻² were effective only after a prolonged period of exposure.Differential effects of UVB radiation on G. skottsbergii, S. crispata and M. laminarioides could interfere with the natural populations of these economically important macroalgal species in southern Chile, where they occur under the annual influence of the Antarctic Ozone Hole and the general thinning of the ozone layer.     

Photosynthetically active and UV radiation act in an antagonistic way in regulating buoyancy of Arthrospira (Spirulina) platensis (cyanobacterium)

Buoyancy provided by gas vesicles has been suggested to play an important role in regulating vertical distribution and nutrient acquisition in cyanobacteria. However, little is known about how changes in UV radiation (UVR, 280–400 nm) would affect the buoyancy. We have shown here that the floatation activity of the economically important cyanobacterium Arthrospira platensis (D-0083) decreased with increased photosynthetic rates associated with increased photosynthetically active radiation (PAR), but it decreased less in the presence of UVR, which resulted in inhibitory effects. When the cells were grown under isoenergetic levels of solar PAR or UVR alone, they migrated downward under PAR but maintained buoyant under UVR. The buoyancy regulation of A. platensis depended on the exposed levels of PAR as well as UVR, which affected photosynthesis and growth in an antagonistic way. The buoyancy of A. platensis in water columns is likely to be dependant on diurnal photosynthetic performance regulated by solar radiation, and can hardly be considered as an active strategy to gain more energy during sunrise/sunset or to escape from harmful irradiation during the noon period.     

UVR defense mechanisms in eurytopic and invasive Gracilaria vermiculophylla (Gracilariales, Rhodophyta)

The invasive success of Gracilaria vermiculophylla has been attributed to its wide tolerance range to different abiotic factors, but its response to ultraviolet radiation (UVR) is yet to be investigated. In the laboratory, carpospores and vegetative thalli of an Atlantic population were exposed to different radiation treatments consisting of high PAR (photosynthetically active radiation) only (P), PAR+UV-A (PA) and PAR+UV-A+UV-B (PAB). Photosynthesis of carpospores was photoinhibited under different radiation treatments but photosystem II (PSII) function was restored after 12 h under dim white light. Growth of vegetative thalli was significantly higher under radiation supplemented with UVR. Decrease in chlorophyll a (Chl a) under daily continuous 16-h exposure to 300 μmol photons m(-2) s(-1) of PAR suggests preventive accumulation of excited chlorophyll molecules within the antennae to minimize the generation of dangerous reactive oxygen species. Moreover, an increase in total carotenoids and xanthophyll cycle pigments (i.e. violaxanthin, antheraxanthin and zeaxanthin) further suggests effective photoprotection under UVR. The presence of the ketocarotenoid β-cryptoxanthin also indicates protection against UVR and oxidative stress. The initial concentration of total mycosporine-like amino acids (MAAs) in freshly-released spores increased approximately four times after 8-h laboratory radiation treatments. On the other hand, initial specific MAAs in vegetative thalli changed in composition after 7-day exposure to laboratory radiation conditions without affecting the total concentration. The above responses suggest that G. vermiculophylla have multiple UVR defense mechanisms to cope with the dynamic variation in light quantity and quality encountered in its habitat. Beside being eurytopic, the UVR photoprotective mechanisms likely contribute to the current invasive success of the species in shallow lagoons and estuaries exposed to high solar radiation.     

Effects of solar UV stress on chlorophyll fluorescence kinetics of intertidal macroalgae from southern Spain: a case study in Gelidium species

Photoinhibition and recovery kinetics after short exposure to solar radiation following three different irradiance treatments of irradiances (PAR, PAR+UVA and PAR+UVA+UVB) was assessed in two intertidal species of the genus Gelidium, Gelidium sesquipedale and G. latifolium, collected from Tarifa (southern Spain) using in vivo chlorophyll fluorescence (PAM fluorometry). After 3 h UV radiation exposure, optimal quantum efficiency (Fv/Fm) in G. sesquipedale decreased between 25 and 35% relative to the control. Under PAR alone, values decreased to 60%. In G. latifolium, photoinhibition did not exceed 40%. Similar results were found for the effective quantum yield (ΔF/Fm′), however, no marked differences in relation to light treatments were seen. When plants were shaded for recovery from stress, only in G. latifolium a significant increase in photosynthesis was observed (between 80 and 100% of control). In contrast, photosynthesis of G. sesquipedale suffered a chronic photoinhibition or photodamage under the three light irradiances. Full solar radiation (PAR+UVA+UVB) affected also the electron transport rate in both species. Here, initial slopes of electron transport vs. irradiance curves decreased up to 60% of controls. Although the recovery kinetic under PAR+UVA+UVB conditions was delayed in G. latifolium, after 24 h recovery this species reached significantly higher than G. sesquipedale. PAR impaired electron trasport only in G. sesquipedale. Overall, both species are characterized by different capacity to tolerate enhanced solar radiation. G. latifolium is a sun adapted plant, well suited to intertidal light conditions, whereas G. sesquipedale, growing at shaded sites in the intertidal zone, is more vulnerable to enhanced UV radiation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Motility and photosynthetic responses of the green microalga Tetraselmis subcordiformis to visible and UV light levels

To test the effects of photosynthetic active radiation (PAR, 400–700 nm) and ultraviolet radiation (UVR, 280–400 nm) on phototaxis and photosynthesis of free swimming microalgae, experiments were performed with Tetraselmis subcordiformis (Wille) Butcher under a solar simulator. In particular, we evaluated the effects of different PAR levels and radiation regimes (i.e., PAR only and PAR+UVR) on those two processes. We found that the cells preferred to move to a particular area (e.g., receiving 100 W m^?2 PAR) with little photochemical suppression or inhibition of carbon fixation. Adding UV-A to high PAR decreased its swimming capacity and photosynthetic capability, and further adding UV-B led to more inhibition. The suppression of the moving capability of T. subcordiformis was reversible but the cells exposed to PAR combined with UVR needed longer time intervals to recover their motility as compared with those irradiated only with PAR. Based on the above results, we postulate that in nature, the motile capability and photosynthesis of free swimming the green microalga might be impaired by enhanced solar UVR. On the other hand, the cells can reduce the damage caused by high irradiances (and even get the optimum light level for photosynthesis) by a behavioral swimming response.     

Exposure of eggs to solar UV‐B leads to reduced hatching rates in two sparid fishes,red sea bream Pagrus major and black sea bream Acanthopagrus schlegeli

Hatching success was examined under exposure to solar ultraviolet radiation (UVR) using filters to give three different light conditions [C1: UV‐B, UV‐A and photosynthetically active radiation (PAR), C2: UV‐A and PAR, C3: PAR] in red Pagrus major and black Acanthopagrus schlegeli sea bream. Hatching rate of both species was reduced by an exposure over a 2 day period to UVR and was not significantly different between two species under the three light conditions.     

Comparing the response of Antarctic,tropical and temperate microalgae to ultraviolet radiation (UVR) stress

The response of Antarctic, tropical and temperate microalgae of similar taxonomic grouping to ultraviolet radiation (UVR) stress was compared based on their growth and fatty acid profiles. Microalgae of similar taxa from the Antarctic (Chlamydomonas UMACC 229, Chlorella UMACC 237 and Navicula UMACC 231), tropical (Chlamydomonas augustae UMACC 246, Chlorella vulgaris UMACC 001 and Amphiprora UMACC 259) and temperate (Chlamydomonas augustae UMACC 247, Chlorella vulgaris UMACC 248 and Navicula incerta UMACC 249) regions were exposed to different UVR conditions. The cultures were exposed to the following conditions: PAR (42 μmol photons m⁻² s⁻¹), PAR + UVA (854 μW cm⁻²) and PAR + UVA + UVB (117 μW cm⁻²). The cultures were subjected to UVA doses of 46.1, 92.2 and 184.4 J cm⁻² and UVB doses of 6.3, 12.6 and 25.2 J cm⁻² by varying the duration of their exposure (1.5, 3 and 6 h) to UVR during the light period (12:12 h light-dark cycle). UVA did not affect the growth of the microalgae, even at the highest dose. In contrast, growth was adversely affected by UVB, especially at the highest dose. The dose that caused 50% inhibition (ID₅₀) in growth was used to assess the sensitivity of the microalgae to UVB. Sensitivity of the microalgae to UVB was species-dependent and also dependent on their biogeographic origin. Of the nine microalgae, the Antarctic Chlorella was most tolerant to UVB stress (ID₅₀ = 21.0 J cm⁻²). Except for this Chlorella, the percentage of polyunsaturated fatty acids of the microalgae decreased in response to high doses of UVB. Fatty acid profile is a useful biomarker for UVB stress for some microalgae. Presented at the 6th Meeting of the Asian Pacific Society of Applied Phycology, Manila, Philippines.