Responses of Antarctic Iridaea cordata (Rhodophyta) tetraspores exposed to ultraviolet radiation

In Antarctica ozone depletion is highest during spring, coinciding with the reproduction of many seaweed species. Propagules are the life-stage of an alga most susceptible to environmental perturbations. Therefore, fertile thalli of Iridaea cordata (Turner) Bory (Rhodophyta) were collected in the eulittoral of King George Island (Antarctica) to examine spore susceptibility to ultraviolet radiation (UVR). In the laboratory, freshly released tetraspores were exposed to photosynthetically active radiation (PAR) (400–700 nm), PAR+UV-A (320–700 nm) or PAR+UV-A+UV-B (280–700 nm). Photosynthetic efficiency was measured during 1–8 h of exposure and after 48 h of recovery. Additionally, mycosporine-like amino acids (MAAs) and DNA damage were determined. Saturating irradiance of photosynthesis of freshly released tetraspores was 57 µmol photons m⁻² s⁻¹. Exposure to increasing fluence of PAR reduced photosynthetic efficiency. UVR further decreased the photosynthetic efficiencies of the tetraspores but spores were able to recover completely after UVR exposure and 2 days post-cultivation under low PAR. DNA damage was minimal and lesions were effectively repaired under photoreactivating light. Concentrations of the MAAs shinorine and palythine were higher in tetraspores treated with UVR than in spores only exposed to PAR. Generally, the tetraspores show a good UV tolerance. This flexible response of the tetraspores of this species to changing radiation conditions enables the alga to grow along a considerable depth gradient from the sublittoral to the eulittoral where they can be exposed to enhanced UVBR under conditions of stratospheric ozone depletion.     

Natural ultraviolet radiation and photosynthetically active radiation induce formation of mycosporine-like amino acids in the marine macroalga Chondrus crispus (Rhodophyta)

The UV-absorbing mycosporine-like amino acids (MAAs) are hypothesized to protect organisms against harmful UV radiation (UVR). Since the physiology and metabolism of these compounds are unknown, the induction and kinetics of MAA biosynthesis by various natural radiation conditions were investigated in the marine red alga Chondrus crispus collected from Helgoland, Germany. Three photosynthetically active radiation (PAR, 400–700 nm) treatments without UVR and three UV-A/B (290–400 nm) treatments without PAR were given. Chondrus crispus collected from 4–6 m depth contained only traces of the MAA palythine. After 24 h exposure to 100% ambient PAR, traces of three additional MAAs, shinorine, palythinol and palythene, were detected, and their concentrations increased strongly during a one-week exposure to all PAR treatments. The concentration of all MAAs varied directly with PAR dose, with palythine and shinorine being four- to sevenfold higher than palythinol and palythene. Likewise, naturally high doses of both UV-A and UV-B resulted in a strong accumulation of all MAAs, in particular shinorine. While shinorine accumulation was much more stimulated by UVR, the content of all other MAAs was more affected by high PAR, indicating an MAA-specific induction triggered by UVR or PAR. Received: 24 September 1997 / Accepted: 17 December 1997     

Isolate-specific effects of ultraviolet radiation on photosynthesis,growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium <Emphasis Type="Italic">Microcoleus </Emphasis><Emphasis Type="Italic">chthonoplastes</Emphasis>

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12–15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 μmol photon m⁻² s⁻¹) = P; PAR + UV-A (8 W m⁻²) = PA; PAR + UV-A + UV-B (0.4 W m⁻²) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F _v/F _m) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F _v/F _m during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F _v/F _m sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.     

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.     

BLUE LIGHT AND UV-A RADIATION CONTROL THE SYNTHESIS OF MYCOSPORINE-LIKE AMINO ACIDS IN CHONDRUS CRISPUS (FLORIDEOPHYCEAE)

The induction of UV-absorbing compounds known as mycosporine-like amino acids (MAAs) by red, green, blue, and white light (43% ambient radiation greater than 390 nm) was examined in sublittoral Chondrus crispus Stackh. Fresh collections or long-term cultures of sublittoral thalli, collected from Helgoland, North Sea, Germany, and containing no measurable amounts of MAAs, were exposed to filtered natural radiation for up to 40 days. The MAA palythine (λ_max 320 nm) was synthesized in thalli in blue light to the same extent observed in control samples in white light. In contrast, thalli in green or red light contained only trace amounts of MAAs. After the growth and synthesis period, the photosynthetic performance of thalli in each treatment, measured as pulse amplitude modulated chlorophyll fluorescence, was assessed after a defined UV dose in the laboratory. Thalli with MAAs were more resistant to UV than those without, and exposure to UV-A+B was more damaging than UV-A in that optimal (F_v/F_m) and effective (φ_II) quantum yields were lower and a greater proportion of the primary electron acceptor of PSII, Q, became reduced at saturating irradiance. However, blue light-grown thalli were generally more sensitive than white light control samples to UV-A despite having similar amounts of MAAs. The most sensitive thalli were those grown in red light, which had significantly greater reductions in F_v/F_m and φ_II and greater Q reduction. Growth under UV radiation alone had been shown previously to lead to the synthesis of the MAA shinorine (λ_max 334 nm) rather than palythine. In further experiments, we found that preexposure to blue light followed by growth in natural UV-A led to a 7-fold increase in the synthesis of shinorine, compared with growth in UV-A or UV-A+B without blue light pretreatment. We hypothesize that there are two photoreceptors for MAA synthesis in C. crispus, one for blue light and one for UV-A, which can act synergistically. This system would predispose C. crispus to efficiently synthesize UV protective compounds when radiation levels are rising, for example, on a seasonal basis. However, because the UV-B increase associated with artificial ozone reduction will not be accompanied by an increase in blue light, this triggering mechanism will have little additional adaptive value in the face of global change unless a global UV-B increase positively affects water column clarity.     

The effect of ultraviolet radiation on photosynthesis and ultraviolet-absorbing substances in the endemic Arctic macroalga Devaleraea ramentacea (Rhodophyta)

In field studies conducted at the Kongsfjord (Spitsbergen), the effect of filtered natural radiation conditions (solar without ulraviolet [UV]-A+UV-B, solar without UV-B, solar) on photosynthesis and the metabolism of UV-absorbing mycosporine-like amino acids (MAAs) in the marine red alga Devaleraea ramentacea have been studied. While solar treatment without UV-A+UV-B did not affect photosynthesis during the course of a day, solar without UV-B and the full solar spectrum led to a strong inhibition. However, after offset of the various radiation conditions, all algae fully recovered. Isolates collected from different depths were exposed in the laboratory to artificial fluence rates of photosynthetic active radiation (PAR), PAR+UV-A, and PAR+UV-A+UV-B. The photosynthetic capacity was affected in accordance with the original sampling depth, i.e. shallow-water isolates were more resistant than algae from deeper waters, indicating that D. ramentacea is able to acclimate to changes in irradiance. Seven different UV-absorbing MAAs were detected in this alga, namely mycosporine-glycine, shinorine, porphyra-334, palythine, asterina-330, palythinol, and palythene. The total amount of MAAs continuously decreased with increasing collecting depth when sampled in mid June, and algae taken in late August from the same depths contained on average 30–45% higher MAA concentrations, indicating a seasonal effect as well. The presence of increasing MAA contents with decreasing depth correlated with a more insensitive photosynthetic capacity under both UV-A and UV-B treatments. Populations of D. ramentacea collected from 1 m depth, with one fully exposed to solar radiation and the other growing protected as understorey vegetation underneath the kelp Laminaria saccharina, exhibited quantitatively different MAA compositions in the apices. The exposed seaweeds contained 2.5-fold higher MAA values compared with the more shaded algae. Moreover, the exposed isolates showed a strong tissue gradient in MAAs, pigments, and proteins. The green apices contained 5-fold higher MAA contents than the red bases. Transplantation of D. ramentacea from 2 m depth to the surface induced the formation and accumulation of MAAs after 1 week exposure to the full solar spectrum. Control samples which were treated with the solar spectrum without UV-A+B or with solar without UV-B showed unchanged MAA contents, indicating a strong UV-B effect on MAA metabolism. All data well supported the suggested physiological function of MAAs as natural UV sunscreens in macroalgae.     

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.     

Impact of ultraviolet radiation on cell structure, UV-absorbing compounds, photosynthesis, DNA damage, and germination in zoospores of Arctic Saccorhiza dermatodea

Stratospheric ozone depletion leads to enhanced UV-B radiation. Therefore, the capacity of reproductive cells to cope with different spectral irradiance was investigated in the laboratory. Zoospores of the upper sublittoral kelp Saccorhiza dermatodea were exposed to varying fluence of spectral irradiance consisting of photosynthetically active radiation (PAR, 400-700 nm; =P), PAR+UV-A radiation (UV-A, 320-400 nm; =PA), and PAR+UV-A+UV-B radiation (UV-B, 280-320 nm; =PAB). Structural changes, localization of phlorotannin-containing physodes, accumulation of UV-absorbing phlorotannins, and physiological responses of zoospores were measured after exposure treatments as well as after 2-6 d recovery in dim white light (8 mumol photon m(-2) s(-1)). Physodes increased in size under PAB treatment. Extrusion of phlorotannins into the medium and accumulation of physodes was induced not only under UVR treatment but also under PAR. UV-B radiation caused photodestruction indicated by a loss of pigmentation. Photosynthetic efficiency of spores was photoinhibited after 8 h exposure to 22 and 30 mumol photon m(-2) s(-1) of PAR, while supplement of UVR had a significant additional effect on photoinhibition. A relatively low recovery of photosystem II function was observed after 2 d recovery in spores exposed to 1.7 x 10(4) J m(-2) of UV-B, with a germination rate of only 49% of P treatment after 6 d recovery. The amount of UV-B-induced DNA damage measured as cyclobutane-pyrimidine dimers (CPDs) increased with the biologically effective UV-B dose (BED(DNA)). Significant removal of CPDs indicating repair of DNA damage was observed after 2 d in low white light. The protective function of phlorotannins has restricted efficiency for a single cell. Within a plume of zoospores, however, each cell can buffer each other and protect the lower layer of spores from excessive radiation. Exudation of phlorotannins into the water can also reduce the impact of UV-B radiation on UV-sensitive spores. The results of this study showed that the impact of UVR on reproductive cells can be mitigated by protective and repair mechanisms.     

Physiological, biochemical, and ultrastructural responses of the green macroalga Urospora penicilliformis from Arctic Spitsbergen to UV radiation

Exposure of the filamentous turf green alga Urospora penicilliformis to ambient and artificial ultraviolet radiation (UVR) revealed a considerable resilient species. This explains the ability of this alga to thrive in the middle–upper intertidal zones of the Arctic sea where it is periodically exposed to environmental extremes. A transient UVR effect on photosynthesis under photosynthetically active radiation (PAR) + UV-A and PAR + UV-A + UV-B was found, but dynamic recovery of photoinhibition was observed immediately after reduction of the photon fluence rate of PAR in the absence or presence of background UVR under laboratory and natural solar radiation, respectively. Chlorophylls, carotenoids, and xanthophyll cycle pigments (violaxanthin, antheraxanthin, and zeaxanthin) concentrations were not significantly different between freshly collected samples and filaments exposed to additional laboratory radiation treatment. The ultrastructure of the U. penicilliformis gametophytes showed that the cells are well adapted to UVR. No significant ultrastructural alterations were observed in filaments exposed to different spectral irradiance in the laboratory compared to in situ acclimated specimen. The antioxidant α-tocopherol was detected in minute quantity while the search for flavonoid-like compounds was negative. Other UV screening strategies or certain genetically fixed physiological protective mechanism could be operating in this species responsible for their occurrence in higher shoreline and ecological success. Further molecular and biochemical studies are needed to elucidate the stress resistance in this turf alga. There is an indication that the extremely thick cell wall of U. penicilliformis gametophytes covered with mucilage sheath and dense layer of mineral depositions may provide a shield against unfavorable environmental conditions in general and against UVR in particular.     

Interactive Effects of Temperature and UV Radiation on Photosynthesis of Chlorella Strains from Polar,Temperate and Tropical Environments: Differential Impacts on Damage and Repair

Global warming and ozone depletion, and the resulting increase of ultraviolet radiation (UVR), have far-reaching impacts on biota, especially affecting the algae that form the basis of the food webs in aquatic ecosystems. The aim of the present study was to investigate the interactive effects of temperature and UVR by comparing the photosynthetic responses of similar taxa of Chlorella from Antarctic (Chlorella UMACC 237), temperate (Chlorella vulgaris UMACC 248) and tropical (Chlorella vulgaris UMACC 001) environments. The cultures were exposed to three different treatments: photosynthetically active radiation (PAR; 400–700 nm), PAR plus ultraviolet-A (320–400 nm) radiation (PAR + UV-A) and PAR plus UV-A and ultraviolet-B (280–320 nm) radiation (PAR + UV-A + UV-B) for one hour in incubators set at different temperatures. The Antarctic Chlorella was exposed to 4, 14 and 20°C. The temperate Chlorella was exposed to 11, 18 and 25°C while the tropical Chlorella was exposed to 24, 28 and 30°C. A pulse-amplitude modulated (PAM) fluorometer was used to assess the photosynthetic response of microalgae. Parameters such as the photoadaptive index (E_k) and light harvesting efficiency (α) were determined from rapid light curves. The damage (k) and repair (r) rates were calculated from the decrease in ΦPSII_eff over time during exposure response curves where cells were exposed to the various combinations of PAR and UVR, and fitting the data to the Kok model. The results showed that UV-A caused much lower inhibition than UV-B in photosynthesis in all Chlorella isolates. The three isolates of Chlorella from different regions showed different trends in their photosynthesis responses under the combined effects of UVR (PAR + UV-A + UV-B) and temperature. In accordance with the noted strain-specific characteristics, we can conclude that the repair (r) mechanisms at higher temperatures were not sufficient to overcome damage caused by UVR in the Antarctic Chlorella strain, suggesting negative effects of global climate change on microalgae inhabiting (circum-) polar regions. For temperate and tropical strains of Chlorella, damage from UVR was independent of temperature but the repair constant increased with increasing temperature, implying an improved ability of these strains to recover from UVR stress under global warming.     

Natural UVR Does Not Affect Decomposition by Aquatic Hyphomycetes

We performed field and laboratory experiments to evaluate the effect of solar radiation (UVR and PAR) on leaf litter decomposition, fungal biomass and sporulation rates, in the Andean Patagonia, where high UVR levels are common. Leaves of Alnus glutinosa exposed to three treatments, normal radiation (PAR + UVR), protected from UVR and protected from total radiation (SHADE) by plastic films lost 31–37% of their mass. Leaves of Nothofagus pumilio lost 61–64% of their mass under the same conditions. For both leaf species, differences in mass losses among treatments were not statistically significant. Sporulation rates were significantly lower in the SHADE treatment. Fungal biomass accounted for 6.2 to 7.1% of leaf mass, without significant differences among treatments. In the laboratory, leaf discs of A. glutinosa colonized by single species of aquatic hyphomycetes (Articulospora tetracladia, Flagellospora curta or Lunulospora curvula) and exposed to or protected from UVR did not differ in mass loss and sporulation rates. Pure cultures of two fungal species grew at the same rates when exposed to light (PAR and PAR + UVR) or to the SHADE. In summary, we found no evidences that current high levels of UV radiation affect litter decomposition mediated by aquatic hyphomycetes. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

SYNTHESIS OF MYCOSPORINE-LIKE AMINO ACIDS IN CHONDRUS CRISPUS (FLORIDEOPHYCEAE) AND THE CONSEQUENCES FOR SENSITIVITY TO ULTRAVIOLET B RADIATION

The induction and protective role of the UV-absorbing compounds known as mycosporine-like amino acids (MAAs) were examined in sublittoral Chondrus crispus Stackh. transplanted for 2 weeks in the spring and summer to shallow water under three irradiance conditions: PAR (photosynthetically active radiation; 400–700 nm), PAR + UVA (PAR + 320– 400 nm), PAR + UVA + UVB (PAR + UVA + 280– 320 nm). Sublittoral thalli collected around Helgoland, North Sea, Germany, from 6 m below the mean low water of spring tides contained less than 0.1 mg·g⁻¹ dry weight (DW) total MAAs, whereas eulittoral samples contained over 1 mg·g⁻¹ DW. Transplantation to shallow water led to the immediate synthesis of three MAAs in the following temporal order: shinorine (λ_max 334 nm), asterina (λ_max 330 nm), and palythine (λ_max 320 nm), with the shinorine content peaking and then declining after 2 days (exposure to 100 mol photons·m⁻²). Maximum total MAA content (2 mg·g⁻¹ DW) also occurred after 2 days of induction, exceeding the content normally found in eulittoral samples. Furthermore, the relative proportion of the different MAAs at this time was different than that in eulittoral samples. After 2 days the total content declined to the eulittoral value, with palythine as the principal MAA. Similar data were obtained for all treatments, indicating that MAA synthesis in C. crispus was induced by PAR and not especially stimulated by UV radiation. The ability of photosystem II (PSII) to resist damage by UVB was tested periodically during the acclimation period by exposing samples to a defined UVB dose in the lab. Changes in chlorophyll fluorescence (F_v/F_m and effective quantum yield, φ_II) indicated that PSII function was inhibited during the initial stage of acclimation but gradually improved with time. No difference among screening treatments was detected except in spring for the samples acclimating to PAR + UVA + UVB. In this treatment F_v/F_m and φ_II were significantly lower than in the other treatments. During the first week of each experiment, growth rates were also significantly reduced by UVB. The reductions occurred despite maximum MAA content, indicating an incomplete protection of photosynthetic and growth-related processes.     

Short- and long-term effects of solar ultraviolet radiation on the red algae Porphyridium cruentum (S. F. Gray) N?geli.

During spring 2002 and fall 2003 we carried out experiment in tropical southern China to determine the short- and long-term effects of solar ultraviolet radiation (UVR, 280-400 nm) on photosynthesis and growth in the unicellular red alga Porphyridium cruentum. During the experimentation, cells of P. cruentum were exposed to three radiation treatments: (a) samples exposed to PAR (400-700 nm) + UV-A (315-400 nm) + UV-B (280-315 nm)(PAB treatment); (b) samples exposed to PAR + UV-A (PA treatment) and, (c) samples exposed only to PAR (P treatment). To assess the short-term impact of UVR as a function of irradiance, we determined photosynthesis versus irradiance (Pvs.E) curves. From these curves the maximum carbon uptake rate (P(max)) and the light saturation parameter (E(k)) were obtained, with values of approximately 12.8-14.4 microg C (microg chl a)(-1) h(-1), and approximately 250 micromol m(-2) s(-1), respectively. A significant UVR effect on assimilation numbers was observed when samples were exposed at irradiances higher than E(k), with samples exposed to full solar radiation having significant less carbon fixation than those exposed only to PAR. Biological weighting functions of P. cruentum were used to evaluate the UVR impact per unit energy received by the cells; the data indicate that the species is as sensitive as natural phytoplankton from the southern China Sea; however, it is much more resistant than Antarctic assemblages. When evaluating the combined effects of mixing speed and UVR, it was seen that samples rotating fast within the upper mixed layer were less inhibited by UVR as compared to those under slow mixing or in fixed samples. Growth of P. cruentum over a week-long experiment was not affected by neither UVR nor UV-A; additionally, low photoinhibition was found at the end as compared to that at the beginning of this experiment. Our results thus indicate that, although on short-term basis P. cruentum is affected by solar UVR, it can acclimate to minimize UVR-induced effects when given enough time.     

Interaction Effects of Ambient UV Radiation and Nutrient Limitation on the Toxic Cyanobacterium <Emphasis Type="Italic">Nodularia spumigena</Emphasis>

Nodularia spumigena is one of the dominating species during the extensive cyanobacterial blooms in the Baltic Sea. The blooms coincide with strong light, stable stratification, low ratios of dissolved inorganic nitrogen, and dissolved inorganic phosphorus. The ability of nitrogen fixation, a high tolerance to phosphorus starvation, and different photo-protective strategies (production of mycosporine-like amino acids, MAAs) may give N. spumigena a competitive advantage over other phytoplankton during the blooms. To elucidate the interactive effects of ambient UV radiation and nutrient limitation on the performance of N. spumigena, an outdoor experiment was designed. Two radiation treatments photosynthetic active radiation (PAR) and PAR +UV-A + UV-B (PAB) and three nutrient treatments were established: nutrient replete (NP), nitrogen limited (−N), and phosphorus limited (−P). Variables measured were specific growth rate, heterocyst frequency, cell volume, cell concentrations of MAAs, photosynthetic pigments, particulate carbon (POC), particulate nitrogen (PON), and particulate phosphorus (POP). Ratios of particulate organic matter were calculated: POC/PON, POC/POP, and PON/POP. There was no interactive effect between radiation and nutrient limitation on the specific growth rate of N. spumigena, but there was an overall effect of phosphorus limitation on the variables measured. Interaction effects were observed for some variables; cell size (larger cells in −P PAB compared to other treatments) and the carotenoid canthaxanthin (highest concentration in −N PAR). In addition, significantly less POC and PON (mol cell⁻¹) were found in −P PAR compared to −P PAB, and the opposite radiation effect was observed in −N. Our study shows that despite interactive effects on some of the variables studied, N. spumigena tolerate high ambient UVR also under nutrient limiting conditions and maintain positive growth rate even under severe phosphorus limitation.     

Photosynthetic response of Nodularia spumigena to UV and photosynthetically active radiation depends on nutrient (N and P) availability

Biomass of N. spumigena is distributed within the dynamic photic zone that changes in both light quantity and quality. This study was designed to determine whether nutrient status can mitigate the negative impacts of experimental radiation treatments on the photosynthetic performance of N. spumigena. Cyanobacterial suspensions were exposed to radiation consisting of photosynthetically active radiation (PAR=400-700 nm), PAR+UV-A (=PA, 320-700 nm), and PAR+UV-A+UV-B (=PAB, 280-700 nm) under different nutrient media either replete with external dissolved nitrate (N) and orthophosphate (P; designated as +N/+P), replete with P only (-N/+P), or replete with N only (+N/-P). Under low PAR (75 micromol photons m(-2) s(-1)), nutrient status had no significant effect on the photosynthetic performance of N. spumigena in terms of rETRmax, alpha, and E(k). Nodularia spumigena was able to acclimate to high PAR (300 micromol photons m(-2) s(-1)), with a corresponding increase in rETRmax and E(k). The photosynthetic performance of N. spumigena cultured with supplemental nitrogen was more susceptible to experimental PAR irradiance. Under UVR, P-enrichment in the absence of additional external N (-N/+P) induced lower photoinhibition of photosynthesis compared with +N/-P cultures. However, the induction of NPQ may have provided PSII protection under P-deplete and PAR+UVR conditions. Because N. spumigena are able to fix nitrogen, access to available P can render them less susceptible to photoinhibition, effectively promoting blooms. Under a P-deficient condition, N. spumigena were more susceptible to radiation but were capable of photosynthetic recovery immediately after removal of radiation stress. In the presence of an internal P pool in the Baltic Sea, which may be seasonally available to the diazotrophic cyanobacteria, summer blooms of the resilient N. spumigena will persist.     

阳光紫外辐射对室内水培发状念珠藻生理特性的影响

发状念珠藻（Nostoc flagelliforme Bornet & Flahault）是一种重要的陆生经济蓝藻,室内培育出的原植体如何适应阳光辐射的问题尚需探讨。为此,作者将室内水培发菜置于阳光下培养,测定了其生长、有效光化学效率（F/Fm&#900;）和色素的变化。结果表明,较高的可见光（PAR,395－700 nm）和紫外辐射（UVR,280－395 nm）均导致水培发菜的F/Fm&#900;下降。第1天中午,PAR和UVR分别使F/Fm&#900;下降了54％和13％;傍晚,F/Fm&#900;有部分恢复。UVR对发菜适应阳光2d后的生长无负面作用。发菜在适应全阳光辐射期间,紫外吸收物质（Scytonemin和Mycosporine-like amino acids）含量不断增加,9d后,分别增加了124倍和9倍。这些紫外吸收物质的增加对发菜细胞降低光抑制,适应阳光辐射,起到了重要作用。本研究的结果可为水培发菜室外培养方法的建立提供一定的理论依据。     

Effects of N supply on the accumulation of photosynthetic pigments and photoprotectors in <Emphasis Type="Italic">Gracilaria tenuistipitata</Emphasis> (Rhodophyta) cultured under UV radiation

We have studied the effects of nitrate supply under photosynthetic active radiation (PAR) plus ultraviolet radiation (UVR) exposure on photosynthetic pigments (chlorophyll a and carotenoids), photoprotective UV screen mycosporine-like amino acids (MAAs), and photosynthetic parameters, including the maximum quantum yield (F _v/F _m) and electron transport rate (ETR) on the red agarophyte Gracilaria tenuistipitata. Apical tips of G. tenuistipitata were cultivated under ten different concentrations of NO₃⁻ for 7 days. It has been shown that G. tenuistipitata cultured under laboratory conditions has the ability to accumulate high amounts of MAAs following a nitrate concentration-dependent manner under PAR + UVR. Two MAAs were identified, shinorine and porphyra-334. The relative concentration of the first increased under high concentrations of nitrate, while the second one decreased. The presence of antheraxanthin is reported for the first time in this macroalgae, which also contains zeaxanthin, lutein, and β-carotene. The accumulation of pigments, photoprotective compounds, and photosynthetic parameters of G. tenuistipitata is directly related to N availability. All variables decreased under low N supplies and reached constant maximum values with supplements higher than 0.5 mM NO₃⁻. Our results suggest a high potential to acclimation and photoprotection against stress factors (including high PAR and UVR) directly related to N availability for G. tenuistipitata.     

Mycosporine-like amino acids (MAAs) profile of a rice-field cyanobacterium Anabaena doliolum as influenced by PAR and UVR

The mycosporine-like amino acid (MAA) profile of a rice-field cyanobacterium, Anabaena doliolum, was studied under PAR and PAR + UVR conditions. The high-performance liquid chromatographic analysis of water-soluble compounds reveals the biosynthesis of three MAAs, mycosporine-glycine (lambda (max) = 310 nm), porphyra-334 (lambda (max) = 334 nm) and shinorine (lambda (max) = 334 nm), with retention times of 4.1, 3.5 and 2.3 min, respectively. This is the first report for the occurrence of mycosporine-glycine and porphyra-334 in addition to shinorine in Anabaena strains studied so far. The results indicate that mycosporine-glycine (monosubstituted) acts as a precursor for the biosynthesis of the bisubstituted MAAs shinorine and porphyra-334. Mycosporine-glycine was under constitutive control while porphyra-334 and shinorine were induced by UV-B radiation, indicating the involvement of UV-regulated enzymes in the biotransformation of MAAs. It seems that A. doliolum is able to protect its cell machinery from UVR by synthesizing a complex set of MAAs and thus is able to survive successfully during the summer in its natural brightly lit habitats.     

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.     

Photosynthesis and growth of Arthrospira (Spirulina) platensis (Cyanophyta) in response to solar UV radiation, with special reference to its minor variant

The minor variant of the economically important cyanobacterium, Arthrospira platensis, usually appears in commercial production ponds under solar radiation. However, how sensitive the minor variant to solar UVR and whether its occurrence relates to the solar exposures are not known. We investigated the photochemical efficiency of PSII and growth rate of D-0083 strain and its minor variant in semi-continuous cultures under PAR (400–700 nm) alone, PAR + UV-A (320–400 nm) and PAR + UV-A + UV-B (280–700 nm) of solar radiation. The effective quantum yield of D-0083 at 14:00 p.m. decreased by about 86% under PAR, 87% under PAR + UV-A and 92% under PAR + UV-A + UV-B (280–315 nm), respectively. That of the minor variant was reduced by 93% under PAR and to undetectable values in the presence of UV-A or UV-A + UV-B. Diurnal change of the yield showed constant pattern during long-term (10 days) exposures, high in the early morning and late afternoon but the lowest at noontime in both strains, with the UVR-related inhibition being always higher in the variant than D-0083. During the long-term exposures, cells of D-0083 acclimated faster to solar UV radiation and showed paralleled growth rates among the treatments with or without UVR at the end of the experiment; however, growth of the minor variant was significantly reduced by UV-A and UV-B throughout the period. Comparing to the major strain D-0083, the minor variant was more sensitive to UVR in terms of its growth, quantum yield and acclimation to solar radiation.