Effect of nitrate concentration and UVR on photosynthesis,respiration, nitrate reductase activity,and phenolic compounds in <Emphasis Type="Italic">Ulva rigida</Emphasis> (Chlorophyta)

Seaweeds growing in the intertidal zone are exposed to fluctuating nitrate and ultraviolet radiation (UVR) levels. While it has been shown that elevated UVR levels and the decrease of nitrate concentration can reduce photosynthetic levels in seaweeds, less is known about the combined effect of nitrate levels and UVR on metabolism and photoprotection mechanisms of intertidal species. Consequently, the objective of this study was to evaluate the effect of nitrate concentration and UVR treatments on photosynthesis, respiration, nitrate reductase activity and phenolic compound levels of Ulva rigida (Chlorophyta). There was a two- to threefold increase in maximal gross photosynthesis (GP_max) and respiration rates, as nitrate increased from 0 to 50 μM NO₃⁻. Similarly, nitrate reductase activity increased linearly from low values in algae incubated at 0 μM NO₃ to high values in tissue incubated at 50 μM NO₃⁻. Phenolic compounds in the tissue of U. rigida increased approximately 60% under 50 μM NO₃⁻ relative to those incubated at 0 μM NO₃⁻. Algae exposed to UVR (8 h) showed a significant decrease in the effective quantum yield and respiration, however, no effect was observed in the phenolic compounds levels. Full recovery of effective quantum yield was observed after U. rigida was transferred for 48 h to low PAR. Nitrate reductase also decreased after an 8-h UVR exposure, but no differences were observed among the nitrate treatments. This study shows that high nitrate levels reduced the negative effect of UVR on the effective quantum yield and increased the recovery of key metabolic enzymes. It is possible that the increase of phenolic compounds in the thallus of U. rigida under high nitrate levels provide a photoprotective mechanism when exposed to high UV levels during low tides.     

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.     

Photosynthetic and biochemical characterization of pigments and UV-absorbing compounds in <Emphasis Type="Italic">Phormidium tenue</Emphasis> due to UV-B radiation

We report the effect of UV-B radiation (0.8 ± 0.1 mW cm⁻²) and UV-B radiation supplemented with low-intensity PAR (∼80 μmol photons m⁻² s⁻¹) on the photosynthesis, photosynthetic pigments, phosphoglycolipids, oxidative damage, enzymatic antioxidants, and UV-absorbing compounds in Phormidium tenue, a marine cyanobacterium. UV-B radiation resulted in a decline in photosynthesis and photosynthetic pigments leading to lower biomass. P. tenue synthesized UV-absorbing compounds like mycosporine-like amino acids (MAAs) and scytonemin in response to UV-B radiation. Quantity of MAAs and scytonemin was higher when UV-B was supplemented with low-level PAR. UV-B treatment also resulted in quantitative changes in phosphoglycolipids of the membrane. The UV-B treatment resulted in a slight increase in the level of peroxidation of cell membrane and very little increase in the activity of superoxide dismutase (SOD). Results indicate that UV-B affected photosynthesis and that the main protective system was the synthesis of MAAs and scytonemin-like compounds rather than antioxidant enzymes such as SOD.     

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.     

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     

Effects of UV Radiation on Photosynthesis,Antioxidant Capacity and the Accumulation of Bioactive Compounds in Gracilariopsis longissima,Hydropuntia cornea and Halopithys incurva (Rhodophyta)

The red macroalgae Hydropuntia cornea, Gracilariopsis longissima and Halopithys incurva were cultured for 14 d under laboratory conditions, in enriched seawater with a high nutrient content (N‐NH₄⁺ and P‐PO₄^3?) and two radiation regimes: PAR (400–700 nm) and PAB (280–700 nm). The UV radiation effects under high availability of nutrients on growth, photosynthetic pigments (chlorophyll a, carotenoids and phycobiliproteins), photosynthetic activity and biochemical composition were studied. Maximum quantum yield (F_v/F_m) was not significantly different among the PAR and PAB treatments during the experiment. However, the maximum electronic transport rate (ETR_max) increased over time, showing the highest values in PAR for H. incurva and H. cornea, whereas for G. longissima it was found in PAB. Photosynthetic efficiency (α_ETR) decreased over time in the first two species, but increased in G. longissima. Saturation irradiance (Ek_ETR) and maximum nonphotochemical quenching (NPQ_max) increased in PAB with time up to 80% and 30%, respectively, indicating a photosynthetic acclimatization like that of sun‐type algae. Five MAAs were identified in all species using high performance liquid chromatography (HPLC). The total content of MAAs increased over time, being 30% higher in H. incurva, 40% in G. longissima and 50% in H. cornea in PAB than in the PAR treatment. Finally, the antioxidant activity was also higher in the PAB treatment. All of the species presented an effective mechanism of photoprotection based on the accumulation of photoprotective compounds with antioxidant activity, as well as a high dissipation of excitation energy (high NPQ_max).     

The role of ammonium in photoprotection against high irradiance in the red alga Grateloupia lanceola

Combined and/or interactive effects of inorganic nitrogen (as ammonium) and irradiance on the accumulation of nitrogenous compounds, like UV-absorbing mycosporine-like amino acids (MAAs), chlorophyll a and phycobiliproteins, were examined in the red alga Grateloupia lanceola (J. Agardh) J. Agardh in a high irradiance laboratory exposure and a subsequent recovery period under low light. Also, photosynthetic activity as in vivo chlorophyll fluorescence of photosystem II, i.e. optimum quantum yield (F_v/F_m), electron transport rate (ETR) and quantum efficiency, were examined. Photosynthetic activity, phycobiliproteins and internal nitrogen content declined during the 3-day PAR (photosynthetically active radiation; 600 μmol s⁻¹ m⁻²) and PAR + UVR (ultraviolet radiation; UVB 280–315 nm 0.8 W m⁻², UVA 315–400 nm 16 W m⁻²) exposure. Ammonium supplied in the culture medium (0, 100 and 300 μM NH₄Cl) modified the responses of the alga to high irradiance exposures in a concentration dependent manner, mainly with respect to recovery, as the highest recovery during a 10-day low light period was produced under elevated concentration of ammonium (300 μM). The recovery of photosynthetic activity and phycobiliproteins was enhanced in the algae previously incubated under PAR + UVR as compared to exposure to only PAR, suggesting a beneficial effect of UVR on recovery or photoprotective processes under enriched nitrogen conditions. However, the content of MAAs did not follow the same pattern and thus it could not be concluded as the cause of observed enhanced recovery.     

Alterations in architecture and metabolism induced by ultraviolet radiation-B in the carragenophyte <Emphasis Type="Italic">Chondracanthus teedei</Emphasis> (Rhodophyta,Gigartinales)

The in vivo effect of ultraviolet radiation-B (UVBR) in apical segments of Chondracanthus teedei was examined. Over a period of 7 days, the segments were cultivated and exposed to photosynthetically active radiation (PAR) at 80 μmol photons m⁻² s⁻¹ and PAR + UVBR at 1.6 W m⁻² for 3 h per day. The samples were processed for electron microscopy and histochemistry; also was analyzed growth rates, mitochondrial activity, protein levels, content of photosynthetic pigments and photosynthetic performance. UVBR elicited increased cell wall thickness and accumulation of plastoglobuli, changes in mitochondrial organization and destruction of chloroplast internal organization. Compared to controls, algae exposed to PAR + UVBR showed a growth rate reduction of 55%. The content of photosynthetic pigments, including chlorophyll a and phycobiliproteins, decreased after exposure to PAR + UVBR. This result agrees with the decreased photosynthetic performance observed after exposing algae to PAR + UVBR. Irradiation also elicited increased activity of the antioxidant enzyme glutathione peroxidase and decreased mitochondrial NADH dehydrogenase activity, which correlated with the decreased protein content in plants exposed to PAR + UVBR. Taken together, these findings strongly indicate that UVBR negatively affects the architecture and metabolism of the carragenophyte C. teedei.     

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.     

UV effects on photosynthesis and DNA in propagules of three Antarctic seaweeds (<Emphasis Type="Italic">Adenocystis utricularis</Emphasis>, <Emphasis Type="Italic">Monostroma hariotii</Emphasis> and <Emphasis Type="Italic">Porphyra endiviifolium</Emphasis>)

Ozone depletion is highest during spring and summer in Antarctica, coinciding with the seasonal reproduction of most macroalgae. Propagules are the life-stage of an alga most susceptible to environmental perturbations therefore, reproductive cells of three intertidal macroalgal species Adenocystis utricularis (Bory) Skottsberg, Monostroma hariotii Gain, and Porphyra endiviifolium (A and E Gepp) Chamberlain were exposed to photosynthetically active radiation (PAR), PAR + UV-A and PAR + UV-A + UV-B radiation in the laboratory. During 1, 2, 4, and 8 h of exposure and after 48 h of recovery, photosynthetic efficiency, and DNA damage were determined. Saturation irradiance of freshly released propagules varied between 33 and 83 μmol photons m⁻² s⁻¹ with lowest values in P. endiviifolium and highest values in M. hariotii. Exposure to 22 μmol photons m⁻² s⁻¹ PAR significantly reduced photosynthetic efficiency in P. endiviifolium and M. hariotii, but not in A. utricularis. UV radiation (UVR) further decreased the photosynthetic efficiency in all species but all propagules recovered completely after 48 h. DNA damage was minimal or not existing. Repeated exposure of A. utricularis spores to 4 h of UVR daily did not show any acclimation of photosynthesis to UVR but fully recovered after 20 h. UVR effects on photosynthesis are shown to be species-specific. Among the tested species, A. utricularis propagules were the most light adapted. Propagules obviously possess good repair and protective mechanisms. Our study indicates that the applied UV dose has no long-lasting negative effects on the propagules, a precondition for the ecological success of macroalgal species in the intertidal.     

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.     

EFFECTS OF ULTRAVIOLET‐A RADIATION AND NUTRIENT AVAILABILITY ON THE CELLULAR COMPOSITION OF PHOTOPROTECTIVE COMPOUNDS IN GLENODINIUM FOLIACEUM (DINOPHYCEAE)1

The photoprotective response in the dinoflagellate Glenodinium foliaceum F. Stein exposed to ultraviolet‐A (UVA) radiation (320–400 nm; 1.7 W · m²) and the effect of nitrate and phosphate availability on that response have been studied. Parameters measured over a 14 d growth period in control (PAR) and experimental (PAR + UVA) cultures included cellular mycosporine‐like amino acids (MAAs), chls, carotenoids, and culture growth rates. Although there were no significant effects of UVA on growth rate, there was significant induction of MAA compounds (28 ± 2 pg · cell^?1) and a reduction in chl a (9.6 ± 0.1 pg · cell^?1) and fucoxanthin (4.4 ± 0.1 pg · cell^?1) compared to the control cultures (3 ± 1 pg · cell^?1, 13.3 ± 3.2 pg · cell^?1, and 7.4 ± 0.3 pg · cell^?1, respectively). In a second investigation, MAA concentrations in UVA‐exposed cultures were lower when nitrate was limited (P < 0.05) but were higher when phosphate was limiting. Nitrate limitation led to significant decreases (P < 0.05) in cellular concentration of chls (chl c₁, chl c₂, and chl a), but other pigments were not affected. Phosphate availability had no effect on final pigment concentrations. Results suggest that nutrient availability significantly affects cellular accumulation of photoprotective compounds in G. foliaceum exposed to UVA.     

Photoregulation of morphological structure and its physiological relevance in the cyanobacterium <Emphasis Type="Italic">Arthrospira</Emphasis> (<Emphasis Type="Italic">Spirulina</Emphasis>) <Emphasis Type="Italic">platensis</Emphasis>

The spiral structure of the cyanobacterium Arthrospira (Spirulina) platensis (Nordst.) Gomont was previously found to be altered by solar ultraviolet radiation (UVR, 280–400 nm). However, how photosynthetic active radiation (PAR, 400–700 nm) and UVR interact in regulating this morphological change remains unknown. Here, we show that the spiral structure of A. platensis (D-0083) was compressed under PAR alone at 30°C, but that at 20°C, the spirals compressed only when exposed to PAR with added UVR, and that UVR alone (the PAR was filtered out) did not tighten the spiral structure, although its presence accelerated morphological regulation by PAR. Their helix pitch decreased linearly as the cells received increased PAR doses, and was reversible when they were transferred back to low PAR levels. SDS-PAGE analysis showed that a 52.0 kDa periplasmic protein was more abundant in tighter filaments, which may have been responsible for the spiral compression. This spiral change together with the increased abundance of the protein made the cells more resistant to high PAR as well as UVR, resulting in a higher photochemical yield.     

The role of ion regulation in the control of the distribution of Gammarus tigrinus (Sexton) in salt-polluted rivers

Fluctuating salinities at different sites on the German salt-polluted rivers Werra and Weser were compared with extracellular ion levels of specimens of Gammarus tigrinus (Sexton; Amphipoda, Crustacea), collected at the same sites. G. tigrinus regulated haemolymph concentrations of inorganic anions (Cl⁻, SO²⁻ ₄, PO³⁻ ₄) and cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) during fluctuations of salt pollution in the upper Weser. This capacity to regulate varying levels of salt pollution in the upper Weser, correlated well with the distribution of the brackish amphipods in this river ecosystem. G. tigrinus tolerated periods of Na⁺ and Cl⁻ stress (>380 mmol l⁻¹) without compensating these maxima by regulating extracellular Na⁺ and Cl⁻. However, during such bursts of Na⁺ and Cl⁻ stress in Werra and Weser, the ability to regulate extracellular [K⁺] at river water K⁺ stress of ≥6.0 mmol l⁻¹ may explain why this brackish species has been more successful in these rivers than its competitors like Gammarus pulex. The present investigation demonstrates that the water salinity affects the [NO⁻ ₃] in the haemolymph of G. tigrinus. With increasing hypo-osmotic stress the animals accumulate increasing amounts of NO⁻ ₃. A simultaneous increase in stream water [NO⁻ ₃] causes an additional accumulation of NO⁻ ₃ in the haemolymph. The high extent of accumulation indicates that active ion transport systems may be involved. The accumulation of NO⁻ ₃ in the haemolymph has low physiological consequences to G. tigrinus, but when hypo-osmotically stressed under anoxic conditions, nitrite formed by the reduction of nitrate may have an adverse affect on the metabolism of G. tigrinus. Accepted: 4 October 1999     

Differential growth response of <Emphasis Type="Italic">Ulva lactuca</Emphasis> to ammonium and nitrate assimilation

Controlled cultivation of marine macroalgal biomass such as Ulva species, notably Ulva lactuca, is currently studied for production of biofuels or functional food ingredients. In a eutrophic environment, this macrophyte is exposed to varying types of nutrient supply, including different and fluctuating levels of nitrogen sources. Our understanding of the influences of this varying condition on the uptake and growth responses of U. lactuca is limited. In this present work, we examined the growth response of U. lactuca exposed to different sources of nitrogen (NH₄⁺; NO₃⁻; and the combination NH₄NO₃) by using photo-scanning technology for monitoring the growth kinetics of U. lactuca. The images revealed differential increases of the surface area of U. lactuca disks with time in response to different N-nutrient enrichments. The results showed a favorable growth response to ammonium as the nitrogen source. The NH₄Cl and NaNO₃ rich media (50 μM of N) accelerated U. lactuca growth to a maximum specific growth rate of 16.4 ± 0.18% day⁻¹ and 9.4 ± 0.72% day⁻¹, respectively. The highest biomass production rate obtained was 22.5 ± 0.24 mg DW m⁻²·day⁻¹. The presence of ammonium apparently discriminated the nitrate uptake by U. lactuca when exposed to NH₄NO₃. Apart from showing the significant differential growth response of U. lactuca to different nitrogen sources, the work exhibits the applicability of a photo-scanning approach for acquiring precise quantitative growth data for U. lactuca as exemplified by assessment of the growth response to two different N-sources.     

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.     

Growth and Nitrogen Uptake Characteristics Reveal Outbreak Mechanism of the Opportunistic Macroalga Gracilaria tenuistipitata

Macroalgae has bloomed in the brackish lake of Shenzhen Bay, China continuously from 2010 to 2014. Gracilaria tenuistipitata was identified as the causative macroalgal species. The aim of this study was to explore the outbreak mechanism of G. tenuistipitata, by studying the effects of salinity and nitrogen sources on growth, and the different nitrogen sources uptake characteristic. Our experimental design was based on environmental conditions observed in the bloom areas, and these main factors were simulated in the laboratory. Results showed that salinity 12 to 20 ‰ was suitable for G. tenuistipitata growth. When the nitrogen sources'' (NH₄ ⁺, NO₃ ⁻) concentrations reached 40 µM or above, the growth rate of G. tenuistipitata was significantly higher. Algal biomass was higher (approximately 1.4 times) when cultured with NH₄ ⁺ than that with NO₃ ⁻ addition. Coincidentally, macroalgal bloom formed during times of moderate salinity (∼12 ‰) and high nitrogen conditions. The NH₄ ⁺ and NO₃ ⁻ uptake characteristic was studied to understand the potential mechanism of G. tenuistipitata bloom. NH₄ ⁺ uptake was best described by a linear, rate-unsaturated response, with the slope decreasing with time intervals. In contrast, NO₃ ⁻ uptake followed a rate-saturating mechanism best described by the Michaelis-Menten model, with kinetic parameters V_max = 37.2 µM g⁻¹ DM h⁻¹ and K_s = 61.5 µM. Further, based on the isotope ¹⁵N tracer method, we found that ¹⁵N from NH₄ ⁺ accumulated faster and reached an atom% twice than that of ¹⁵N from NO₃ ⁻, suggesting when both NH₄ ⁺ and NO₃ ⁻ were available, NH₄ ⁺ was assimilated more rapidly. The results of the present study indicate that in the estuarine environment, the combination of moderate salinity with high ammonium may stimulate bloom formation.     

Effects of ozone exposure on sub-tropical evergreen Cinnamomum camphora seedlings grown in different nitrogen loads

The present study aims in investigating the individual and combined effects of ozone (O₃) exposure and nitrogen (N) load on the growth and photosynthetic characters of Cinnamomum camphora seedlings, a dominant evergreen broadleaf tree species in sub-tropical regions. The seedlings were supplied with N as NH₄NO₃ solution at 0, 30 and 60 kg ha⁻¹ year⁻¹ (simplified as N0, N30, N60, respectively) and were exposed to ambient O₃ concentration (AA) or elevated [O₃] (E-O₃, AA +60 ppb) for one growth season. E-O₃ induced significant negative effects on foliar photosynthesis, including lower photosynthetic rate, reduced carboxylation efficiency, quantum yield of PSII and photosynthetic pigment contents, despite no effect on growth. In contrast, N load acted as fertilization effects. Medium N (N30) increased photosynthetic pigments and stem-base diameter growth relative to N0, whereas high N load (N60) significantly enhanced the growth, photosynthetic pigments, and dark and light action of photosynthesis of C. camphora seedlings. No significant interactive effects of O₃ and N load on the growth, net photosynthetic rate and pigment contents of the seedlings were found, suggesting that N supply to the soil at ≤60 kg ha⁻¹ year⁻¹ does not significantly change the sensitivity of C. camphora to ozone.     

Regulation of inorganic carbon acquisition by nitrogen and phosphorus levels in the Nannochloropsis sp.

Nannochloropsis sp. was grown to the exponential phase and transferred to the high CO₂ (2,800 μl l⁻¹) and irradiance (100 μmol photons m⁻² s⁻¹) condition with different levels of nitrate and phosphate for 72 h, then the photosynthetic activity and inorganic carbon acquisition of the alga were measured. The apparent photosynthetic efficiency (α) of Nannochloropsis sp. decreased with increasing NO₃ ⁻ concentration from 150 to 3,000 μM, and the high nitrate-grown cells showed the lowest levels of light-saturated photosynthetic rate (P _m), while the low nitrate-grown cells showed the highest levels of dark respiration rate (R _d). The maximal light-saturated photosynthetic rate and the minimal dark respiration rate were seen under the middle nitrate condition. When the nitrate concentration ranged from 150 to 3,000 μM, the affinity for inorganic carbons of Nannochloropsis sp. increased sharply with the increasing NO₃ ⁻ concentration to 300 μM and then decreased significantly. The middle phosphate-grown cells exhibited the highest light-saturated photosynthetic rate and apparent photosynthetic efficiency, however, the affinity for inorganic carbons of Nannochloropsis sp. was the maximum under the low phosphate condition. It was shown that the appropriate nitrogen and phosphorus levels were of vital importance to the photosynthesis of cells.     

Estimating the uptake of traffic-derived NO2 from 15N abundance in Norway spruce needles

The ¹⁵N ratio of nitrogen oxides (NO_x) emitted from vehicles, measured in the air adjacent to a highway in the Swiss Middle Land, was very high [δ¹⁵N(NO₂) = +5.7‰]. This high ¹⁵N abundance was used to estimate long-term NO₂ dry deposition into a forest ecosystem by measuring δ¹⁵N in the needles and the soil of potted and autochthonous spruce trees [Picea abies (L.) Karst] exposed to NO₂ in a transect orthogonal to the highway. δ¹⁵N in the current-year needles of potted trees was 2.0‰ higher than that of the control after 4 months of exposure close to the highway, suggesting a 25% contribution to the N-nutrition of these needles. Needle fall into the pots was prevented by grids placed above the soil, while the continuous decomposition of needle litter below the autochthonous trees over previous years has increased δ¹⁵N values in the soil, resulting in parallel gradients of δ¹⁵N in soil and needles with distance from the highway. Estimates of NO₂ uptake into needles obtained from the δ¹⁵N data were significantly correlated with the inputs calculated with a shoot gas exchange model based on a parameterisation widely used in deposition modelling. Therefore, we provide an indication of estimated N inputs to forest ecosystems via dry deposition of NO₂ at the receptor level under field conditions. Received: 7 November 1997 / Accepted: 16 September 1998