Thallus morphology and optical characteristics affect growth and DNA damage by UV radiation in juvenile Arctic Laminaria sporophytes

Growth of young sporophytes of the brown algae Laminaria digitata, L. saccharina and L. solidungula from Spitsbergen were measured in the laboratory after being exposed for 21 days to either photosynthetically active radiation (PAR=P) or to full light spectrum (PAR + UV-A + UV-B=PAB) using of cutoff glass filters. The plants were grown at 8±2°C and 16 h light : 8 h dark cycles with 6 h additional ultraviolet radiation (UVR) exposure in the middle of the light period. Growth was measured every 10 min using growth chambers with online video measuring technique. Tissue morphology and absorption spectra were measured in untreated young sporophytes while chlorophyll (Chl) a content and DNA damage were measured in treated thalli at the end of the experiment. In all species, growth rates were significantly higher in sporophytes exposed to P alone compared to sporophytes exposed to PAB. Tissue DNA damage is dependent on thallus thickness and absorption spectra characteristics of pigments and UV-absorbing compounds. In sporophytes exposed to UVR, energy demands for repair of DNA damage and synthesis of UV-absorbing compounds for protection effectively diverts photosynthate at the expense of growth. Photosynthetic pigment was not significantly different between treatments suggesting a capacity for acclimation to moderate UVR fluence. The general growth pattern in sporophytes exposed to P alone showed an increasing growth rate from the onset of light (0500–0900 hours) to a peak at the middle of the light phase (0900–1500 hours), a decline towards the end of the light phase (1500–2100 hours) and a minimum “low” growth in the dark (2100–0500 hours) relative to growth during the entire light phase. Under PAB, different growth patterns were observed such as growth compensation at night in L. digitata, delayed growth recovery in L. saccharina and minimal but continuous growth in L. solidungula. Growth as an integrative parameter of all physiological processes showed that the effect of UVR is correlated to the depth distribution of these species.     

Effects of solar ultraviolet radiation on the periphyton community in lotic systems: comparison of attached algae and bacteria during their development

The effects of solar ultraviolet radiation (UVR) on the development of a periphyton community were studied in an outdoor artificial stream apparatus. Algal biomass, species composition, and bacterial cell density were measured under full sunlight and non-UVR (photosynthetically active radiation [PAR]-only) conditions. Attachment of algae was detected on days 6–9. Although the chlorophyll-a concentration under non-UVR conditions was 2–4 times that under full sunlight (PAR + UVR) throughout the experiment, neither net algal growth rate nor species composition differed significantly between the two light conditions. The relative carotenoid pigment contents of attached algae in the PAR + UVR condition were 1.1–1.3 times those in the non-UVR condition. Rates of increase of bacterial cell densities under the PAR + UVR condition were depressed by solar UVR for the first few days, although there were no apparent differences in the rates of increase between the light conditions later in the experiment. The small effect of UVR on the development of this periphyton community may be attributable to low UV flux at this study site and to the experimental conditions under which the algae were kept: a high physiological state with high nutrient conditions. Attached bacteria and algae that colonize substrata first are likely to be sensitive to solar UVR, and the negative effects of UVR are mitigated by the development of a periphyton community.     

Screening capacity of UV-absorbing compounds in spores of Arctic Laminariales

The functional significance of phlorotannins as ultraviolet radiation screens in brown algae is presented. Spectral analysis of zoospore suspensions of the three Arctic Laminariales Saccorhiza dermatodea, Alaria esculenta and Laminaria digitata showed strong absorption in the UV waveband, characteristic of phlorotannins. An induction in the synthesis of the UV-absorbing compound in zoospore suspensions of S. dermatodea and A. esculenta was observed as an increase in absorbance in the UV region after 8 h exposure to the whole light spectrum. Transmission of UVR was also negatively correlated with zoospore density in both these species but not in L. digitata. ‘Biofilters’ constructed from UV-transparent acrylic sheet, containing zoospore suspensions or solutions of phloroglucinol showed varying capacity to protect zoospore cultures from the lethal effects of ultraviolet radiation. Phloroglucinol protects the zoospores from damage by screening out the much harmful shorter UV-B spectra (280-290 nm). Cultured spores of A. esculenta and L. digitata after exposure to the whole light spectrum covered by filters containing phloroglucinol showed high rates of germination, unlike controls covered by seawater-only filters that showed 100% mortality. Biofilters containing zoospore suspensions act as buffers and showed variable UV-protection properties on the germination of its conspecies. At highest zoospore density (∼ 4 × 10⁶ spores ml^− 1), zoospores were observed to screen UV radiation maintaining viability among shielded spores in all species investigated. The protective function of zoospore film is, however, density-dependent in L. digitata. At lower spore density, UV-screening function in S. dermatodea and A. esculenta is attributed to their capacity to accumulate and release UV-absorbing compounds into the medium. Ultraviolet radiation transmission by zoospore suspensions of Saccorhiza and Alaria decreased during exposure to the whole light spectrum which is consistent with the earlier observation of enlarged phenolic vesicles following UVR exposure. The increase in vesicle size and the corresponding increase in UV-absorbing capacity may contribute to greater tolerance of UVR exposure in both species.     

UV-susceptibility of zoospores of the brown macroalga Laminaria digitata from Spitsbergen

The UV-susceptibility of zoospores of the lower sublittoral kelp Laminaria digitata was studied in the laboratory under 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). In vivo absorption of phlorotannin, localisation of phlorotannin-containing physodes, structural changes, DNA damage and repair, photosynthesis and germination of zoospores were measured after exposure treatments and after 2–6 days of recovery in dim white light. Photodegradation of phlorotannins was observed after extended exposure to ultraviolet radiation (UVR). The UV-protective function of extra- and intracellular phlorotannins was, therefore, observed only after 8 h, but not after 16-h UVR exposure. The energetic cost of photoprotection may have caused the delay in ontogenic development of zoospores after 8-h exposure to PA and PAB treatment; longer exposure time corresponding to 16-h PA and PAB treatment eventually lead to cell degeneration at 6 days post-cultivation. The formation of cyclobutane–pyrimidine dimers (CPDs), as indicator of DNA damage, was not blocked by the UV-absorbing phlorotannins during the 16-h PAB exposure and the inability for DNA damage repair was likely responsible for low photosynthetic recovery and spore mortality. The higher sensitivity of L. digitata zoospores to UVR compared to other kelps such as Saccorhiza dermatodea and Alaria esculenta confirmed our hypothesis that the depth distribution of adult sporophytes in the field correlates to the sensitivity of their corresponding early life history stages to different stress factors in general and UVR in particular.     

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

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

UV‐radiation effects on photosynthesis and photoprotection in gametophytic and sporophytic stages of the freshwater red alga Kumanoa ambigua (Rhodophyta,Batrachospermales)

The aim of this study was to analyze the photosynthetic performance of gametophytic and sporophytic (‘Chantransia’) stages of Kumanoa ambigua in culture under UV radiation. We hypothesized that both life history stages of K. ambigua would exhibit different photosynthetic responses to UVR exposure. Experiments were performed under three conditions: (i) photosynthetically active radiation (PAR) only (400–700 nm), P control; (ii) PAR + UVA (320–700 nm), PA treatment; and (iii) PAR + UVA + UVB (280–700 nm), PAB treatment. The photosynthetic parameters were measured as in vivo chlorophyll a fluorescence. Differences were found between life stages, observing higher values of NPQ and effective quantum yields (ΔF/F_m′) under UVA and PAR in gametophytes compared to sporophytes. One type of mycosporine‐like amino acid (MAA) was detected in the gametophyte in all treatments, but not in the ‘Chantransia’ stage. The increased photosynthetic performance for some parameters and the presence of MAA in gametophyte suggest that it is less sensitive to UV radiation, particularly UVA, in comparison to sporophyte under culture conditions. This approach is relevant for a better understanding of the adaptation and physiological acclimation of freshwater Rhodophyta to varying light climates in terms of global changes.     

Influence of ultraviolet radiation, copper, and zinc on microcystin content in Microcystis aeruginosa (Cyanobacteria)

Cyanobacterial toxin production is allied to some unknown trigger resulting in the production of toxins such as microcystin. We hypothesize that microcystins serve as metal ligands to control bioavailability and toxicity of ambient metals. Since ultraviolet radiation (UVR) promotes photo-oxidation of organic metal ligands and influences trace metal bioavailability, the present study aimed to investigate the influence of UVR, Cu, and Zn on specific growth rates, biomass, photosynthetic capacity, and microcystin content in Microcystis aeruginosa. Two toxigenic strains of Microcystis were cultivated using either Lake Erie filtered water or a chemically defined medium, with realistic concentrations of Cu and Zn combined with natural or artificial UVR exposure. Cu was more toxic than Zn on the basis of free ion concentration of trace metals in synthetic medium, although in Lake Erie water total added Zn (10 nM) or Zn plus Cu (10 nM) had a more detrimental effect on biomass and specific growth rate. Natural UVR delivered at 25% ambient levels caused no decrease on the parameters measured (chlorophyll-a, photosynthetic rate), yet artificial levels of UVR (up to 5.9 μmol UVB photons m⁻² s⁻¹) negatively affected biomass and specific growth rate. Cellular levels of microcystin (per unit chlorophyll-a) were concomitant with specific growth rather than being triggered in response either of these stressors (UVR, Zn, and Cu) alone or in combination, in agreement with a purported constitutive production of microcystins.     

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.     

EFFECT OF HIGH IRRADIANCE ON RECRUITMENT OF THE GIANT KELP MACROCYSTIS (PHAEOPHYTA) IN SHALLOW WATER1

Laboratory and field experiments were done hi Still-water Cove, Carmel Bay, California, and Monterey Harbor, California, to determine the effect of photosynthetically active radiation (PAR) on the shallow (upper) limit of giant kelp, Macrocystis pyrifera (L.) C. Agardh. At shallow depths, M. pyrifera did not recruit or grow to macroscopic size from gametophytes or embryonic sporophytes transplanted to vertical buoy lines; sharp decreases in PAR with depth coincided with observed recruitment and sporophyte distributions. Shade manipulations indicated that settlement of M. pyrifera zoospores was decreased, but not prohibited, by high PAR. Postsettlement stages (gametophytes and embryonic sporophytes), however, survived only under shade. These results suggest that high PAR can inhibit the recruitment of M. pyrifera to shallow water by killing its postsettlement stages; whether or not ultraviolet (UV) radiation also inhibits recruitment was not tested. In either case, however, it appears that high irradiance (PAR and/or UV) regulates the shallow limit of M. pyrifera prior to temperature and desiccation stresses inherent to intertidal regions. In an additional experiment, recruitment or growth of transplanted gametophytes or embryonic sporophytes of Macrocystis integrifolia Bory also did not occur at shallow depths, suggesting that this shallow water species accesses high irradiance regions via a method other than sexual reproduction.     

SHORT‐TERM AND LONG‐TERM EFFECTS OF TEMPERATURE ON PHOTOSYNTHESIS IN THE DIATOM THALASSIOSIRA PSEUDONANA UNDER UVR EXPOSURES1

Temperature is expected to modify the effects of ultraviolet radiation (UVR) on photosynthesis by affecting the rate of repair. We studied the effect of short‐term (1 h) and long‐term (days) acclimation to temperature on UVR photoinhibition in the diatom Thalassiosira pseudonana Hasle et Heimdal. Photosynthesis was measured during 1 h exposures to varying irradiances of PAR and UVR + PAR at 15, 20, and 25°C, the latter corresponding to the upper temperature limit for optimal growth in T. pseudonana. The exposures allowed the estimation of photosynthesis–irradiance (P–E) curves and biological weighting functions (BWFs) for photoinhibition. For the growth conditions used, temperature did not affect photosynthesis under PAR. However, photoinhibition by UVR was highly affected by temperature. For cultures preacclimated to 20°C, the extent of UVR photoinhibition increased with decreasing temperature, from 63% inhibition of PAR‐only photosynthesis at 25°C to 71% at 20°C and 85% at 15°C. These effects were slightly modified after several days of acclimation: UVR photoinhibition increased from 63% to 75% at 25°C and decreased from 85% to 80% at 15°C. Time courses of photochemical efficiency (Φ_PSII) under UVR + PAR were also fitted to a model of UVR photoinhibition, allowing the estimation of the rates of damage (k) and repair (r). The r/k values obtained for each temperature treatment verified the responses observed with the BWF (R² = 0.94). The results demonstrated the relevance of temperature in determining primary productivity under UVR exposures. However, the results suggested that temperature and UVR interact mainly over short (hours) rather than long (days) timescales.     

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.     

Sporogenic and vegetative tissues of Saccharina latissima (Laminariales,Phaeophyceae) exhibit distinctive sensitivity to experimentally enhanced ultraviolet radiation : photosynthetically active radiation ratio

Fertile Saccharina latissima sporophytes, collected in the Kongsfjorden, Ny‐Ålesund, Spitsbergen, Norway (78°56.87′ N, 11°51.64′ E) were investigated in relation to its sensitivity to experimentally enhanced ultraviolet radiation : photosynthetically active radiation (UVR : PAR) ratios. Irradiance of UVR were 4.30 W m^?2 of UV‐A (320–400 nm) and 0.40 W m^?2 of UV‐B (280–320 nm), and PAR (400–700 nm) was ～4.30 W m^?2 (=20 µmol photons m^?2 s^?1). Excised soral (sporogenic) and non‐soral (vegetative) tissues were separately irradiated for 16 h at 7°C. Transmission electron microscopy showed abundant occurrence of physodes, electron dense particles (～300–600 nm) in the sorus. Paraphysis cells, with partly crystalline content, large mitochondria and abundant golgi bodies were towering over the sporangia. In soral tissue, cells were not visibly altered by the PAR + UVR irradiation. The chloroplasts, flagella and nucleus of unreleased meiospores inside the sporangial parent cells were visibly intact. Severe changes in the chloroplast structure of vegetative tissue occurred after PAR + UVR irradiation. These changes included wrinkling and dilatation of the thylakoid membranes, and appearance of electron translucent areas inside the chloroplasts. In vegetative cells exposed to PAR + UVR, the total amount of physodes, was slightly higher as in cells exposed to PAR only. Initial values of optimum quantum yield of photosystem II (F_v/F_m) were 0.743 ± 0.04 in non‐soral and 0.633 ± 0.04 in soral tissue. Vegetative tissue was observed to be more sensitive to radiant exposure of PAR and PAR + UVR compared to reproductive tissue. Under PAR, a 20% reduction in Fv/Fm was observed in non‐soral compared to no reduction in soral tissue, whereas under PAR + UVR, 60% and 33% reduction in Fv/Fm was observed in non‐soral and soral tissues, respectively. This can be attributed to the corresponding three times higher antiradical power (ARP) capacity in soral compared to non‐soral tissue.     

Influence of UV radiation on growth of sporelings of three non-geniculate coralline red algae from Southern Iberian Peninsula

The effects of UV radiation (UVR) on growth of sporelings of Melobesia membranacea (Esper) Lamouroux, Lithophyllum incrustans Philippi and Mesophyllum lichenoides (Ellis) Lemoine, were investigated by culturing the algae under different doses of photosynthetically active radiation (PAR) only and PAR + UVR. Under natural conditions, the light fields occurring in the habitats of the three species differ substantially. Whereas M. lichenoides and L. incrustans inhabit sun‐exposed places in the eulittoral and upper part of the sublittoral, M. membranacea grows as an epiphyte in shady crevices in the eulittoral, where irradiance is < 10% of that in sun‐exposed places. The relative growth rate (RGR) of sporelings of these non‐geniculate coralline algae was affected by the UVR. The extent of harmful UVR effects on growth rate showed a similar increase as a function of the logarithm of the dose in the three species, inferred by a similar slope in all the linear regressions for a given action spectrum. The inhibition of growth under the PAR + UVR showed similar features in the two species of non‐geniculate coralline species from sun‐exposed places, that is, similar intercepts and slopes in the linear regressions of RGR as a function of the logarithm of the biologically effective dose.     

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.     

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.     

Acclimation of brown algal photosynthesis to ultraviolet radiation in Arctic coastal waters (Spitsbergen, Norway)

In field studies conducted at the Kongsfjord (Spitsbergen) changes of the irradiance in the atmosphere and the sublittoral zone were monitored from the beginning of June until the end of August 1997, to register the minimum and maximum fluxes of ultraviolet and photosynthetically active radiation and to characterise the underwater light climate. Measurements of photosynthesis in three abundant brown algal species (Alaria esculenta, Laminaria saccharina, Saccorhiza dermatodea) were conducted to test whether their photosynthetic performance reflects changing light climate in accordance with depth. Plants sampled at various depths were exposed to controlled fluence rates of photosynthetically active radiation (400–700 nm), UV-A (320–400 nm) and UV-B (280–320 nm). Changes in photosynthetic performance during the treatments were monitored by measuring variable chlorophyll fluorescence of photosystem II. In each species, the degree of inhibition of photosynthesis was related to the original collection depth, i.e. shallow-water isolates were more resistant than plants from deeper waters. The results show that macroalgae acclimate effectively to increasing irradiance levels for both photosynthetically active and ultraviolet radiation. However, the kinetics of acclimation are different within the different species. It is shown that one important strategy to cope with higher irradiance levels in shallow waters is the capability for a faster recovery from high light stress compared to isolates from deeper waters. Received: 13 March 1998 / Accepted: 16 May 1998     

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.     

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.     

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.     

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.