Interactive effects of ultraviolet radiation and salinity on the ecophysiology of two Arctic red algae from shallow waters

In a comparative ecophysiological study, the abundant red macroalgae Devaleraea ramentacea (L.) Guiry and Palmaria palmata (L.) O. Kuntze from shallow waters of the Arctic Kongsfjord (Spitsbergen) were exposed to hyposaline and hypersaline media, in combination with and without artificial UV radiation, to evaluate the interactive effects of both environmental parameters on optimum quantum yield of photosynthesis, as well as on the physiological capability to synthesise and accumulate photoprotective mycosporine-like amino acids (MAAs). While D. ramentacea exhibited euryhaline features and acclimated well to the UV radiation applied, P. palmata can be characterised as a stenohaline plant because of its high mortality even under mild hyposaline conditions (15 PSU). In addition, the latter species showed a limited ability to acclimate to changing PAR/UV radiation, pointing to a relatively low physiological plasticity. Both species synthesised and accumulated MAAs after UV treatment. However, only in D. ramentacea was a correlation between increasing MAA concentration and decreasing photosynthetic sensitivity under UV observed. All ecophysiological data from the laboratory correlate well with field observations, where both red-algal species co-exist in the same shallow-water habitat of the Kongsfjord. However, while P. palmata becomes more often greenish, sometimes slightly bleached over the summer months, D. ramentacea appears much more healthy under the prevailing environmental conditions.     

Photosynthetic performance of Arctic macroalgae after transplantation from deep to shallow waters

Transplantation experiments conducted in the Arctic Kongsfjord (Spitsbergen) in summer 1997 investigated the effects of various types of filtered natural radiation (solar, solar without UV-B, solar without UV-A/B) on photosynthesis of various macroalgae. Two brown algal species (Laminaria solidungula, Saccorhiza dermatodea) and four red algal species (Palmaria palmata, Phycodrys rubens, Phyllophora truncata, Ptilota plumosa) were collected from deeper waters, kept in UV-transparent plexiglass tubes wrapped with different spectral cut-off filter foils and positioned at fixed depths in shallow waters for 7-9 days. At regular intervals, chlorophyll fluorescence of photosystem II (optimum quantum yield, F_v/F_m) was determined, as an indicator of photosynthetic performance. The data demonstrate that shallow-water species such as P. palmata are much less affected by natural photosynthetically active radiation (PAR) and UV radiation near the surface than extremely sensitive deep-water species such as Phyc. rubens which exhibited strong decreases in photosynthetic performance, as well as photobleaching of part of the thallus. The other species showed intermediate response patterns. In most species investigated inhibition of photosynthesis was mainly caused by the UV-B wavelengths. Interpretation of the data clearly indicates species-specific tolerances of photosynthesis to ambient solar radiation which can be explained by broad physiological acclimation potentials and/or genetic adaptation to certain (low or high) irradiances. The species-specific photosynthetic performance under radiation stress is in good accordance with the vertical distribution of the macroalgae on the shore.     

Nutritional value of proteins from edible seaweed Palmaria palmata (dulse)

Palmaria palmata (Dulse) is a red seaweed that may be a potential protein source in the human diet. Its protein content, amino acid composition, and protein digestibility were studied with algae collected every month over a 1-year period. Significant variations in protein content were observed according to the season: The highest protein content (21.9 +/- 3.5%) was found in the winter-spring period and the lowest (11.9 +/- 2.0%) in the summer-early autumn period. Most of the essential amino acids were present throughout the year. After 6-hour in vitro digestion in a cell dialysis using porcine pepsin and porcine pancreatin, the digestibility of proteins from Palmaria palmata crude powder, represented by dialyzed nitrogen, was estimated at 29.52 +/- 1.47%. Relative digestibility was 56%, using casein hydrolysis as 100% reference digestibility. In vitro digestibility of proteins extracted in water was analyzed by sodium dodecylsulfate polyacrylamide gel electrophoresis using either bovine trypsin, bovine chymotrypsin, pronase from Streptomyces griseus, or human intestinal juice. Dulse proteins were hydrolyzed to a limited extent, which confirmed a rather low digestibility. Hydrolysis rate was higher with trypsin and lower with chymotrypsin compared with the two other enzymatic systems, pronase and intestinal juice, respectively. The association of algal powder and protein extract to casein and bovine serum albumin, respectively, produced a significant decrease in the hydrolysis rate of the standard proteins. In conclusion, the digestibility of Palmaria palmata proteins seems to be limited by the algae non-proteic fraction.     

Response of oxidative stress parameters and sunscreening compounds in Arctic amphipods during experimental exposure to maximal natural UVB radiation

The paper investigates tolerance to UV radiation (UVR) in 3 amphipod species from the Arctic Kongsfjord, Spitsbergen: the herbivore Gammarellus homari (0- to 5-m water depth), the strictly carnivore scavenger Anonyx nugax (2- to 5-m water depth) and the detritivore/carnivore Onisimus edwardsi (2- to 5-m water depth). In previous radiation exposure experiments, both carnivore species displayed elevated mortality rates already at moderate UVR levels. Therefore, the concentrations of sunscreening compounds (mycosporine-like amino acids, MAAs, and carotenoids) and two antioxidant enzymes (superoxide dismutase, catalase) were studied in the animals under control conditions and following moderate as well as high UVR exposure.In both carnivore amphipods elevated sensitivity to experimental UVR exposure went along with a degradation of the tissue carotenoid and MAAs and a decrease of the enzymatic antioxidant defence, which resulted in increased lipid peroxidation in exposed animals. In contrast, the herbivore G. homari seems well protected by high concentrations of MAAs absorbed from its algal diet, and no oxidative stress occurred under experimental UVR. The species-specific degree of UV tolerance correlates well with the animals' typical vertical distribution in the water column.     

Exposure to ultraviolet radiation delays photosynthetic recovery in Arctic kelp zoospores

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

Photoinhibition of Photosynthesis and its Recovery in Red Algae

In different marine red algae (Chondrus crispus, Delesseria sanguinea, Membranoptera alata, Phycodrys rubens, Phyllophora truncata, Polyneura hilliae) photoinhibition of photosynthesis has been investigated by means of both fluorescence and oxygen measurements. Measurements of absolute oxygen production show that photoinhibition causes a decline in the initial slope and in the rate of bending of the fluence rate-response curve (i.e. the photosynthetic efficiency at non-saturating fluence rates), as well as a decline in the photosynthetic capacity (Pm) at saturating fluence rates. Fluorescence data (Fv/Fm) were consistent with the results of oxygen measurements. Under excessive light photoinhibition protects photosynthesis against photo-damage in red algae. However, an increase in the initial fluorescence (Fo) after photoinhibitory treatment indicates that it could not prevent photodamage entirely. Action spectra of photoinhibition demonstrate that the main photoinhibition site in Polyneura hiliae is PS II, because far red light absorbed by PS I was ineffective. The strong increase of Fo in the blue wavelength range and the slight and partial recovery in weak blue light indicate that blue light especially causes photodamage. Recovery of photosynthesis requires dim white light conditions. Experiments with monochromatic light also show a wavelength dependence of recovery. Moreover, the recovery of photosynthesis after a photoinhibitory treatment is strongly temperature dependent, indicating participation of enzymatic processes. The comparison of fluorescence and oxygen measurement of the recovery shows different results in some species. The rate of oxygen production in red control light increased immediately after photoinhibited algae were exposed to weak light conditions. Surprisingly, the ratio of variable to maximum fluorescence (Fv/Fm) of Phyllophora truncata and the maximum fluorescence (Fm) of Polyneura hilliae show first a delay of the recovery under weak light conditions. Thus, in recovery experiments fluorescence and oxygen data are not quite consistent.     

The supramolecular architecture, function, and regulation of thylakoid membranes in red algae: an overview

Red algae are a group of eukaryotic photosynthetic organisms. Phycobilisomes (PBSs), which are composed of various types of phycobiliproteins and linker polypeptides, are the main light-harvesting antennae in red algae, as in cyanobacteria. Two morphological types of PBSs, hemispherical- and hemidiscoidal-shaped, are found in different red algae species. PBSs harvest solar energy and efficiently transfer it to photosystem II (PS II) and finally to photosystem I (PS I). The PS I of red algae uses light-harvesting complex of PS I (LHC I) as a light-harvesting antennae, which is phylogenetically related to the LHC I found in higher plants. PBSs, PS II, and PS I are all distributed throughout the entire thylakoid membrane, a pattern that is different from the one found in higher plants. Photosynthesis processes, especially those of the light reactions, are carried out by the supramolecular complexes located in/on the thylakoid membranes. Here, the supramolecular architecture, function and regulation of thylakoid membranes in red algal are reviewed.     

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     

Photoinhibition and its wavelength dependence in the cyanobacterium Anabaena variabilis

In the cyanobacterium Anabaena variabilis the dependence of photoinhibition on fluence rate, duration and wavelength of irradiation were studied by measurements of oxygen production and fluorescence emission spectra. The analysis of the photosynthetic activity revealed that photoinhibition affects exclusively photosystem II (PS II), whereas photosystem I (PS I) remained largely unimpaired. Furthermore, PS II fluorescence emission decreased much faster in bleached than in unbleached controls.Studying the wavelength dependence of photoinhibition it was found that only radiation between 520 and 680 nm causes photoinhibition. This is about the same range of wavelengths which causes photobleaching. Fluorescence emission spectra of samples exposed to high fluence rates of 582 and 662 nm, respectively, essentially agree with those samples exposed to high fluence rates of white light, whereas the fluorescence emission spectra of samples exposed to blue light resemble those exposed to dim white light.NaN₃, a substance which prevents photobleaching, inhibits the photosynthetic O₂ production of Anabaena and, hence, enhances the photoinhibitory effect.     

ACCLIMATION OF THE PHOTOSYNTHETIC APPARATUS OF PALMARIA PALMATA (RHODOPHYTA) TO LIGHT QUALITIES THAT PREFERENTIALLY EXCITE PHOTOSYSTEM I OR PHOTOSYSTEM II1

Acclimation of the photosynthetic apparatus to light absorbed primarily by phycobilisomes (which transfer energy predominantly to photosystem II) or absorbed by chlorophyll a (mainly present in the antenna of photosystem I) was studied in the macroalga Palmaria palmata L. In addition, the influence of blue and yellow light, exciting chlorophyll a and phycobilisomes, respectively, ivas investigated. All results were compared to a white light control. Complementary chromatic adaptation in terms of an enhanced ratio of phycoerythrin to phycocyanin under green light conditions was observed. Red light (mainly absorbed by chlorophyll a) and green light (mainly absorbed by phycobilisomes) caused an increase of the antenna system, which was not preferentially excited. Yellow and blue light led to intermediate states comparable to each other and white light. Growth was reduced under all light qualities in comparison to white light, especially under conditions preferably exciting phycobilisomes (green light-adapted algae had a 58% lower growth rate compared to white light-adapted algae). Red and blue light-adapted algae showed maximal photosynthetic capacity with white light excitation and significantly lower values with green light excitation. In contrast, green and yellow light-adapted algae exhibited comparable photosynthetic capacities at all excitation wavelengths. Low-temperature fluorescence emission analysis showed an increase of photosystem II emission in red light-adapted algae and a decrease in green light-adapted algae. A small increase of photosystem I emission teas also found in green light-adapted algae, but this was much less than the photosystem II emission increase observed in red light-adapted algae (both compared to phycobilisome emission). Efficiency of energy transfer from phycobilisomes to photosystem II was higher in red than in green light-adapted algae. The opposite was found for the energy transfer efficiency from phycobilisomes to photosystem I. Zeaxanthin content increased in green and blue light-adapted algae compared to red, white, and yellow light-adapted algae. Results are discussed in comparison to published data on unicellular red algae and cyanobacteria.     

Temperature and light requirements for growth of two diatom species (Bacillariophyceae) isolated from an Arctic macroalga

In the present study, two abundant epiphytic diatom taxa were isolated from the assimilation hairs of the brown macroalga Chordaria flagelliformis collected in the Arctic Kongsfjorden (Spitsbergen, Norway), established as unialgal cultures and their growth rates determined under controlled photon fluence rate and temperature conditions. Using morphological (light and scanning electron microscopy) and SSU rRNA gene data both isolates (ROS D99 and ROS D125) were identified as members of a Fragilaria–Synedropsis clade. The molecular data of ROS D99 and ROS D125 were not identical to any other published sequence. While ROS D99 has been identified as Fragilaria barbararum mainly due to the SEM characteristics, ROS D125 could not be definitely identified although morphological data speak for Fragilaria striatula. Both diatom species showed similar growth rates at all temperatures and photon fluence rates tested. They grew well between 0 and 15°C with optimum temperatures of 12–14°C, but did not survive 20°C. Therefore, compared to Antarctic diatoms both taxa from Kongsfjorden can be characterised as eurythermal organisms. Increasing photon fluence rates between 2 and 15 μmol m⁻² s⁻¹ were accompanied with an almost twofold increase in growth rates, but photon fluence rates >15 μmol m⁻² s⁻¹ did not further enhance growth pointing to low light requirements. From these data optimum, minimum and maximum photon fluence rates and temperatures for growth can be assessed indicating that both diatoms are well acclimated to the fluctuating environmental conditions in the Arctic habitat.     

DAY AND NIGHT KINETICS OF GROWTH RATE IN GREEN,BROWN, AND RED SEAWEEDS1

Growth rates in terms of area increase per 30 min were measured in flat thalli of several seaweed, species by means of computer-assisted image analysis, at 12 h light per day and a photon fluence rate of 20 μmol · m^-2· s^?1. Light fields included white fluorescent, imitated underwater, blue, green, and red light. In the green alga Ulva pseudocurvata Koeman et Hoek, blue light caused an immediate reduction of thallus area and growth rate after the onset of light, whereas green light and red light resulted in an initial peak in growth rate followed by inhibition 60 min after the onset of light. More growth was observed in darkness than in blue light in U. pseudocurvata. All brown and red algae tested, with Laminaria saccharina (L.) Lamour. and Palmaria palmata Stackh. as the main investigated species, grew faster during the day than during the night, irrespective of light quality during the main light phase. The upper intertidal red alga Porphyra umbilicalis (L.) J. Ag. achieved most of its thallus expansion per 24 h during the first 3 h of the light phase, with maximum growth rates of 2–3% increase in area per hour. Maximal growth rates were 0.7% for juvenile laminarian sporophytes and were lower than this in Palmaria palmata and other perennial red algae. The temporary growth inhibition by light in Ulva pseudocurvata suggests photomorphogenetic events, similar to the kinetics of stem elongation in higher plant seedlings after blue or red light pulses in darkness.     

Temperature dependence of UV radiation effects in Arctic and temperate isolates of three red macrophytes

The temperature dependence of UV effects was studied for Arctic and temperate isolates of the red macrophytes Palmaria palmata, Coccotylus truncatus and Phycodrys rubens. The effects of daily repeated artificial ultraviolet B and A radiation (UVBR: 280–320?nm, UVAR: 320–400?nm) treatments were examined for all isolates at 6, 12 and 18?°C by measuring growth, optimal quantum yield of PSII (F_v/F_m) and cyclobutane-pyrimidine dimer (CPD) accumulation. Furthermore, possible ecotypic differences in UV sensitivity between Arctic and temperate isolates were evaluated. Large species-specific differences in UV sensitivity were observed for all parameters: the lower subtidal species C. truncatus and P. rubens were highly sensitive to the UV treatments, whereas P. palmata, which predominantly occurs in the upper subtidal zone, was not affected by these treatments. Only minor differences were found between Arctic and temperate isolates, suggesting that no differences in UV sensitivity have evolved in these species. Relative growth rates were temperature-dependent, whereas species-specific UV effects on growth rates were relatively independent of temperature. In contrast, the species-specific decrease in F_v/F_m and its subsequent recovery were temperature-dependent in all species. UV effects on F_v/F_m were lower at 12 and 18?°C compared with 6?°C. In addition, UV effects on F_v/F_m decreased in the course of the experiment at all temperatures, indicating acclimation to the UV treatments. CPDs accumulated during the experiment in both isolates of P. rubens, whereas CPD concentrations remained low for the other two species. CPD accumulation appeared to be independent of temperature. The results suggest that summer temperatures occurring in temperate regions facilitate repair of UV-induced damage and acclimation to UV radiation in these algae compared with Arctic temperatures. Because the differences in UV effects on F_v/F_m, growth and CPD accumulation were relatively small over a broad range of temperatures, it was concluded that the influence of temperature on UV effects is small in these species.     

Axillary bud proliferation and ethylene production as controlled by radiation of different spectral composition and exogenous phytohormones

The effect of radiation of different spectral composition on axillary proliferation of lavandin (Lavandula officinalis Chaix ×Lavandula latifolia Villars cv. Grosso) was studied in combination with application of exogenous benzyladenine (BA) and putrescine (Put) and endogenous ethylene production. The effect of BA was predominant over the radiation. Continuous far-red showed a fluence rate-dependent promotion of shoot proliferation in the presence of BA. On BA-free medium, shoot number was enhanced under blue, white, and red radiation, at low photon fluence rates. BA, however, could reduce the inhibiting effect of blue and ultraviolet radiation, at high photon fluence rates. Exogenous Put stimulated axillary bud proliferation under some radiation treatments in the presence of BA. Moreover, Put, analogously to BA, could overcome the detrimental effect of ultraviolet radiation. A positive correlation between biotic ethylene production and shoot formation was evidenced under far-red at high photon fluence rate in the presence of BA, and under white, red and blue radiation at low photon fluence rate in the BA-free medium. However, when abiotic ethylene (released from the agarized medium) was stimulated by UV, no improvement of shoot formation was observed.     

In situ photosynthetic differentiation of the green algal and the cyanobacterial photobiont in the crustose lichen Placopsis contortuplicata

In situ photosynthetic activity in the green algal and the cyanobacterial photobionts of Placopsis contortuplicata was monitored within the same thallus using chlorophyll a fluorescence methods. It proved possible to show that the response to hydration of the green algal and the cyanobacterial photobionts is different within the same thallus. Measurements of the photochemical efficiency of PS II, Fv/Fm, reveal that in the dry lichen thallus photosynthetic activity could be induced in the green algal photobiont by water vapour uptake, in the cyanobacterial photobiont only if it was hydrated with liquid water. However, rates of apparent electron flow through PS II as well as rates of CO₂ gas exchange were suboptimal after hydration with water vapour alone and maximum rates could only be observed when the thallus was saturated with liquid water. The differences in the waterrelated photosynthetic performance and different light response curves of apparent electron transport rate through PS II indicate that the two photobionts act highly independently of each other. It was shown that the cyanobacteria from the cephalodia in P. contortuplicata act as photobiont. The rate of electron flow through PS II was found to be saturated at 1500 mol photon m^–2 s^–1, despite a considerable increase of non-photochemical quenching in the green algal photobiont which is lacking in the cyanobacterial photobiont. No evidence of photoinhibition could be found in either photobiont. Pronounced competition between the green algal and the cyanobacterial thallus can be observed in the natural habitat, indicating that the symbiosis in P. contortuplicata should be regarded as a very variable adaptation to the extreme environmental conditions in the maritime Antarctic.Abbreviations DR dark respiration - ETR apparent rate of electron flow of PS II (=F/Fm×PFD) - F difference in yield of fluorescence and maximal Fm and steady state Fs under ambient light - Fo minimum level of fluorescence yield in dark-adapted state - Fo minimum level of fluorescence yield after transient darkening and far-red illumination - Fm maximum level of dark-adapted fluorescence yield - Fm maximum yield of fluorescence under ambient light - Fs yield of fluorescence at steady state - Fv difference in minimum fluorescence and maximum fluorescence in dark-adapted state - NP net photosynthesis - NPQ coefficient for non-photochemical quenching - PAR photosynthetically active radiation (400–700 nm) - PFD photon flux density in PAR - PS II photosystem II - qN coefficient for non-photochemical quenching - qP coefficient for photochemical quenching     

遮光对不同基因型玉米光合特性的影响

采用盆栽试验,研究了遮光对4个基因型玉米光合特性的影响.结果表明:4个基因型玉米叶片的光饱和点、净光合速率(Pn)、电子传递速率(ETR)、光系统Ⅱ最大光化学效率(Fv/Fm)和实际光化学效率(ΦPSⅡ)均受光强的影响.遮光降低了玉米的光饱和点,苗期遮光处理豫玉2号和丹玉13分别在光量子通量密度(PFD)为1400μmol·m-2·s-1和1100μmol·m-2·s-1时达到饱和.遮光还降低了玉米的Pn、ETR、Fv/Fm和ΦPSⅡ,但不同基因型玉米表现不同,豫玉2号和掖单22的下降幅度较小,而丹玉13和掖单6号的下降幅度较大.     

Structural and functional properties of the coleoptile chloroplast: Photosynthesis and photosensory transduction

Recent studies have shown that guard cell and coleoptile chloroplasts appear to be involved in blue light photoreception during blue light-dependent stomatal opening and phototropic bending. The guard cell chloroplast has been studied in detail but the coleoptile chloroplast is poorly understood. The present study was aimed at the characterization of the corn coleoptile chloroplast, and its comparison with mesophyll and guard cell chloroplasts. Coleoptile chloroplasts operated the xanthophyll cycle, and their zeaxanthin content tracked incident rates of solar radiation throughout the day. Zeaxanthin formation was very sensitive to low incident fluence rates, and saturated at around 800–1000 mol m^–2 s^–1. Zeaxanthin formation in corn mesophyll chloroplasts was insensitive to low fluence rates and saturated at around 1800 mol m^–2 s^–1. Quenching rates of chlorophyll a fluorescence transients from coleoptile chloroplasts induced by saturating fluence rates of actinic red light increased as a function of zeaxanthin content. This implies that zeaxanthin plays a photoprotective role in the coleoptile chloroplast. Addition of low fluence rates of blue light to saturating red light also increased quenching rates in a zeaxanthin-dependent fashion. This blue light response of the coleoptile chloroplast is analogous to that of the guard cell chloroplast, and implicates these organelles in the sensory transduction of blue light. On a chlorophyll basis, coleoptile chloroplasts had high rates of photosynthetic oxygen evolution and low rates of photosynthetic carbon fixation, as compared with mesophyll chloroplasts. In contrast with the uniform chloroplast distribution in the leaf, coleoptile chloroplasts were predominately found in the outer cell layers of the coleoptile cortex, and had large starch grains and a moderate amount of stacked grana and stroma lamellae. Several key properties of the coleoptile chloroplast were different from those of mesophyll chloroplasts and resembled those of guard cell chloroplasts. We propose that the common properties of guard cell and coleoptile chloroplasts define a functional pattern characteristic of chloroplasts specialized in photosensory transduction.Abbreviations Ant or A antheraxanthin - dv/dt fluorescence quenching rate - Fm maximum yield of fluorescence with all PS II reaction centers closed - Fo yield of instantaneous fluorescence with all PS II reaction centers open - Vio or V violaxanthin - Zea or Z zeaxanthin     

Photosynthesis of two Arctic macroalgae under different ambient radiation levels and their sensitivity to enhanced UV radiation

The change in optimal quantum efficiency (F _v/F _m) of the Arctic species Laminaria saccharina and Palmaria palmata was investigated in a long-term experiment in situ under different radiation levels during the summer of 1997 in the Kongsfjord (Ny-Ålesund, Spitsbergen, Norway, 78°55.5′N, 11°56.0′E). Whole plants were incubated in an open box system made of UV-transparent Perspex and exposed to solar radiation (λ>295?nm), solar radiation excluding UVB (λ?>?320?nm) and solar radiation excluding UVA?+ UVB (λ?>?400?nm). Increasing radiation levels were simulated by transplantation of the pre-adapted algae from their growth depth at 2?m to a water depth of 1?m. Sensitivity to artificially increased UV radiation was determined by exposure of algae from the three treatments to 6?h of strong UV radiation. P. palmata was relatively insensitive to increasing UV radiation and recovered very fast and almost completely in 2?h. Even plants pre-cultured in ambient radiation levels excluding UVA?+?UVB or UVB only showed no photoinhibition after exposure to extra UV radiation in the laboratory. L. saccharina was, in comparison to P. palmata, more sensitive and showed photoinhibition under solar radiation and solar minus UVB radiation after transplantation from 2 to 1?m water depth. However, after 3?weeks at 1?m depth, F _v/F _m of L. saccharina was equal in all treatments and restored to the original values at the start of the experiment. Sensitivity to extra UV radiation in the laboratory increased in time, although recovery was also fast and occurred within 20?h.     

pEffects of UV radiation on the ultrastructure of several red algae

The effect of ultraviolet (UV) radiation on the ultrastructure of four red algae, the endemic Antarctic Palmaria decipiens (Reinsch) Ricker and Phycodrys austrogeorgica Skottsberg, the Arctic‐cold temperate Palmaria palmata (Linnaeus) O. Kuntze and the cosmopolitan Bangia atropurpurea (Roth) C. Agardh was studied. All four species showed a formation of ‘inside‐out’ vesicles from the chloroplast thylakoids upon exposure to artificial UV‐radiation. In P. decipiens, most vesicles were developed after 8 h and in P. palmata, after 48 h of UV exposure. In B. atropurpurea, vesi‐culation of thylakoids was observed after 72 h of UV irradiation. In Ph. austrogeorgica, the chloroplast envelope and thylakoid membranes were damaged and the phycobilisomes became detached from the thylakoids after 12 h of UV exposure. Ultraviolet‐induced changes in the membrane structure of mitochondria were observed in P. decipiens and P. palmata. However, in P. decipiens they were reversible as was the damage in chloroplast fine structure after 12 h of UV treatment. Protein crystals in Ph. austrogeorgica showed degradation after exposure to UV radiation. Different methods of fixation and embedding macroalgal material are discussed. These findings give insight into the fine structural changes which occur during and after UV exposure and indicate a relationship between the species dependent sensitivity to UV‐exposure and the depth distribution of the different species.     

Loss of Functional Photosystem II Reaction Centres in Zooxanthellae of Corals Exposed to Bleaching Conditions: Using Fluorescence Rise Kinetics

Mass coral bleaching is linked to elevated sea surface temperatures, 1-2 degrees C above average, during periods of intense light. These conditions induce the expulsion of zooxanthellae from the coral host in response to photosynthetic damage in the algal symbionts. The mechanism that triggers this release has not been clearly established and to further our knowledge of this process, fluorescence rise kinetics have been studied for the first time. Corals that were exposed to elevated temperature (33 degrees C) and light (280 mumol photons m(-2) s(-1)), showed distinct changes in the fast polyphasic induction of chlorophyll-a fluorescence, indicating biophysical changes in the photochemical processes. The fluorescence rise over the first 2000ms was monitored in three species of corals for up to 8 h, with a PEA fluorometer and an imaging-PAM. Pocillopora damicornis showed the least impact on photosynthetic apparatus, while Acropora nobilis was the most sensitive, with Cyphastrea serailia intermediate between the other two species. A. nobilis showed a remarkable capacity for recovery from bleaching conditions. For all three species, a steady decline in the slope of the initial rise and the height of the J-transient was observed, indicating the loss of functional Photosystem II (PS II) centres under elevated-temperature conditions. A significant loss of PS II centres was confirmed by a decline in photochemical quenching when exposed to bleaching stress. Non-photochemical quenching was identified as a significant mechanism for dissipating excess energy as heat under the bleaching conditions. Photophosphorylation could explain this decline in PS II activity. State transitions, a component of non-photochemical quenching, was a probable cause of the high non-photochemical quenching during bleaching and this mechanism is associated with the phosphorylation-induced dissociation of the light harvesting complexes from the PS II reaction centres. This reversible process may account for the coral recovery, particularly in A. nobilis.