Seasonal changes in photosystem II organisation and pigment composition in Pinus sylvestris

Conifers of the boreal zone encounter considerable combined stress of low temperature and high light during winter, when photosynthetic consumption of excitation energy is blocked. In the evergreen Pinus sylvestris L. these stresses coincided with major seasonal changes in photosystem II (PSII) organisation and pigment composition. The earliest changes occurred in September, before any freezing stress, with initial losses of chlorophyll, the D1-protein of the PSII reaction centre and of PSII light-harvesting-complex (LHC II) proteins. In October there was a transient increase in F₀, resulting from detachment of the light-harvesting antennae as reaction centres lost D1. The D1-protein content eventually decreased to 90%, reaching a minimum by December, but PSII photochemical efficiency [variable fluorescence (F_v)/maximum fluorescence (F_m)] did not reach the winter minimum until mid-February. The carotenoid composition varied seasonally with a twofold increase in lutein and the carotenoids of the xanthophyll cycle during winter, while the epoxidation state of the xanthophylls decreased from 0.9 to 0.1 from October to January. The loss of chlorophyll was complete by October and during winter much of the remaining chlorophyll was reorganised in aggregates of specific polypeptide composition, which apparently efficiently quench excitation energy through non-radiative dissipation. The timing of the autumn and winter changes indicated that xanthophyll de-epoxidation correlates with winter quenching of chlorophyll fluorescence while the drop in photochemical efficiency relates more to loss of D1-protein. In April and May recovery of the photochemistry of PSII, protein synthesis, pigment rearrangements and zeaxanthin epoxidation occurred concomitantly. Indoor recovery of photosynthesis in winter-stressed branches under favourable conditions was completed within 3 d, with rapid increases in F₀, the epoxidation state of the xanthophylls and in light-harvesting polypeptides, followed by recovery of D1-protein content and F_v/F_m, all without net increase in chlorophyll. The fall and winter reorganisation allow Pinus sylvestris to maintain a large stock of chlorophyll in a quenched, photoprotected state, allowing rapid recovery of photosynthesis in spring.Abbreviations Elips early light-induced proteins - EPS epoxidation state - F₀ instantaneous fluorescence - F_m maximum fluorescence - F_v variable fluorescence - LHC II light-harvesting complex of PSII - LiDS lithium dodecyl sulfate This research was supported by the Swedish Natural Science Research Council. We wish to thank Dr. Adrian Clarke¹ (Department of Plant Physiology, University of Umeå, Sweden) for advice on electrophoresis, valuable discussion and providing antibodies. Dr. Stefan Jansson¹ and Dr. Torill Hundal (Department for Biochemistry, University of Stockholm, Sweden) provided antibodies. Jan Karlsson¹ helped with the HPLC, Dr. Marianna Krol gave advice on green gels and Dr. Vaughan Hurry (Cooperative Research Centre for Plant Sciences, Australian National University, Canberra, Australia) provided valuable discussion.     

Response of the Photosynthetic Apparatus of the Diatom Thallassiosira weisflogii to High Irradiance Light

The response of the photosynthetic apparatus to high irradiance illumination (440–2200 W/m²) was studied in the diatom Thallassiosira weisflogii by fluorescence methods. Changes in the photosynthetic apparatus were monitored by measuring characteristics of chlorophyll fluorescence F ₀, F _m, F _v/F _m, and qN for several hours after illumination of the alga with high-intensity light. Incubation of the alga with 2 mM DTT, an inhibitor of de-epoxidase of carotenoids in the diadinoxanthin cycle, led to a decrease in the nonphotochemical quenching of chlorophyll fluorescence and a drop in the F _v/F _m ratio, a characteristic that reflects the quantum efficiency of the functioning of the photosynthetic apparatus. Light-induced absorption changes associated with transformations of carotenoids of diadinoxanthin cycle were recorded in vivo in algal suspensions in the absence and in the presence of DTT. Using the microfluorometric method, we measured cell distribution over the efficiency of the primary processes of photosynthesis (F _v/F _m) after illumination. We found cells with a high tolerance of their photosynthetic apparatus to photooxidative damage. The relatively high tolerance of a portion of the cell population to high-light illumination can be related to light-induced transformation of carotenoids and to the functioning of other protective systems of the photosynthetic apparatus in diatoms.     

Chlorophyll fluorescence and leaf chlorophyll content of bean leaves injured by spider mites (Acari: Tetranychidae)

The use of chlorophyll fluorescence as a method for detecting and monitoring plant stress arising from Tetranychus urticae (Koch) feeding injury was investigated. The effect of mite density (1–32 mites per 1.5 cm² of leaf) and the duration of the feeding period (1–5 days) on the chlorophyll fluorescence parameters of bean (Phaseolus vulgaris) leaves were examined. Changes in chlorophyll fluorescence parameters were dependent both on mite density and duration of feeding. Decreases in F _o, the initial fluorescence and F _m, the maximum fluorescence led to a decrease in the ratio of variable to maximum fluorescence, F _v/F _m. The decrease in F _v/F _m is typical of the response of many plants to a wide range of environmental stresses and indicates a reduced efficiency of photosystem II (PSII) photochemistry. T _1/2, which is proportional to the pool size of electron acceptors on the reducing side of PSII, was also reduced in response to mite-feeding injury. The leaf chlorophyll content decreased with increasing mite density and duration of feeding but did not appear to contribute to the decrease in F _v/F _m. Chlorophyll fluorescence is an effective method for detecting and monitoring stress in T. urticae-injured bean leaves.     

Biomass production in mixotrophic culture of Euglena gracilis under acidic condition and its growth energetics

When Euglena gracilis was grown in the heterotrophic condition with glucose and (NH₄)₂SO₄ as the carbon and nitrogen source, a high cell yield (4.28–4.48 g l^–1) was obtained and the culture pH decreased to 1.6–2. The biomass production in the heterotrophic culture was compared to those in the autotrophic and mixotrophic cultures. Autotrophic growth was 4.7–6.3% of the heterotrophic one, whereas about 15–19% higher growth was obtained in the mixotrophic culture. Moreover, good production of chlorophyll (39.4 mg l^–1) and carotenoids (13.8 mg l^–1) were attained in the mixotrophic culture, giving the highest fermenter productivity with respect to biomass as well as chlorophyll and carotenoids. Through an energetic analysis in the mixotrophic culture, it was estimated about 25–28% of the total ATP requirement is formed in the photochemical reactions. This resulted in an improved biomass production in the mixotrophic culture of E. gracilis.     

Effect of drought stress on chlorophyll a fluorescence and electrical admittance of shoots in Norway spruce seedlings

Effects of mild and severe soil drought on the water status of needles, chlorophyll a fluorescence, shoot electrical admittance, and concentrations of photosynthetic pigments in needles of seedlings of Picea abies (L.) Karst. were examined under controlled greenhouse conditions. Drought stress reduced shoot admittance linearly with a decrease in shoot water potential (_w) and increase in water deficit (WD) and led to a decrease in concentrations of chlorophyll a, b and carotenoids. Severe water stress (shoot _w=–2.4 MPa) had a negative effect on chlorophyll a fluorescence parameters including PSII activity (F_v/F_m), and the vitality index (R_fd). Variations in these parameters suggest an inhibition of the photosynthetic electron transport in spruce needles. Water stress led to a decrease in the mobility of electrolytes in tissues, which was reflected by decreased shoot electrical admittance. After re-watering for 21 days the WD in needles decreased and the shoot water potential increased. In the re-watered plants, the chloroplast function was restored and chlorophyll a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in the seedlings triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. We conclude that the shoot electrical admittance and photosynthetic electron transport in leaves are closely linked to changes in water status and their decrease is among the initial responses of seedlings to water stress.     

Effect of a Short Photoinhibition Stress on Photosynthesis,Chlorophyll a Fluorescence,and Pigment Contents of Different Maize Cultivars. Can a Rapid and Objective Stress Indicator be Found?

The effect of a short cold stress in combination with photoinhibition stress, similar to a low temperature and a high irradiance situation during early morning in the spring time, was examined on four maize cultivars common for Belgium, that differ in early vigour. After 1 h of 2 °C and 500 mol(photon) m^–2 s^–1, quantum efficiency and maximum photosynthesis rate at saturating irradiance decreased on average by 11 and 8 %, respectively. For one cultivar, Magister, the decrease was the largest: by 23 and 10 %, respectively. For this cultivar it was combined with a decrease of the water vapour conductance after the stress. The decrease of F_v/F₀ due to the cold/light stress was dependent on the cold tolerance (early vigour) of the cultivars. F_v/F₀ changed with –45.5 and –40.2 % for the cultivars Ardiles and Banguy, respectively (cultivars with a less good early vigour) in comparison to –36.3 and –35.9 % for Fjord and Magister, which have a good early vigour. Also the ratio of total chlorophylls/total carotenoids changed in dependence on cold tolerance of the cultivars. For more cold tolerant cultivars, the relative amount of total carotenoids (x+c) was higher, indicating a higher protective state. Both the parameter F_v/F₀ and the ratio of total chlorophylls to total carotenoids can be used to differentiate the cold tolerant cultivars from the cold non-tolerant ones. F_v/F₀ has the advantage because its resolving power is larger and the measurement is less expensive than determination of the pigment ratio.     

Multicolour Fluorescence Imaging of Sugar Beet Leaves with Different Nitrogen Status by Flash Lamp UV-Excitation

Fluorescence images of leaves of sugar beet plants (Beta vulgaris L. cv. Patricia) grown on an experimental field with different fertilisation doses of nitrogen [0, 3, 6, 9, 12, 15 g(N) m^–2] were taken, applying a new multicolour flash-lamp fluorescence imaging system (FL-FIS). Fluorescence was excited by the UV-range (280–400 nm, _max = 340 nm) of a pulsed Xenon lamp. The images were acquired successively in the four fluorescence bands of leaves near 440, 520, 690, and 740 nm (F₄₄₀, F₅₂₀, F₆₉₀, F₇₄₀) by means of a CCD-camera. Parallel measurements were performed to characterise the physiological state of the leaves (nitrogen content, invert-sugars, chlorophylls and carotenoids as well as chlorophyll fluorescence induction kinetics and beet yield). The fluorescence images indicated a differential local patchiness across the leaf blade for the four fluorescence bands. The blue (F₄₄₀) and green fluorescence (F₅₂₀) were high in the leaf veins, whereas the red (F₆₉₀) and far-red (F₇₄₀) chlorophyll (Chl) fluorescences were more pronounced in the intercostal leaf areas. Sugar beet plants with high N supply could be distinguished from beet plants with low N supply by lower values of F₄₄₀/F₆₉₀ and F₄₄₀/F₇₄₀. Both the blue-green fluorescence and the Chl fluorescence rose at a higher N application. This increase was more pronounced for the Chl fluorescence than for the blue-green one. The results demonstrate that fluorescence ratio imaging of leaves can be applied for a non-destructive monitoring of differences in nitrogen supply. The FL-FIS is a valuable diagnostic tool for screening site-specific differences in N-availability which is required for precision farming.     

Interplay between photochemical activities and pigment composition in an outdoor culture of Haematococcus pluvialis during the shift from the green to red stage

The transfer of laboratory cultures of H. pluvialis to high irradiance outdoors caused a substantial decline in the maximum quantum yield of photosystem II (PSII), from 0.65 in the morning to 0.45 at midday, as measured by the ratio of variable to maximum fluorescence yields (F_v/F_m), and a steep rise in non-photochemical quenching (NPQ). Chlorophyll fluorescence induction curves of morning samples showed a clear I-step, reflecting a certain PSII heterogeneity. Single turnover flash measurements on samples taken from the outdoor photobioreactor in the middle of day showed an increase in the reoxidation time constant of the reduced plastoquinone Q_A ^–, i.e., the time required for electron transfer from the primary plastoquinone acceptor of PSII Q_A ^– to the secondary plastoquinone acceptor Q_B. Photosynthesis rates were almost constant during the day. Along with the increase in non-photochemical quenching, there was a slight increase in zeaxanthin and antheraxanthin contents and decrease in violaxanthin, showing the presence of an operative xanthophyll cycle in this microalga. A marked increase of secondary carotenoids was found at the end of the first day of exposure to sunlight, mainly astaxanthin monoester, which reached 15.5% of the total carotenoid content. Though cells turned reddish during the second day, the decline in the fluorescence parameter F_v/F_m in the middle of the day was less than during the first day, and there was no further increase in the value for NPQ. Similar behaviour was observed during the third day when the culture was fully red. After four days of exposure to sunlight, the dry weight reached 800 mg L^–1 and the concentration of secondary carotenoids (81% astaxanthin monoester) reached 4.4% dry weight.     

Deep chlorophyll a maxima in the Red Sea observed by in-vivo flash fluorometry

An investigation of the hydrography and the population, production and biomass of plankton in the Red Sea, carried out during the METEOR cruise in summer 1987, aimed to describe the ecosystem characteristics during the SW monsoon period. Vertical profiles of in-vivo chlorophyll fluorescence intensity, measured in the presence of chlorphenyl-dimethyl-urea (CMU), are presented. Variations in the fluorescence pattern were observed and assumed to be due to the influence of a reef and surface influx of nutrient rich water from the Gulf of Aden into the Red Sea. This northward influx was driven by SE winds, caused by an unusual northward shift of the innertropical convergence zone up to 20°N in summer 1987. Integrated chlorophyll a values were calculated from fluorescence data. They showed a slight increase from north to south and higher pigment contents in August (8.7–20.2 mg m^–2) than in July (3.3–9.0 mg m^–2), the latter was attributed to the above mentioned influx. Calibration of the fluorescence measurements using cultures of a green alga and cyanobacterium indicated that there may have been an underestimate of the contribution of Oscillatoria populations to the chlorophyll a concentration of the samples. Fluorescence peaks were recorded in the lower part of the euphotic zone, indicating a deep maximum of phytoplankton and/or an increase in chlorophyll fluorescence per unit biomass at these depths.     

Changes in Photosystem II fluorescence in Chlamydomonas reinhardtii exposed to increasing levels of irradiance in relationship to the photosynthetic response to light

The effects of a 60 min exposure to photosynthetic photon flux densities ranging from 300 to 2200 mol m^–2s^–1 on the photosynthetic light response curve and on PS II heterogeneity as reflected in chlorophyll a fluorescence were investigated using the unicellular green alga Chlamydomonas reinhardtii. It was established that exposure to high light acts at three different regulatory or inhibitory levels; 1) regulation occurs from 300 to 780 mol m^–2s^–1 where total amount of PS II centers and the shape of the light response curve is not significantly changed, 2) a first photoinhibitory range above 780 up to 1600 mol m^–2s^–1 where a progressive inhibition of the quantum yield and the rate of bending (convexity) of the light response curve can be related to the loss of Q_B-reducing centers and 3) a second photoinhibitory range above 1600 mol m^–2s^–1 where the rate of light saturated photosynthesis also decreases and convexity reaches zero. This was related to a particularly large decrease in PS II centers and a large increase in spill-over in energy to PS I.Abbreviations Chl chlorophyll - DCMU 3,(3,4-dichlorophenyl)-1,1-dimethylurea - F_M maximal fluorescence yield - F_pl intermediate fluorescence yield plateau level - F₀ non-variable fluorescence yield - F_v total variable fluorescence yield (F_M-F₀) - initial slope to the light response curve, used as an estimate of initial quantum yield - convexity (rate of bending) of the light response curve of photosynthesis - LHC light-harvesting complex - P_max maximum rate of photosynthesis - PQ plastoquinone - Q photosynthetically active photon flux density (400–700 nm, mol m^–2s^–1) - PS photosystem - Q_A and Q_B primary and secondary quinone electron acceptor of PS II     

Functioning of photosystems I and II in pea leaves exposed to heat stress in the presence or absence of light

Fluorimetric, photoacoustic, polarographic and absorbance techniques were used to measure in situ various functional aspects of the photochemical apparatus of photosynthesis in intact pea leaves (Pisum sativum L.) after short exposures to a high temperature of 40 ° C. The results indicated (i) that the in-vivo responses of the two photosystems to high-temperature pretreatments were markedly different and in some respects opposite, with photosystem (PS) II activity being inhibited (or down-regulated) and PSI function being stimulated; and (ii) that light strongly interacts with the response of the photosystems, acting as an efficient protector of the photochemical activity against its inactivation by heat. When imposed in the dark, heat provoked a drastic inhibition of photosynthetic oxygen evolution and photochemical energy storage, correlated with a marked loss of variable PSII-chlorophyll fluorescence emission. None of the above changes were observed in leaves which were illuminated during heating. This photoprotection was saturated at rather low light fluence rates (around 10 W · m^–2). Heat stress in darkness appeared to increase the capacity for cyclic electron flow around PSI, as indicated by the enhanced photochemical energy storage in far-red light and the faster decay of P ₇₀₀ ⁺ (oxidized reaction center of PSI) monitored upon sudded interruption of the far-red light. The presence of light during heat stress reduced somewhat this PSI-driven cyclic electron transport. It was also observed that heat stress in darkness resulted in the progressive closure of the PSI reaction centers in leaves under steady illumination whereas PSII traps remained largely open, possibly reflecting the adjustment of the photochemical efficiency of undamaged PSI to the reduced rate of photochemistry in PSII.Abbreviations B₁ and B₂ fraction of closed PSI and PSII reaction centers, respectively - ES photoacoustically measured energy storage - F_o, F_m and F_s initial, maximal and steady-state levels of chlorophyll fluorescence - P₇₀₀ reaction center of PSI - PS (I, II) photosystem (I, II) - V = (F_s – F_o)/(F_m – F_o) relative variable chlorophyll fluorescence We wish to thank Professor R. Lannoye (ULB, Brussels) for the use of this photoacoustic spectrometer and Mrs. M. Eyletters for her help.     

Analysis of xanthophyll cycle carotenoids and chlorophyll fluorescence in light intensity-dependent chlorophyll-deficient mutants of wheat and barley

Three light intensity-dependent Chl b-deficient mutants, two in wheat and one in barley, were analyzed for their xanthophyll cycle carotenoids and Chl fluorescence characteristics under two different growth PFDs (30 versus 600 mol photons·m^–2 s^–1 incident light). Mutants grown under low light possessed lower levels of total Chls and carotenoids per unit leaf area compared to wild type plants, but the relative proportions of the two did not vary markedly between strains. In contrast, mutants grown under high light had much lower levels of Chl, leading to markedly greater carotenoid to Chl ratios in the mutants when compared to wild type. Under low light conditions the carotenoids of the xanthophyll cycle comprised approximately 15% of the total carotenoids in all strains; under high light the xanthophyll cycle pool increased to over 30% of the total carotenoids in wild type plants and to over 50% of the total carotenoids in the three mutant strains. Whereas the xanthophyll cycle remained fairly epoxidized in all plants grown under low light, plants grown under high light exhibited a considerable degree of conversion of the xanthophyll cycle into antheraxanthin and zeaxanthin during the diurnal cycle, with almost complete conversion (over 90%) occurring only in the mutants. 50 to 95% of the xanthophyll cycle was retained as antheraxanthin and zeaxanthin overnight in these mutants which also exhibited sustained depressions in PS II photochemical efficiency (F_v/F_m), which may have resulted from a sustained high level of photoprotective energy dissipation activity. The relatively larger xanthophyll cycle pool in the Chl b-deficient mutant could result in part from the reported concentration of the xanthophyll cycle in the inner antenna complexes, given that the Chl b-deficient mutants are deficient in the peripheral LHC-II complexes.Abbreviations A antheraxanthin - Chl chlorophyll - F_o and F_m minimal yield (at open PS II reaction centers) and maximal yield (at closed centers) of chlorophyll fluorescence in darkness - F level of fluorescence during illumination with photosynthetically active radiation - F_m maximal yield (at closed centers) of chlorophyll fluorescence during illumination with photosynthetically active radiation - (F_m–F)/F_m actual efficiency of PS II during illumination with photosynthetically active radiation - F_v/F_m+(F_m–F_o)/F_m intrinsic efficiency of PS II in darkness - LHC_II light-harvesting chlorophyll-protein complex of Photosystem II - PFD photon flux density (between 400 and 700 nm) - PS I Photosystem I - PS II Photosystem II - V violaxanthin - Z zeaxanthin     

The Effect of In Vitro Culture Conditions on the Pattern of Photoinhibition during Acclimation of Gardenia Plantlets to Ex Vitro Conditions

We tested the effect of growing conditions during micropropagation on the fast kinetics of chlorophyll (Chl) fluorescence of Gardenia jasminoides Ellis plantlets during a 4-week acclimation to ex vitro. We studied whether photoautotrophic growing in vitro produced plantlets with less photoinhibition impairment during acclimation. Of the growing conditions stimulating photoautotrophy in vitro, only loose tube caps had a positive effect, whereas low sucrose or sucrose-free content in the medium and high PPFD showed a negative effect. Thus, plantlets cultured with 3 % (m/v) of sucrose were subsequently less photoinhibited throughout acclimation than those cultured with low sucrose (0.5 %) or sucrose-free media. Moreover, at the end of acclimation the former plantlets showed F_v/F_m and F_v/F₀ ratios typical of unstressed ex vitro plants as well as a higher Chl content and ratio of Chls to carotenoids. Plantlets cultured at a photosynthetic photon fluence density (PPFD) of 50 µmol m^–2 s^–1 also showed a better performance at the end of acclimation than those cultured at a higher (110 µmol m^–2 s^–1) PPFD. Thus except in the case of loose-tube closure, gardenia plantlets cultured in vitro under conventional sucrose concentration and PPFD are the least photoinhibited during acclimation. Nevertheless, significant interactions between the in vitro growing factors were observed at the end of acclimation.     

Rearrangement of light harvesting bacteriochlorophyll homologues as a response of green sulfur bacteria to low light intensities

The pigment composition of two species of green-colored BChl c-containing green sulfur bacteria (Chlorobium limicola and C. chlorovibrioides) and two species of brown-colored BChl e-containing ones (C. phaeobacteroides and C. phaeovibrioides) incubated at different light intensities have been studied. All species responded to the reduction of light intensity from 50 to 1 Einstein(E) m^–2 s^–1 by an increase in the specific content of light harvesting pigments, bacteriochlorophylls and carotenoids. At critical light intensities (0.5 to 0.1 E m^–2 s^–1) only brown-colored chlorobia were able to grow, though at low specific rates (0.002 days^–1 mg prot^–1). High variations in the relative content of farnesyl-bacteriochlorophyll homologues were found, in particular BChl e ₁ and BChl e ₄, which were tentatively identified as [M, E] and [I, E] BChl_F e, respectively. The former was almost completely lost upon reduction of light intensity from 50 to 0.1 E m^–2 s^–1, whereas the latter increased from 7.2 to 38.4% and from 13.6 to 42.0% in C. phaeobacteroides and C. phaeovibrioides, respectively. This increase in the content of highly alkylated pigment molecules inside the chlorosomes of brown species is interpreted as a physiological mechanism to improve the efficiency of energy transfer towards the reaction center. This study provides some clues for understanding the physiological basis of the adaptation of brown species to extremely low light intensities.Abbreviations BChl bacteriochlorophyll - [M, E] BChl_F e 8-methyl, 12-ethyl BChl e, esterified with farnesol (F). Analogously: I - isobutyl - Pr propyl - Car carotenoids - Chlb chlorobactene - HPLC high performance liquid chromatography - Isr isorenieratene - LHP light harvesting pigments - PDA photodiode array detector - RC reaction center - RCH relative content of homologues     

In situ monitoring of chlorophyll fluorescence to assess the synergistic effect of low temperature and high irradiance stresses inSpirulina cultures grown outdoors in photobioreactors

A chlorophyll fluorescence technique was applied to anin situ study on the effects of low temperature and high light stresses onSpirulina cultures grown outdoors in controlled tubular photobioreactors at high (1.1 g L^–1) and low (0.44 g L^–1) biomass concentrations. Diurnal changes in PSII photochemistry (F _v/F _m) after 15 min of darkness, or in the light (dF/F _m), and non-photochemical (qN) quenching were measured using a portable, pulse-amplitude-modulated fluorometer. The depression of theF _v/F _m ratio ofSpirulina cultures grown outdoors at 25°C (i.e. 10°C below optimum for growth) and 0.44 g L^–1, reached 30% at the middle of the day. At the same time of the day thedF/F _m ratio showed a reduction of up to 52%. The depression of bothF _v/F _m anddF/F _m was lower in the cultures grown at 1.1 g L^–1. Photoinhibition reduced the daily productivity of the culture grown at 0.44 g L^–1 and 25°C by 33% with respect to that grown at 35°C. Changes in the growth yields of the cultures grown under different temperatures and growth rates correlate well with analogous changes in photon yield (dF/F _m). Simple measurements of photochemical yield (F _v/F _m) can be used to test the physiological status ofSpirulina cultures. The results indicate that the saturating pulse fluorescence technique, when usedin situ, is a powerful tool for assessment of the photosynthetic characteristics of outdoor cultures ofSpirulina.     

Oxidative Processes in Photosystem I of Thermophilic Cyanobacteria at High Temperatures

We studied the mechanisms of the relationships between the generation of millisecond delayed fluorescence in photosystem I (DF) and the oxidative destruction of chlorophyll in the membranes of a thermophilic cyanobacteria Synechococcus elongatusin the temperature range 60–80°C at various irradiation levels and in the presence of substances affecting the intensity of DF. Light and temperature dependencies of the chlorophyll oxidation rates were similar to those of the DF of PSI. Anions Cl^–, Br^–, and NO^– ₃, which quench the triplet states of chlorophyll, almost completely inhibited the chlorophyll oxidation and reduced the intensity of the DF maximum by 70%. Under anaerobic conditions and in the presence of sodium ascorbate, the rate of chlorophyll oxidation also markedly decreased. We found that the long-wavelength chlorophyll forms were the most susceptible to oxidation and related the temperature-dependent changes in the DF of PSI and in the oxidative processes in the membranes of thermophilic cyanobacteria to an increase in the concentration of the triplet states of P₇₀₀and other chlorophyll forms. The latter result from the temperature-dependent inactivation of carotenoids and the inhibition of electron transfer to ferredoxin in PSI.     

Photobiont-related differences in carbon acquisition among green-algal lichens

The photosynthetic properties of a range of lichens (eight species) containing green algal primary photobionts of either the genus Coccomyxa, Dictyochloropsis or Trebouxia were examined with the aim of obtaining a better understanding for the different CO₂ acquisition strategies of lichenized green algae. Fast transients of light/dark-dependent CO₂ uptake and release were measured in order to screen for the presence or absence of a photosynthetic CO₂-concentrating mechanism (CCM) within the photobiont. It was found that lichens with Trebouxia photobionts (four species) were able to accumulate a small pool of inorganic carbon (DIC; 70–140 nmol per mg chlorophyll (Chl)), in the light, which theoretically may result in, at least, a two to threefold increase in the stromal CO₂ concentration, as compared to that in equilibrium with ambient air. The other lichens (four species), which were tripartite associations between a fungus, a cyanobacterium (Nostoc) and a green alga (Coccomyxa or Dictyochloropsis) accumulated a much smaller pool of DIC (10–30 nmol·(mg Chl)^–1). This pool is most probably associated with the previously documented CCM of Nostoc, inferred from the finding that free-living cells of Coccomyxa did not show any signs of DIC accumulation. In addition, the kinetics of fast CO₂ exchange for free-living Nostoc were similar to those of intact tripartite lichens, especially in their responses to the CCM and the carbonic anhydrase (CA) inhibitor ethoxyzolamide. Trebouxia lichens had a higher photosynthetic capacity at low and limiting external CO₂ concentrations, with an initial slope of the CO₂-response curve of 2.6–3.9 mol·(mg Chl)^–1·h^–1·Pa^–1, compared to the tripartite lichens which had an initial slope of 0.5–1.1 mol-(mg Chl)^–1·h^–1·-Pa^–1, suggesting that the presence of a CCM in the photobiont affects the photosynthetic performance of the whole lichen. Regardless of these indications for the presence or absence of a CCM, ethoxyzolamide inhibited the steady-state rate of photosynthesis at low CO₂ in all lichens, indicating a role of CA in the photosynthetic process within all of the photobionts. Measurements of CA activity in photobiont-enriched homogenates of the lichens showed that Coccomyxa had by far the highest activity, while the other photobionts displayed only traces or no activity at all. As the CCM is apparently absent in Coccomyxa, it is speculated that this alga compensates for this absence with high internal CA activity, which may function to reduce the CO₂-diffusion resistance through the cell.Abbreviations CA carbonic anhydrase (EC 4.2.1.1) - CCM CO₂-concentrating mechanism - Chl chlorophyll - DIC dissolved inorganic carbon - EZ ethoxyzolamide or 6-ethoxy-2-benzo-thiazole-2-sulfonamide - GA glycolaldehyde - Hepps 4-(2-hydroxyethyl)-l-piperazinepropanesulfonic acid - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) This research was supported by a grant from the Swedish Natural Sciences Resource Council to K.P.     

Optimum growth conditions and light utilization efficiency of Spirulina platensis (= Arthrospira fusiformis) (Cyanophyta) from Lake Chitu, Ethiopia

Spirulina platensis (= Arthrospira fusiformis) was isolated from Lake Chitu, a saline, alkaline lake in Ethiopia, where it forms an almost unialgal population. Optimum growth conditions were studied in a turbidostat. Cultures grown in modified Zarrouk's medium and exposed to a range of light intensities (20–500 µmol photons m^–2s^–1) showed a maximum specific growth rate (µ_max) of 1.78 d^–1. Quantum yield for growth (µ) was 3.8% at the optimum light for growth of 330 µmol photons m^–2s^–1, and ranged from 2.8 to 9.4%. With increase in irradiance, the chlorophyll a concentration decreased, and the carotenoids/chlorophyll a ratio increased by a factor of 2.4. The phosphorus to carbon ratio (P/C) showed some variation, while the nitrogen to carbon ratio (N/C) remained relatively constant, thus causing fluctuations in the N:P ratio (7–11) of cells. An optimum N:P ratio of about 7 was attained in cells growing at the optimum light for growth. Results from the continuous culture experiments agreed well with maximum values of photosynthetic efficiency given in the literature for natural populations of S. platensis in the soda lakes of East Africa, Lake Arenguade (Ethiopia), and Lake Simbi (Kenya).     

Combined Effects of CO2 and O3 on Antioxidative and Photoprotective Defense Systems in Needles of Ponderosa Pine

Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings were exposed to near ambient or elevated CO₂ (average concentrations during the last growing season 446 versus 699 mol mol^–1), combined with low or elevated O₃ for three seasons. Ozone exposure during the last growing season (accumulated dose above threshold 0.06 mol mol^–1) was 0.05 versus 26.13 mol mol^–1 h. Needles of the youngest age class were harvested after the dormancy period. Ozone exposure decreased needle contents of chlorophyll a, chlorophyll b, and ascorbate, and resulted in a more oxidized total ascorbate and a more de-epoxidized xanthophyll cycle pool irrespective of the CO₂ level. Trees under elevated CO₂ had a more oxidized glutathione pool and lower chlorophyll a content. Contents of glutathione, tocopherol, and carotenoids were not affected by the CO₂ or O₃ treatments. There were no interactive effects between elevated CO₂ and elevated O₃ on any of the parameters measured. The results suggest that elevated atmospheric CO₂ concentration does not compensate for ozone stress by increasing antioxidative capacity in ponderosa pine.