Photosynthetic enhancement at high light intensities

Enhancement of photosynthetic oxygen evolution by Chlorella pyrenoidosa (211-8b) decreased almost linearly as the rate of photosynthesis was increased. The observed decrease was predicted most accurately by a model assuming exponential rate versus intensity curves as opposed to a model assuming hyperbolic light curves. The decrease in enhancement was not changed significantly upon changing from 0.03% to 5% carbon dioxide.     

Culture conditions and the development of the photosynthetic mechanism; influence of light intensity on photosynthetic characteristics of Chlorella

1. Chlorella pyrenoidosa has been grown in a continuous-culture apparatus under various light intensities provided by incandescent lamps, other conditions of culture being maintained constant. Light intensity curves for cells immersed in the No. 11 Warburg buffer and in Knop''s solution + 4.4 per cent CO₂ at a saturating light intensity were determined as characteristics of the photosynthetic mechanism. These characteristics were referred to the centrifuged cell volume as an index of quantity of cellular material. 2. Cells grown at intensities in the range of about 35 f.-c. develop a capacity for a high rate of photosynthesis (c.mm. O₂/hour/c.mm. cells). At culture intensities above or below this range the cells produced have a lower capacity for photosynthesis. A similar effect is observed for rate of photosynthesis per unit dry weight or rate per unit cell nitrogen. 3. The rate of photosynthesis per cell or rate per unit chlorophyll shows no maximum at any light intensity of culture but increases continuously throughout the range of light intensities studied. 4. Maximum rate of growth is attained at a light intensity of about 100 f.-c. The hypothesis is advanced that at culture intensities above that needed to give maximum rate of growth (100 f.-c.) a mechanism is developed which opposes the photosynthetic process and removes the photosynthetic products. 5. The low capacity for photosynthesis shown by cells grown at culture intensities below 35 f.-c. finds no immediate explanation. 6. The shape of the light intensity curve is markedly affected by the light intensity at which the cells have been cultured. Cells grown at lower intensities give light intensity curves approaching the Blackman type with a short transitional region between light limitation and light saturation.     

Modelling time-series of photosynthesis and comparisons with the fluorescence yield in Chlorella vulgaris: a study of adaptation, inhibition and recovery

Time-series of ¹⁴C uptake and fluorescence yield (i.e., thefluorescence enhancement after addition of the photosyntheticinhibitor DCMU) were measured in Chorella vulgaris at variouslight intensities. Adaptation and recovery processes after alterationof the light intensity were also studied. At a constant lightintensity, both the rate of ¹⁴C uptake and the fluorescenceyield decreased with time. Comparison of time-series data of¹⁴C uptake at different light intensities showed that this phenomenonconsisted of several processes (i.e., at low light intensitiessmall changes in uptake rates were mainly due to photoadaptation,while at higher light intensities relatively larger changesoccurred, as result of photoinhibition). Transfer of an algalsample to low light intensities after a period of exposure toinhibiting light intensities resulted in an exponential recoveryof the ¹⁴C uptake rate with time, coupled with an exponentialrecovery of the fluorescence yield. A mechanistic model is presented,which describes the algal ¹⁴C uptake rate as a function of timeand light intensity. The model includes adaptation, inhibitionand recovery. Six parameters, characterising the algal suspension,have to be estimated from the results of one P versus I curveand one time-series ¹⁴C uptake, which includes a period of recovery.Using these parameters the model can predict the time-courseof ¹⁴C uptake at every constant light intensity, as well aswhen the light intensity is changed during the experiment. Whenapplied to a culture of C. vulgaris, the theoretical valuesclosely approach the actual measurements. The resemblance betweenthe measured time-series of fluorescence yield and the rateof ¹⁴C uptake indicates, that the changes in the rate of ¹⁴Cuptake are due to changes in the photosynthetic apparatus, ratherthan to changes of diffusion of ¹⁴C into the cell.     

Time-dependent upregulation of electron transport with concomitant induction of regulated excitation dissipation in <Emphasis Type="Italic">Haslea</Emphasis> diatoms

Photoacclimation by strains of Haslea “blue” diatom species H. ostrearia and H. silbo sp. nov. ined. was investigated with rapid light curves and induction–recovery curves using fast repetition rate fluorescence. Cultures were grown to exponential phase under 50 µmol m^?2 s^?1 photosynthetic available radiation (PAR) and then exposed to non-sequential rapid light curves where, once electron transport rate (ETR) had reached saturation, light intensity was decreased and then further increased prior to returning to near growth light intensity. The non-sequential rapid light curve revealed that ETR was not proportional to the instantaneously applied light intensity, due to rapid photoacclimation. Changes in the effective absorption cross sections for open PSII reaction centres (σ_PSII′) or reaction centre connectivity (ρ) did not account for the observed increases in ETR under extended high light. σ_PSII′ in fact decreased as a function of a time-dependent induction of regulated excitation dissipation Y(NPQ), once cells were at or above a PAR coinciding with saturation of ETR. Instead, the observed increases in ETR under extended high light were explained by an increase in the rate of PSII reopening, i.e. Q_A^? oxidation. This acceleration of electron transport was strictly light dependent and relaxed within seconds after a return to low light or darkness. The time-dependent nature of ETR upregulation and regulated NPQ induction was verified using induction–recovery curves. Our findings show a time-dependent induction of excitation dissipation, in parallel with very rapid photoacclimation of electron transport, which combine to make ETR independent of short-term changes in PAR. This supports a selective advantage for these diatoms when exposed to fluctuating light in their environment.     

Toxin Production by Microcystis aeruginosa as a Function of Light in Continuous Cultures and Its Ecological Significance

The effects of light intensity and light quality on toxin production by Microcystis aeruginosa were examined in continuous cultures. Light intensity had a pronounced effect on toxicity and the toxin production rate. Toxicity and the toxin production rate increased with light intensity up to an intensity of about 40 microeinsteins m^-2 s^-1 and decreased at higher light intensities, while the ratio of toxin to protein was constant at intensities of more than 40 microeinsteins m^-2 s^-1. Light quality had only slight effects on toxicity. The results of our laboratory experiments were supported by the results of field work in which we examined toxin production at different depths in a lake. Our observations explain the mixed pattern of high and low toxicity found in a surface bloom of M. aeruginosa. Our findings also indicate that production of the peptide toxin can be uncoupled from general protein synthesis.     

Effects of Light Intensity on Photosynthetic Carboxylative Phase Enzymes and Chlorophyll Synthesis in Greening Leaves of Hordeum vulgare L

The effects of various light intensities on in vivo increases in activities of phosphoriboisomerase, phosphoribulokinase and ribulose-1, 5-diP carboxylase and on synthesis of chlorophyll were studied in greening leaves of Hordeum vulgare L.

Each enzyme was already present in dark-grown plants, but further increases in activities required both a light treatment of the intact plant and a favorable temperature. The amount of enzymatic activity and chlorophyll developed was governed by light intensity.

Measured activities of phosphoriboisomerase and ribulose 1,5-diP carboxylase were highly correlated with synthesis of chlorophyll at all intensities studied. Measured activity of phosphoribulokinase was correlated with synthesis of chlorophyll only at saturating or near saturating light intensities. At decreasing light intensities the response curves of this enzyme differed from those of chlorophyll and of phosphoriboisomerase and ribulose-1, 5-diP carboxylase. A lag period of phosphoribulokinase increased with decreasing light intensity. After the lag period a rapid rate of increase occurred which did not level off during 48 hours of illumination. Thus, a different control mechanism may be operative in inducing increased activity of this enzyme.

     

Growth of Spirulina platensis enhanced under intermittent illumination

The growth characteristics of microalgae under different light conditions (continuous or intermittent) are essential information for photobioreactor design and operation. In this study, we constructed a thin-layer (10 mm) flat plate photobioreactor device with a light/dark (L/D) alternation system to investigate the growth of Spirulina platensis under two different light regimes: (1) continuous illumination in a wide range of light intensities (1.00-77.16 mW cm⁻²); (2) intermittent illumination in medium frequency (0.01-20 Hz). Specific growth rate and light efficiency based on biomass production were determined for each round of experiment. Four regions (light limited region, intermediate region, light saturated region and light inhibition region) were recognized according to the results under continuous illumination. Under intermittent illumination, when L/D frequency increased from 0.01 Hz to 20 Hz, specific growth rate and light efficiency were enhanced. However, the enhancement was different, depending on the applied light intensity and light fraction. The higher the light intensity, the greater the enhancement would be when L/D frequency increased from 0.01 Hz to 20 Hz; and the higher the light intensity, the lower the light fractions is needed to maintain light efficiency as high as that under continuous illumination in light limited region.     

Interaction between saline stress and photoinhibition of photosynthesis in the freshwater green algae Chlamydomonas reinhardtii. Implications for glycerol photoproduction

Light intensity is the main limiting factor for the photosynthetic bioconversion of CO₂ into glycerol which takes place when Chlamydomonas reinhardtii cells are exposed to saline stress conditions. Although productivity increases with light intensity for low irradiances, a strong inhibition is observed for high light intensity values. Saline stress enhances the damage caused by excess of light on the photosynthetic apparatus. The aim of this work is to evaluate the effect of high light intensity and saline stress on photosynthetic activity, cell growth and glycerol photoproduction by C. reinhardtii. The effect of light intensity on C. reinhardtii cells was studied immediately after transfer to a saline medium and after 24 h of adaptation to saline stress conditions. The influence of light intensity on the glycerol production rate was also evaluated for C. reinhardtii cultured in bioreactors of different radius. The factors that significantly affected photoinhibition were light intensity, cell density, radius of the bioreactor and time of exposure to the high light intensity. Our results suggest that bioreactors with a high surface/volume ratio will enable the achievement of high productivities with relatively low light intensities on the surface and will miminise the photoinhibition effect.     

Studies of Chloroplast Development in Euglena: XIV. Sequential Interactions of Ultraviolet Light and Photoreactivating Light in Green Colony Formation

Photoreactivation (PR) of green colony-forming ability in Euglena is pH-insensitive from pH 6.0 to 8.0 and temperature-sensitive with a maximum rate at 35°C. There is no PR at 0°C. The rate of PR varies with the growth stage of the cells; PR of exponential phase cells is slower than that of stationary phase cells. The reciprocity rule holds for PR over a 6-fold range of intensity. The shape of PR curves is a function of the UV dose; there appears to be a progressive increase in multiplicity until a limiting multiplicity is reached as indicated by the fact that curves for high doses are superposable. Dark-grown and light-grown cells give the same PR response for comparable UV doses. UV inactivation of cells which have been treated with UV and then with PR light shows that, if the PR dose is sufficiently large, the same UV-inactivation curve is obtained as for nonpretreated control cells. Doses of PR lower than the saturating dose produce UV-inactivation curves, the ultimate slopes of which are parallel to the slope of the control curve, but which show reduced multiplicity. The multiplicity of these curves increases with increasing PR dose. The UV inactivation of green colony-forming ability in Euglena is completely photoreactivable at the doses studied, in contrast with the UV inactivation of colony-forming ability, which occurs at considerably higher UV doses and behaves like most other photoreactivable systems, showing a photoreactivable sector of 0.32.     

Influence of Temperature, Oxygen, and pH on a Metalimnetic Population of Oscillatoria rubescens

Planktonic Oscillatoria spp. often inhabit depths of thermally stratified lakes in which gradients of physical and chemical factors occur. Measurements of photosynthetic rate or photosynthetic carbon metabolism were used to evaluate the importance of vertical gradients of temperature, oxygen, and pH upon Oscillatoria rubescens in Crooked Lake, Ind. At the low light intensities experienced in situ, neither photosynthetic rate nor relative incorporation of carbon dioxide into low-molecular-weight compounds, polysaccharide, or protein was affected by temperature. At a 10-fold-higher light intensity, the photosynthetic rate increased as temperature increased; most of the additional carbon accumulated as polysaccharide. Polysaccharide which was synthesized at high light intensity and temperature was respired when the organisms were placed in the dark, but was not used for protein biosynthesis. When O. rubescens was shifted from high light to low light, a fraction of the polysaccharide was metabolized into protein. Adaptation to growth at lower temperatures by O. rubescens cultures resulted in a decrease in the maximum photosynthetic rate. Oxygen inhibited photosynthesis by only 10 to 15% at concentrations typically found in the lake. The photosynthetic rates at pH values which occurred in Crooked Lake were all near the maximum. Thus, gradients of temperature, oxygen, or pH are not likely to significantly affect the distribution of O. rubescens in Crooked Lake, given the low light intensity at which O. rubescens grows and the range of values for those factors in the lake.     

Whole Leaf Carbon Exchange Characteristics of Phosphate Deficient Soybeans (Glycine max L.)

Low phosphate nutrition results in increased chlorophyll fluorescence, reduced photosynthetic rate, accumulation of starch and sucrose in leaves, and low crop yields. This study investigated physiological responses of soybean (Glycine max [L.] Merr.) leaves to low inorganic phosphate (Pi) conditions. Responses of photosynthesis to light and CO₂ were examined for leaves of soybean grown at high (0.50 millimolar) or low (0.05 millimolar) Pi. Leaves of low Pi plants exhibited paraheliotropic orientation on bright sunny days rather than the normal diaheliotropic orientation exhibited by leaves of high Pi soybeans. Leaves of plants grown at high Pi had significantly higher light saturation points (1000 versus 630 micromole photons [400-700 nanometers] per square meter per second) and higher apparent quantum efficiency (0.062 versus 0.044 mole CO₂ per mole photons) at ambient (34 pascals) CO₂ than did low Pi leaves, yet stomatal conductances were similar. High Pi leaves also had significantly higher carboxylation efficiency (2.90 versus 0.49 micromole CO₂ per square meter per second per pascal), a lower CO₂ compensation point (6.9 versus 11.9 pascals), and a higher photosynthetic rate at 34 pascals CO₂ (19.5 versus 6.7 micromoles CO₂ per square meter per second) than did low Pi leaves. Soluble protein (0.94 versus 0.73 milligram per square centimeter), ribulose-1,5-bisphosphate carboxylase/oxygenase content (0.33 versus 0.25 milligram per square centimeter), and ribulose-1,5-bisphosphate carboxylase/oxygenase specific activity (25.0 versus 16.7 micromoles per square meter per second) were significantly greater in leaves of plants in the high Pi treatment. The data indicate that Pi stress alters the plant's CO₂ reduction characteristics, which may in turn affect the plant's capacity to accommodate normal radiation loads.     

STUDIES ON PHOTOSYNTHESIS : SOME EFFECTS OF LIGHT OF HIGH INTENSITY ON CHLORELLA

1. The effect on oxygen evolution of Chlorella vulgaris produced by light intensities up to about 40,000 f.-c. has been studied by the use of the Warburg technique. 2. Above a certain critical intensity, which is determined by the previous history of the cells, the rate of oxygen evolution decreases from the maximum to another constant rate. This depression is at first a completely reversible effect. 3. With a sufficiently high intensity this constant rate represents an oxygen uptake greater than the rate of dark respiration. During such a constant rate of oxygen uptake a progressive injury to the photosynthetic mechanism takes place. After a given oxygen consumption the rate falls off, approaching zero, and the cells are irreversibly injured. 4. The constant rate of oxygen evolution (2 and 3) decreases in a continuous manner with increasing light intensities, approaching a value which is approximately constant for all lots of cells regardless of previous history. 5. Two alternative hypotheses have been presented to explain the observed phenomena. The more acceptable of these proposes quick inactivation of the photosynthetic mechanism, the extent of inhibition depending on the light intensity. 6. In Chlorella vulgaris solarization is influenced by the previous history of the cells.     

The use of outdoor phytoplankton continuous cultures to analyse factors influencing species selection

Natural phytoplankton populations have been grown in outdoor continuous cultures at three dilution rates (D = 0.5, 0.25, and 0.1 · day^?1) under nitrogen (N) or silicon (Si) limitation and two light intensities. At a high specific nutrient flux (high dilution rate) under N limitation an assemblage of primarily small, fast growing centric diatoms such as Skeletonema costatum (Grev.) Cleve and Chaetoceros spp. dominated with a low percentage of flagellates. At a low specific nutrient flux, a mixture of larger, slower growing centric diatoms, small flagellates, and pennate diatoms was obtained. Similar trends were observed under silicate limitation. Decreasing the light intensity at the lowest dilution rate selected for an assemblage similar to that observed at the high dilution rate and high light intensity.The results of these competition experiments suggest that specific nutrient flux (dilution rate) is an important factor in determining between group dominance (e.g., centric and pennate diatoms and small flagellates). Successful competitors representing broad phytoplankton groups can be arranged along a resource gradient of specific nutrient flux (dilution rate), with groups such as centric and pennate diatoms, represented as high and medium flux species, respectively.     

INTENSITY DISCRIMINATION IN THE HUMAN EYE : II. THE RELATION BETWEENΔI/IAND INTENSITY FOR DIFFERENT PARTS OF THE SPECTRUM

1. A new apparatus is described for measuring visual intensity discrimination over a large range of intensities, with white light and with selected portions of the spectrum. With it measurements were made of the intensity ΔI which is just perceptible when it is added for a short time to a portion of a field of intensity I to which the eye has been adapted. 2. For white and for all colors the fraction ΔI/I decreases as I increases and reaches an asymptotic minimum value at high values of I. In addition, with white light the relation between ΔI/I and I shows two sections, one at low intensities and the other at high intensities, the two being separated by an abrupt transition. These findings are contrary to the generally accepted measurements of Koenig and Brodhun; however, they confirm the recent work of Steinhardt, as well as the older work of Blanchard and of Aubert. The abrupt transition is in keeping with the Duplicity theory which attributes the two sections to the functions of the rods and cones respectively. 3. Measurements with five parts of the spectrum amplify these relationships in terms of the different spectral sensibilities of the rods and cones. With extreme red light the relation of ΔI/I to I shows only a high intensity section corresponding to cone function, while with other colors the low intensity rod section appears and increases in extent as the light used moves toward the violet end of the spectrum. 4. Like most of the previously published data from various sources, the present numerical data are all described with precision by the theory which supposes that intensity discrimination is determined by the initial photochemical and chemical events in the rods and cones.     

Control of the Photosynthetic Apparatus of Acetabularia mediterranea by Blue Light : Analysis by Light-Saturation Curves

During growth, Acetabularia mediterranea requires the action of blue light to maintain high rates of photosynthesis. In the present study, blue light-dependent alterations of the photosynthetic apparatus, which can be detected by analysis of light-saturation curves and by measurements of partial reactions of the photosynthetic electron transport chain, are described. Light-saturation curves of photosynthesis in vivo were measured with a new closed oxygen electrode system after culture of Acetabularia in continuous red or blue light. These curves were compared to those of 2,6-dichlorophenol-indophenol reduction by isolated chloroplast membranes. The analysis lead to the following statements: (a) only one reaction limits electron transport rates in vitro (dichlorophenol-indophenol reduction) at all light intensities irrespective of the light quality during growth, and (b) the limiting step is light driven and located in the reaction center of photosystem II. Presumably, this same reaction determines the flow of electrons under low light intensities in vivo in cells from white, blue, and red light. In addition to photosynthesis, the rates of dark respiration changed due to the action of blue light. Concomitantly, the light compensation point of apparent photosynthesis was shifted during monochromatic irradiations.     

Photoinhibition of Growth in Acanthamoeba castellanii Cultures

Light from 350 to 680 nm at intensities up to 1.62 × 10⁵ ergs per sec per cm² slowed exponential growth and lowered the maximum yield in axenic cultures of Acanthamoeba castellanii. Photoinhibition was a linear function of light intensity up to 1.25 × 10⁵ ergs per sec per cm². At higher intensities, growth was too slow to be measured accurately. A photochemical change occurring in the growth medium on irradiation was a function of light dosage and not intensity per se. Light in dosages which appreciably changed the growth-supporting properties of the medium exceeded the dosages received by exponentially growing cultures during irradiation. Consequently, photoinhibition of growth was attributed to a direct effect of light on the amoebae, not to photodegradation of the medium. The growth-supporting properties of irradiated media could be restored by the addition of yeast extract and Proteose peptone. The reduced growth rate in the light was not due to cyst formation or induction of multinuclearity. Light affected the amoebae either through absorption by intracellular pigment(s) or through binding to the amoebae of a photosensitizing compound in the medium.     

<Emphasis Type="Italic">Chirocephalus sarpedonis</Emphasis> sp. nov. (Branchiopoda,Anostraca, Chirocephalidae) from Turkey questions the monophyly of the traditional <Emphasis Type="Italic">Chirocephalus</Emphasis> species-groups

Here we test for the possible coupling of two kairomone-induced, anti-fish defences in Daphnia, life-history changes (LHC) and diel vertical migration (DVM) mediated by the environmental factor light. A gradient of five different light intensities that represents naturally occurring intensities in the lake water column was used in life-history experiments, and we show that LHC of a single Daphnia clone are inversely coupled to the ambient light intensity. Furthermore, we could show that the light intensity has to exceed a threshold to induce the LHC. We also observed an effect of the light intensity on the fish kairomone-mediated expression of a candidate gene (actin 3) in a way that the gene response differs between groups of low and high light intensities. We argue that the ambient light intensity an individual is exposed to and that is dependent on the position in the water column of a lake inversely determines the degree of LHC. These findings suggest a plastic coupling of these two anti-fish defences in Daphnia (LHC and DVM), which allows for an adjustment to fluctuating environments of standing freshwater ecosystems.     

On the State 1-State 2 phenomenon in photosynthesis

The State 1-State 2 phenomenon of photosynthesis was studied in Chlorella by measuring the flash yield (Y) and the modulated rate (v) of oxygen evolution induced by weak modulated 650-nm light. From light intensity curves, intensities of 650 and 710 nm background and preilluminations were chosen to give maximum values of Y and v. Following long preilluminations in 710 nm (State 1) or in 650 nm (State 2), Y and v were measured in background light of chosen wavelength. The resulting plots of v vs Y show a discontinuity between State 1 and State 2. They confirm the predictions of Bonaventura and Myers [(1969) Biochim. Biophys. Acta 189, 366–383] and are consistent with changes in α (fraction of absorbed light captured by System II) as explanation of the State 1–State 2 phenomenon. When the intensity of 710 nm preillumination was too low, the characteristics of State 1 were not fully developed and the results then were similar to those of Delrieu [(1972) Biochim. Biophys. Acta 256, 293–299].     

On the kinetics of phytochrome photoconversion in vivo

Summary Samples for spectrophotometric measurement of phytochrome in vivo are not optically thin. For different cross sections of the sample, the rate constant of a photochemical reaction will, therefore, have different values. We have developed a mathematical model, based on the assumption that the rate of phytochrome phototransformation is proportional to the light intensity and that the light intensity gradient in the sample is exponential. Kinetic curves computed with this model conform closely with the measurements. The simplest explanation of the observed kinetics is that there is only one type of phytochrome and that the light intensity gradient in samples that are not too thin, is close to exponential.309th Communication.