CARBOXYSOME CONTENT OF SYNECHOCOCCUS LEOPOLIENSIS (CYANOPHYTA) IN RESPONSE TO INORGANIC CARBON1

The carboxysome content of chemostat grown Synechococcus leopoliensis (Racib.) Komarek increases under inorganic carbon limitation. At growth rates of ca. 85%μ_max the carboxysome content (±SE) was 0.57 ± 0.09 carboxysomes·cell section^?1. Under severe carbon limitation (ca. 13%μ_max) this increased to 3.4 · 0.3 carboxysomes·cell section^?1. Corresponding to this change is a three order of magnitude decrease in the half-saturation constant of photosynthesis for dissolved inorganic carbon. Nitrogen and phosphorus limitation had no effect on carboxysome content or the kinetics of photosynthesis with respect to inorganic carbon. These results are discussed in light of the apparent lack of photorespiration in these organisms.     

Measurement of photorespiration in algae

The rates of true and apparent photosynthesis of two unicellular green algae, one diatom and four blue-green algae were measured in buffer at pH 8.0 at subsaturating concentrations of dissolved inorganic carbon (13-27 micromolar). Initial rates of depletion from the medium of inorganic carbon and ¹⁴C activity caused by the algae in a closed system were measured by gas chromatography and by liquid scintillation counting, respectively. The rate of photorespiration was calculated as the difference between the rates of apparent and true photosynthesis. The three eucaryotic algae and two blue-green algae had photorespiratory rates of 10 to 28% that of true photosynthesis at air levels of O₂. Reduction of the O₂ level to 2% caused a 52 to 91% reduction in photorespiratory rate. Two other blue-green algae displayed low photorespiratory rates, 2.4 to 6.2% that of true photosynthesis at air levels of O₂, and reduction of the O₂ concentration had no effect on these rates.     

Experimental Evidence for the Isotope Effect in Photorespiration

Recent data on carbon isotope fractionation in photosynthesis are reviewed. Analysis of the carbon isotope composition in photosynthates and derivative products supports the hypothesis of the isotope effect in photorespiration. This hypothesis envisages the existence in a photosynthesizing cell of two carbon flows differing in isotope composition. One of these flows is enriched in ¹²C and associated with the assimilation pathway of photosynthesis. The other flow enriched in ¹³C circulates in the photorespiratory pathway. The relation between stimulated photorespiration and the carbon isotope composition of biomass supports this view. Our hypothesis of two interrelated isotope effects in photosynthesis leads to the conclusion that photosynthesis and photorespiration are coupled processes subject to periodical oscillations, where Rubisco acts as a main switch regulating these two pathways.     

干出和沉水不同条件下羊栖菜光合作用碳源获得机制比较(英文)

经济海洋褐藻羊栖菜(Hizikia fusiforme(Harv.)Okamura)低潮时常常周期性地暴露于空气中。为了认识这种海藻在潮汐循环背景下的光合特征,对其在高潮沉水和低潮干出不同条件下的光合作用碳素获得机制进行了比较。沉水时,羊栖菜主要利用海水中HCO_3~-作为外源无机碳源驱动光合作用;而在干出条件下,其光合作用的主要碳源为空气中的CO_2。在这两种不同环境条件下,光合作用与pH值的关系不同:沉水状态时,羊栖菜在高pH值(10.0)下光合活性很弱;而在干出条件下,羊栖菜在高pH值时仍有较高的光合活性。然而,光合作用无论是在沉水还是在干出条件下,对外源碳源的获得都表现出对胞外碳酸酐酶(CA)强烈的依赖性,并且其光合速率都受周围环境中无机碳源水平的限制。此外,在沉水和干出两种环境条件下,羊栖菜光合作用都表现出对氧气的敏感性。这表明,在羊栖菜中,依赖胞外CA的碳源获得机制不能使细胞内CO_2浓度提高到阻碍其光呼吸的程度。增加空气中或海水中无机碳的浓度,能促进羊栖菜的光合作用,进而增加这种海藻的水产养殖产量。     

Photosynthesis, Photorespiration and Respiration of Detached Spruce Twigs as Influenced by Oxygen Concentration and Light Intensity

The effects of oxygen concentration and light intensity on the rates of apparent photosynthesis, true photosynthesis, photorespiration and dark respiration of detached spruce twigs were determined by means of an infra-red carbon dioxide analyzer (IRCA). A closed circuit system IRCA was filled with either 1 per cent of oxygen in nitrogen, air (21 % O₂) or pure oxygen (100 % O₂). Two light intensities 30 × 10³ erg · cm ^?2· s^?1 and 120 × 10³ erg · cm^?2· s^?1 were applied. It has been found that the inhibitory effect of high concentration of oxygen on the apparent photosynthesis was mainly a result of a stimulation of the rate of CO₂ production in light (photorespiration). In the atmosphere of 100 % O₂, photorespiration accounts for 66–80 per cent of total CO₂ uptake (true photosynthesis). Owing to a strong acceleration of photorespiration by high oxygen concentrations, the rate of true photosynthesis calculated as the sum of apparent photosynthesis and photorespiration was by several times less inhibited by oxygen than the rate of apparent photosynthesis. The rates of dark respiration were essentially unaffected by the oxygen concentrations used in the experiments. An increase in the intensity of light from 30 × 10³ erg · cm^?3· s^?1 to 120 · 10³ erg · cm^?2· s^?1 enhanced the rate of photorespiration in the atmospheres of 21 and 100 % oxygen but not in 1 % O₂. The rate of apparent photosynthesis, however, was little affected by light intensity in an atmosphere of 1 % oxygen.     

Strategies for engineering C4 photosynthesis

C₃ photosynthesis is an inefficient process, because the enzyme that lies at the heart of the Benson–Calvin cycle, ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) is itself a very inefficient enzyme. The oxygenase activity of Rubisco is an unavoidable side reaction that is a consequence of its reaction mechanism. The product of oxygenation, glycollate 2-P, has to be retrieved by photorespiration, a process which results in the loss of a quarter of the carbon that was originally present in glycollate 2-P. Photorespiration therefore reduces carbon gain. Purely in terms of carbon economy, there is, therefore, a strong selection pressure on plants to reduce the rate of photorespiration so as to increase carbon gain, but it also improves water- and nitrogen-use efficiency. Possibilities for the manipulation of plants to decrease the amount of photorespiration include the introduction of improved Rubisco from other species, reconfiguring photorespiration, or introducing carbon-concentrating mechanisms, such as inorganic carbon transporters, carboxysomes or pyrenoids, or engineering a full C₄ Kranz pathway using the existing evolutionary progression in C₃–C₄ intermediates as a blueprint. Possible routes and progress to suppressing photorespiration by introducing C₄ photosynthesis in C₃ crop plants will be discussed, including whether single cell C₄ photosynthesis is feasible, how the evolution of C₃–C₄ intermediates can be used as a blueprint for engineering C₄ photosynthesis, which pathway for the C₄ cycle might be introduced and the extent to which processes and structures in C₃ plant might require optimisation.     

Interactions of blue light and inorganic carbon supply in the control of light-saturated photosynthesis in brown algae

Photosynthetic capacities of five species of brown algae in red light were found to be strongly limited by the inorganic carbon supply of natural sea water. Under these conditions, pH 8·2 and dissolved inorganic carbon concentration (DIG) of 2·1 mol m^?3, a short pulse of blue light was found to increase the subsequent rate of photosynthesis in saturating red light. The degree of blue light stimulation varied between species, ranging from an increase of over 200% of the original rate in Colpomenia peregrins to only 10% in Dictyota dichotoma. Increasing the DIG concentration of sea water by bicarbonate addition resulted in carbon saturation of photosynthesis in all five species. Blue light stimulation was greatly reduced at these higher DIG concentrations. The response in Laminaria digitata was examined in more detail by manipulation of pH and DIG to produce solutions with different concentrations of dissolved CO₂. At a CO₂ concentration typical of normal sea water (12·4 mmol m^?3), blue light treatment increased photosynthetic rate by approximately 50%. Blue light stimulation was increased to over 150% at CO₂ concentrations below that of sea water, whereas at concentrations above that of sea water, the effect was diminished. Therefore, the effect of blue light on photosynthetic capacity appears to involve an increase in the rate of supply of carbon dioxide to the plant.     

干出和沉水不同条件下羊栖菜光合作用碳源获得机制比较

经济海洋褐藻羊栖菜(Hizikia fusiforme(Harv.)Okamura)低潮时常常周期性地暴露于空气中.为了认识这种海藻在潮汐循环背景下的光合特征,对其在高潮沉水和低潮干出不同条件下的光合作用碳素获得机制进行了比较.沉水时,羊栖菜主要利用海水中HCO3-作为外源无机碳源驱动光合作用;而在干出条件下,其光合作用的主要碳源为空气中的CO2.在这两种不同环境条件下,光合作用与pH值的关系不同:沉水状态时,羊栖菜在高pH值(10.0)下光合活性很弱;而在干出条件下,羊栖菜在高pH值时仍有较高的光合活性.然而,光合作用无论是在沉水还是在干出条件下,对外源碳源的获得都表现出对胞外碳酸酐酶(CA)强烈的依赖性,并且其光合速率都受周围环境中无机碳源水平的限制.此外,在沉水和干出两种环境条件下,羊栖菜光合作用都表现出对氧气的敏感性.这表明,在羊栖菜中,依赖胞外CA的碳源获得机制不能使细胞内CO2浓度提高到阻碍其光呼吸的程度.增加空气中或海水中无机碳的浓度,能促进羊栖菜的光合作用,进而增加这种海藻的水产养殖产量.     

Regulation of photorespiration in leaves: evidence that the fraction of ribulose bisphosphate oxygenated is conserved and stoichiometry fluctuates

Under steady-state conditions the combined system of the reductive photosynthetic cycle and the oxidative photorespiratory loop may be defined by two partitioning terms: the fraction of ribulose bisphosphate oxygenated and the fraction of glycolate carbon photorespired (the stoichiometry of photorespiration). A combination of physical and stereochemical methods [K.R. Hanson, and R. B. Peterson, (1985) Arch. Biochem. Biophys. 237,300-310] has been used to estimate these partitionings for tobacco leaf discs. Inverted discs, as compared to normally oriented discs, were found to have greater net photosynthesis; their ratio of photorespiration to net photosynthesis was less, and less of their glycolate carbon was photorespired. An eightfold reduction of irradiance below that of full sunlight for inverted discs in normal air at 32 degrees C reduced both photosynthesis and photorespiration about threefold but had little effect on the partitioning of ribulose bisphosphate and glycolate. Increasing the temperature from 22 to 40 degrees C for inverted discs in normal air and 1000 microE m-2 s-1 irradiance had little effect on net photosynthesis but increased the ratio of photorespiration to net photosynthesis almost threefold; ribulose bisphosphate partitioning was little changed but the fraction of glycolate carbon photorespired more than doubled. If field-grown plants respond to temperature in a similar fashion, genetic intervention to reduce the increase in photorespiration stoichiometry with temperature could increase total daily carbon assimilation and hence improve crop yields.     

Compartmental modelling of photorespiration and carbon metabolism of water stressed leaves

Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO₂ and O₂ concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO₂ is suggested to explain the discrepancy. The model suggests that resistance to CO₂ fixation is mainly in the carboxylation reactions, not in CO₂ transport. Using the steady state model, the kinetics of ¹⁴C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of ¹⁴C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO₂ released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.     

CO2 is the main inorganic C species entering photosynthetically active leaf protoplasts of the freshwater macrophyte Ranunculus penicillatus ssp. pseudofluitans

Submerged aquatic macrophytes growing in water where free CO₂ is unavailable (above pH 8·2) must use mechanisms to supply external dissolved inorganic carbon in a form available to chloroplasts (CO₂). Active transport of HCO₃^– across the plasmalemma has not been proven to be widespread in aquatic macrophytes and catalytic conversion of HCO₃^– to CO₂ is the usual supply mechanism in submerged macrophytes. The interaction of leaf form and function in this respect was investigated in the linear, submerged leaves of Ranunculus penicillatus (Dumort.) Bab ssp. pseudofluitans (Syme) S.Webster. Viable protoplasts were isolated using a mixture of cell wall degrading enzymes optimized for this species. Protoplast viabilities greater than 80% after 5 h of isolation were achieved. Photosynthetic rates of isolated protoplasts were comparable with that of intact plant tissue. Results of carbon isotopic disequilibrium experiments showed that CO₂ was the preferred species of dissolved inorganic carbon for photosynthesis by protoplasts and that HCO₃^– which predominates in the plant’s natural environment mainly contributes by supplying CO₂ outside the cells.     

The mechanism of carbon isotope fractionation in photosynthesis and carbon dioxide component of the greenhouse effect

The relationship between the global climate warming, which is largely induced by increased CO₂ atmospheric concentration, and the changes in carbon isotope fractionation in plants was explained in terms of the previously proposed oscillatory mechanism of photosynthesis, according to which CO₂ assimilation and photorespiration are two reciprocally coupled oscillating mechanisms controlled by ribulose bisphosphate carboxylase/oxygenase switching. This explanation is confirmed by the changes in carbon isotope fractionation in the annual rings of trees and demonstrates that the light carbon isotope ¹²C enrichment before 1990s resulted from increased photosynthetic assimilation of CO₂. The subsequent sharp ¹³C enrichment of the tree ring carbon until the present time suggests that the compensatory role of photosynthesis in boreal forests has been lost for the global climate.     

Palaeoenvironmental significance of lacustrine stromatolites of the Arnstadt Formation (“Steinmergelkeuper”, Upper Triassic, N-Germany)

Lacustrine stromatolites of the Norian Arnstadt Formation (“Steinmergelkeuper”) occur on top of asymmetric flooding–evaporation cycles of a closed lake basin. They have been investigated with regard to associated lithofacies and biota, microfabric and stable carbon and oxygen isotopes. The stromatolites of the “Middle Grey Series” are brecciated and reworked by a flooding event of a subsequent lake cycle. They comprise agglutinated stromatolites rich in fish scales as well as skeletal stromatolites composed of a rhythmically grown dendroid micropeloidal framework. The latter are characterized by a shift towards positive δ¹³C values relative to the associated lake carbonates. This points to an effective photosynthesis in biofilm calcification at low concentrations in dissolved inorganic carbon (DIC) in a perennial fresh to brackish water lake. The stromatolites of the “Upper Red Series” occur on top of perennial lake cycles intercalated between playa lake deposits. The fine-grained stromatolites are poor in microfabric characteristics but show a significant covariation of δ¹⁸O and δ¹³C. This points to evaporation/degassing acting as driving mechanism in biofilm calcification. The lack of biotic effects on carbon isotope fractionation may reflect high concentrations in dissolved inorganic carbon. Skeletal oncoids, which occur as allochthonous components within an intraformational lag deposit of the “Upper Red Series”, are composed of cyanobacterial tubes and probably represent lowest saline lakes with only poor DIC buffering. Stable isotope signatures in conjunction with stromatolite microfabric analyses may be used as a proxy of DIC concentrations in ancient closed lakes.     

Photosynthetic kinetics determine the outcome of competition for dissolved inorganic carbon by freshwater microalgae: implications for acidified lakes

Summary Photosynthetic kinetics with respect to dissolved inorganic carbon were used to predict the outcome of competition for DIC between the green alga Selenastrum minutum and the cyanobacterium Synechococcus leopoliensis at pH 6.2, 7.5, and 10. Based on measured values of the maximum rate of photosynthesis, the half-saturation value of photosynthesis with respect to DIC (K ₁ ^2/DIC ), and the DIC compensation point, it was predicted that S. leopoliensis would lower the steady-state DIC concentration below the DIC compensation point of S. minutum. This should result in competitive displacement of the green alga at a rate equivalent to the chemostat dilution rate. This prediction was validated by carrying out competition experiments over the range of pH. These results suggest that the low levels of DIC in air-equilibrated acidified lakes may be an important rate-limiting resource and hence affect phytoplankton community structure. Furthermore, the low levels of DIC in these systems may be below the DIC compensation point for some species, thereby precluding their growth at acid pH solely as a function of DIC limitation. The potential importance of DIC in shaping phytoplankton community structure in acidified systems is discussed.Abbreviations growth rate - _max maximum growth rate - K concentration of dissolved inorganic carbon required to maintain half-maximal rate of growth - K ₁ ^2/DIC concentration of dissolved inorganic carbon required to maintain half-maximal photosynthesis - DIC dissolved inorganic carbon - P _max maximum rate of photosynthesis - R ^* substrate concentration required for an organism to maintain a growth rate equal to the mortality rate - DIC compensation point (DIC) concentration where gross photosynthesis equals respiration - i.e. net photosynthesis equals zero     

The interactive effects of light and inorganic carbon on aquatic plant growth

Submerged aquatic macrophytes grow across a wide, often coupled, range of light and inorganic carbon availabilities, and each single factor influences photosynthesis and acclimation. Here we examine the interactive effects of light and inorganic carbon on the growth of Elodea canadensis and Callitriche cophocarpa. The plants were grown in the laboratory at a range of light intensities (0–108 μmol m⁻²s⁻¹) and four inorganic carbon regimes in a crossed factorial design. Plant growth rates, measured over 3–4 weeks of incubation, increased in response to increasing light intensity and inorganic carbon availability, and significant interactive effects were observed. The light-use efficiency for growth at low light increased 2-fold for Callitriche and 6-fold for Elodea between the lowest and highest inorganic carbon concentrations applied. Also, the growth rate at the highest light intensity increased with inorganic carbon availability, but the relative increase was smaller than at low light. Both species acclimated to the light and carbon regime such that the chlorophyll content declined at low and high light intensities and the initial slopes of the photosynthetic CO₂ and HCO₃⁻ response curves declined at high levels of CO₂. Callitriche responded less markedly than Elodea to changing inorganic carbon availability during growth, and the initial slope of the photosynthetic HCO₃⁻ response curve, in particular, was greatly reduced (>90%) in Elodea by high CO₂. It is suggested that the coupled responses of aquatic macrophytes to light and inorganic carbon influence their ability to develop dense stands at high light in shallow water and to extend to greater depths in waters rich in inorganic carbon.     

Extracellular Carbonic Anhydrase Facilitates Carbon Dioxide Availability for Photosynthesis in the Marine Dinoflagellate Prorocentrum micans

This study investigated inorganic carbon accumulation in relation to photosynthesis in the marine dinoflagellate Prorocentrum micans. Measurement of the internal inorganic carbon pool showed a 10-fold accumulation in relation to external dissolved inorganic carbon (DIC). Dextran-bound sulfonamide (DBS), which inhibited extracellular carbonic anhydrase, caused more than 95% inhibition of DIC accumulation and photosynthesis. We used real-time imaging of living cells with confocal laser scanning microscopy and a fluorescent pH indicator dye to measure transient pH changes in relation to inorganic carbon availability. When steady-state photosynthesizing cells were DIC limited, the chloroplast pH decreased from 8.3 to 6.9 and cytosolic pH decreased from 7.7 to 7.1. Re-addition of HCO₃⁻ led to a rapid re-establishment of the steady-state pH values abolished by DBS. The addition of DBS to photosynthesizing cells under steady-state conditions resulted in a transient increase in intracellular pH, with photosynthesis maintained for 6 s, the amount of time needed for depletion of the intracellular inorganic carbon pool. These results demonstrate the key role of extracellular carbonic anhydrase in facilitating the availability of CO₂ at the exofacial surface of the plasma membrane necessary to maintain the photosynthetic rate. The need for a CO₂-concentrating mechanism at ambient CO₂ concentrations may reflect the difference in the specificity factor of ribulose-1,5 bisphosphate carboxylase/oxygenase in dinoflagellates compared with other algal phyla.     

Biological fractionation of stable carbon isotopes at the aerobic/anaerobic water interface of meromictic water bodies

Mass-spectrometric investigation of carbon isotope composition (δ¹³C) was carried out for suspended organic matter and dissolved mineral compounds for the water column of some meromictic water bodies differing in salinity and trophic state. As a rule, a more pronounced carbon isotope fractionation (resulting from the metabolism of phytoplankton and anoxygenic phototrophic bacteria) was revealed in the zones of enhanced oxygenic and anoxygenic photosynthesis. Carbon isotope fractionation at the border between oxidized and reduced waters depends both on the activity of microbial communities and on the dominant species of phototrophic microorganisms. Analysis of the distribution profiles of the isotopic composition of suspended organic matter and dissolved mineral carbon revealed active mineralization of the organic matter newly formed via anoxygenic photosynthesis in the monimolimnion by microbial communities, resulting in the release of isotopically light carbon dioxide. Mineral carbon in the anaerobic zones of highly productive meromictic water bodies is therefore enriched with the light ¹²C isotope.     

A Mathematical Model of Photorespiration and Photosynthesis

A comprehensive mathematical model of C₃ leaf carbon metabolism,involving the Calvin cycle and the glycolate and glycerate pathwaysof photorespiration, is formulated in terms of a system of non-lineardifferential equations. A steady state, which is found to beeffectively stable, is derived. The model behaves realisticallywhen tested under varying external carbon dioxide and oxygenconcentrations: photosynthesis is inhibited by higher oxygenlevels, while photorespiration is inhibited by higher carbondioxide levels. Calvin cycle, differential equations, glycolate pathway, mathematical model, photorespiration, photosynthesis     

An isotopic and geochemical study of carbonate-clay mineralization in basaltic caves: abiotic versus microbial processes

Stable isotope and geochemical data are used here to differentiate between contemporaneous abiotic and microbial processes leading to formation of modern carbonate‐ (calcite, aragonite and magnesite) and silicate‐rich (kerolite) mineralization in basaltic sea caves on the island of Kauai, Hawaii. Strontium isotope and Ca/Sr ratios in meteoric water and cave carbonates suggest that the majority of Sr and Ca are derived from rock–water interaction within the host basalts situated above the caves. Oxygen and hydrogen isotope ratios and chemical compositions of cave and surface waters indicate that evaporation does not control cave‐water composition. However, evaporation of drops and thin films of water in microenvironments can lead to precipitation of some phases. This behaviour is suggested by the covariance in δ¹⁸O and δ¹³C values of some carbonates, especially magnesite, which is considered to be a late‐stage evaporative precipitate. Modelling of water evolution suggests that evaporation can be a cause of supersaturation for magnesite, kerolite and some Ca carbonates. However, the highly elevated δ¹³C values (up to +8.2) of some Ca carbonates, compared to average dissolved inorganic carbon δ¹³C values (~?12), are best explained as the product of microbial photosynthesis, in particular by cyanobacteria, present in the upper layers of active microbial mats on cave surfaces. The preferential uptake of ¹²C by cyanobacteria is recorded in the low δ¹³C values (?29.1 to ?22.6) of organic matter in mats and mineralized microbialites. The resulting ¹³C‐enrichment of dissolved inorganic carbon is recorded in the elevated δ¹³C values of these Ca carbonates. A positive correlation exists between the δ¹³C values of the carbonates and coexisting organic matter. The large enrichment in ¹³C of carbonate minerals, relative to dissolved inorganic carbon, and its covariance with the δ¹³C values of coexisting organic matter are useful for identification of carbonate‐rich mineralization resulting from autotrophic microbial activity.     

光呼吸突变体研究进展

光呼吸(photorespiration)是绿色植物在光下吸收氧气并释放CO₂的过程。C₃植物光呼吸可消耗25%光合产物, 故合理改良光呼吸可望提高植物的光合效率。筛选与利用光呼吸突变体是研究光呼吸代谢及其功能的最为有效的途径。该文对光呼吸代谢途径、光呼吸突变体的筛选以及研究进展进行综述, 以期为深入探讨植物光呼吸的生物学功能及进行植物分子改良提供帮助。