Low Ambient CO2 Research Articles

Control of Rubisco function via homeostatic equilibration of CO2 supply.

Rubisco is the most abundant protein on Earth that serves as the primary engine of carbon assimilation. It is characterized by a slow rate and low specificity for CO2 leading to photorespiration. We analyze here the challenges of operation of this enzyme as the main carbon fixation engine. The high concentration of Rubisco exceeds that of its substrate CO2 by 2–3 orders of magnitude; however, the total pool of available carbon in chloroplast, i.e., mainly bicarbonate, is comparable to the concentration of Rubisco active sites. This makes the reactant stationary assumption (RSA), which is essential as a condition of satisfying the Michaelis–Menten (MM) kinetics, valid if we assume that the delivery of CO2 from this pool is not limiting. The RSA is supported by active carbonic anhydrases (CA) that quickly equilibrate bicarbonate and CO2 pools and supply CO2 to Rubisco. While the operation of stromal CA is independent of light reactions, the thylakoidal CA associated with PSII and pumping CO2 from the thylakoid lumen is coordinated with the rate of electron transport, water splitting and proton gradient across the thylakoid membrane. At high CO2 concentrations, CA becomes less efficient (the equilibrium becomes unfavorable), so a deviation from the MM kinetics is observed, consistent with Rubisco reaching its Vmax at approximately 50% lower level than expected from the classical MM curve. Previously, this deviation was controversially explained by the limitation of RuBP regeneration. At low ambient CO2 and correspondingly limited capacity of the bicarbonate pool, its depletion at Rubisco sites is relieved in that the enzyme utilizes O2 instead of CO2, i.e., by photorespiration. In this process, CO2 is supplied back to Rubisco, and the chloroplastic redox state and energy level are maintained. It is concluded that the optimal performance of photosynthesis is achieved via the provision of continuous CO2 supply to Rubisco by carbonic anhydrases and photorespiration.

Feedforward non-Michaelis–Menten mechanism for CO2 uptake by Rubisco: Contribution of carbonic anhydrases and photorespiration to optimization of photosynthetic carbon assimilation

Rubisco, the most abundant protein serving as the primary engine generating organic biomass on Earth, is characterized by a low catalytic constant (in higher plants approx. 3s−1) and low specificity for CO2 leading to photorespiration. We analyze here why this enzyme evolved as the main carbon fixation engine. The high concentration of Rubisco exceeding the concentration of its substrate CO2 by 2–3 orders of magnitude makes application of Michaelis–Menten kinetics invalid and requires alternative kinetic approaches to describe photosynthetic CO2 assimilation. Efficient operation of Rubisco is supported by a strong flux of CO2 to the chloroplast stroma provided by fast equilibration of bicarbonate and CO2 and forwarding the latter to Rubisco reaction centers. The main part of this feedforward mechanism is a thylakoidal carbonic anhydrase associated with photosystem II and pumping CO2 from the thylakoid lumen in coordination with the rate of electron transport, water splitting and proton gradient across the thylakoid membrane. This steady flux of CO2 limits photosynthesis at saturating CO2 concentrations. At low ambient CO2 and correspondingly limited capacity of the bicarbonate pool in the stroma, its depletion at the sites of Rubisco is relieved by utilizing O2 instead of CO2, i.e. by photorespiration, a process which supplies CO2 back to Rubisco and buffers the redox state and energy level in the chloroplast. Thus, the regulation of Rubisco function aims to keep steady non-equilibrium levels of CO2, NADPH/NADP and ATP/ADP in the chloroplast stroma and to optimize the condition of homeostatic photosynthetic flux of matter and energy.

Is the greenhouse theory a fallacy? A paleontological paradox

Over a hundred years ago, Svante Arrhenius (1896), building on earlier work of the French physicist Joseph Fourier, Tyndall (1861), Langley (1884), and others, was the first to model quantitatively the effects of changes in the concentration of atmospheric carbon dioxide on climate (Uppenbrink, 1996; Fleming, 1998). A “greenhouse theory”, or “hothouse theory” as it was then called (Arrhenius, 1908), thus was born and has stood the test of time pretty well since its 19th-century origins, despite falling out of favor for a brief period between 1900 and 1940 (Fleming, 1998). Quaternary ice-core records, now spanning four complete glacial-interglacial cycles (Petit et al., 1997), have provided the strongest support, to date, for the role of greenhouse gasses as primary drivers of global climate change rather than as mere feedback effects (Shackleton, 2000). A number of recent publications based on paleontological and geochemical proxy data sets, however, have challenged the notion that climate and atmospheric CO2 have been coupled throughout pre-Quaternary time (Pagani et al., 1999; Pearson and Palmer, 2000; Viezer et al., 2000; Royer et al., 2001). Intervals of extreme global warmth, such as the late Paleocene and early Eocene, appear to be associated with relatively low ambient CO2 concentrations (Royer et al., 2001), invoking alternative or additional mechanisms, such as altered ocean circulation, for high-latitude warmth during this interval. Similarly, the mid-Miocene climatic optimum (∼ 17 to ∼15 Ma), according to some proxy CO2 records, was not associated with high concentrations of greenhouse gasses (Pagani et al., 1999; Pearson and Palmer, 2000), but rather with concentrations lower than today. An apparent paradox has emerged—the pre-Quaternary history of CO2 and climate interaction, which once appeared to be relatively well coupled based on temporal correlation of …

Drought and oxidative load in the leaves of C3 plants: a predominant role for photorespiration?

Although active oxygen species are produced at high rates in both the chloroplasts and peroxisomes of the leaves of C3 plants, most attention has focused on the potentially damaging consequences of enhanced chloroplastic production in stress conditions such as drought. This article attempts to provide quantitative estimates of the relative contributions of the chloroplast electron transport chain and the glycolate oxidase reaction to the oxidative load placed on the photosynthetic leaf cell. Rates of photorespiratory H2O2 production were obtained from photosynthetic and photorespiratory flux rates, derived from steady-state leaf gas exchange measurements at varying irradiance and ambient CO2. Assuming a 10% allocation of photosynthetic electron flow to the Mehler reaction, photorespiratory H2O2 production would account for about 70% of total H2O2 formed at all irradiances measured. When chloroplastic CO2 concentration rates are decreased, photorespiration becomes even more predominant in H2O2 generation. At the increased flux through photorespiration observed at lower ambient CO2, the Mehler reaction would have to account for more than 35% of the total photosynthetic electron flow in order to match the rate of peroxisomal H2O2 production. The potential signalling role of H2O2 produced in the peroxisomes is emphasized, and it is demonstrated that photorespiratory H2O2 can perturb the redox states of leaf antioxidant pools. We discuss the interactions between oxidants, antioxidants and redox changes leading to modified gene expression, particularly in relation to drought, and call attention to the potential significance of photorespiratory H2O2 in signalling and acclimation.

Acquisition of inorganic carbon by the marine diatom Thalassiosira weissflogii.

Recent data on the physiology of inorganic carbon acquisition by the model marine diatom Thalassiosira weissflogii (Grunow) demonstrate the importance of the catalytic equilibration of HCO3-and CO2by carbonic anhydrases located in the periplasm and in the cytoplasm. These enzymes can use Zn, Co or Cd as their metal centre, and their activity increases at low ambient CO2. The silica frustule provides buffering for extracellular CA activity, The transmembrane transport of CO2 may occur by passive diffusion. Under CO2 limitation, the cytoplasmic HCO3-is used to form malate and oxaloacetic acid via phosphoenolpyruvate carboxylase. It appears that subsequent decarboxylation of these compounds in the chloroplast regenerates CO2 near the site of Rubisco, and thus provides the organism with an effective unicellular C4 photosynthetic pathway. These results, together with other published data, bring up two major questions regarding inorganic carbon acquisition in diatoms: What is the major species of inorganic carbon (CO2 or HCO3-) transported across the membrane under natural conditions? And what is the form of carbon (inorganic or organic) accumulated by the cells?

Comparison of Methods to Estimate Dark Respiration in the Light in Leaves of Two Woody Species.

Dark respiration in the light was estimated in leaves of two woody species (Heteromeles arbutifolia Ait. and Lepechinia fragans Greene) using two different approaches based on gas-exchange techniques: the Kok method and the Laisk method. In all cases, dark respiration in the light was lower (P < 0.05) than respiration in darkness, indicating that dark respiration was inhibited in the light. Rates of dark respiration in the light estimated by the Laisk method were 52% higher (P < 0.05) than those estimated by the Kok method. Differences between the methods could be explained by the low ambient CO2 concentrations required by the Laisk approach. The mean value of the inhibition of respiration by light for the two species, corrected for the ambient CO2 concentration effect, was 55%. Despite the differences in leaf characteristics between the species, values of the CO2 photocompensation point, at which the rate of photosynthetic CO2 uptake equaled that of photorespiratory CO2 evolution, were very constant, suggesting an excellent consistency in the results obtained with the Laisk approach.

Molecular analysis of high CO2 requiring mutants: involvement of genes in the region of rbc, including rbcS, in the ability of cyanobacteria to grow under low CO2

Modifications of the genomic region near (and including) rbc, the operon that codes for the large and small subunits of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), resulted in cyanobacterial mutants that demand high CO2 for growth. Mutant EK6 originated from the fusion of the 3' end of rbcS (which codes for the small subunit of Rubisco) with 84 nucleotides from the 5′ flanking region of nptII (kanamycin-resistance gene), leading to a 17-kDa small subunit, as compared to 14 kDa in the wild type. Mutant D4 originated from substitution of the 1.4-kb PstI fragment, downstream of rbc, with nptII, inactivating several open reading frames in this region. Mutant O105 was obtained by chemical mutagenesis and the mutation was mapped approximately 9 kb upstream of rbc. Mutants EK6 and O105 exhibited a very low apparent photosynthetic affinity for inorganic carbon, whereas D4 had an affinity similar to that observed in wild-type cells grown under high CO2. These mutants, and the constructs used to raise them, can be used to study the role of the small subunit of Rubisco and the genomic region near rbc in cyanobacterial photosynthesis. We propose that this region contains a cluster of genes involved in the ability of cyanobacteria to grow under low ambient CO2..

Leaf Factors Affecting Light‐Saturated Photosynthesis in Ecotypes of Solidago virgaurea from Exposed and Shaded Habitats

AbstractPlants of Solidago virgaurea L. from exposed and shaded habitats differ with respect to the response of the photosynthetic apparatus to the level of irradiance during growth. An analysis was carried out on leaf characteristies which might be responsible for the differences established in the rates of Hght‐saturated CO2 uptake. The clones were grown in controlled environment chambers at high and low levels of irradiance. Light‐saturated rates of photosynthesis and transpiration were measured at natural and lower ambient CO2 concentrations.A low temperature dependence of light‐saturated CO2 uptake at natural CO2 concentrations, and a strong response to changes in stomatal width, suggested that the rate of CO2 transfer from ambient air towards reaetion sites in chloroplasts was mainly limiting the pholosynthetic rate. Resistances to transfer of CO2 for different parts of the pathway were calculated. There was a weak but significant correlation between stomatal conductance and the product stomatal frequency ± pore length. Mesopbyll conductance and dry weight per unit area were highly correlated in leaves not damaged by high irradiance. This suggests that mesophyll conductance increases with increasing cross sectional area (per unit leaf area) of the pathways of CO2 transfer in the mesophyll from cell surfaces to reaction sites.The higher light‐saturated photosynthesis in clones from exposed habitats when grown at high irradiance than when grown at low irradiance was attributable mainly to a lower mesophyll resistance. In shade clones the effect upon CO2 uptake of the increase in leaf thickness when grown at high irradiance was counteracted by the associated inactivation of the photosynthetic apparatus. The difference in CO2 uptake present between clones from exposed and shaded habitats when preconditioned to high irradiance resulted from differences in both mesophyll and stomatal resistances. A few hybrid clones of an F1‐population from a cross between a clone from an exposed habitat and a clone from a shaded habitat reacted, on the whole, in the same way as the exposed habitat parent. When grown at high irradiance, the hybrid clones showed higher photosynthetic rates than either parent; this was largely attributable to the unusually low stomatal resistance of the hybrid leaves.

Carbon Dioxide Efflux from Leaves in Light and Darkness

AbstractEfflux of carbon dioxide in light and darkness was measured at low ambient CO2 concentrations in leaves of Rumex acetosa. Light carbon dioxide production (photo‐respiration) was found to depend on irradiance and to differ from dark production as to the response to temperature and ambient concentrations of O2 and CO2. These observations support previously made suggestions that photorespiration follows a different metabolic pathway to dark respiration.