Efficient CO2 fixation by surface Prochlorococcus in the Atlantic Ocean

Nearly half of the Earth''s surface is covered by the ocean populated by the most abundant photosynthetic organisms on the planet—Prochlorococcus cyanobacteria. However, in the oligotrophic open ocean, the majority of their cells in the top half of the photic layer have levels of photosynthetic pigmentation barely detectable by flow cytometry, suggesting low efficiency of CO₂ fixation compared with other phytoplankton living in the same waters. To test the latter assumption, CO₂ fixation rates of flow cytometrically sorted ¹⁴C-labelled phytoplankton cells were directly compared in surface waters of the open Atlantic Ocean (30°S to 30°N). CO₂ fixation rates of Prochlorococcus are at least 1.5–2.0 times higher than CO₂ fixation rates of the smallest plastidic protists and Synechococcus cyanobacteria when normalised to photosynthetic pigmentation assessed using cellular red autofluorescence. Therefore, our data indicate that in oligotrophic oceanic surface waters, pigment minimisation allows Prochlorococcus cells to harvest plentiful sunlight more effectively than other phytoplankton.     

Specificity factor of Rubisco: estimation in intact leaves by carboxylation at different CO2/O2 ratios

The specificity factor of Rubisco (S _f) was estimated in intact leaves from the carboxylation of ribulose-1,5-bisphosphate (RuBP) at various CO₂/O₂ ratios. As oxygenation is calculated by the difference of the ¹⁴CO₂ uptake by RuBP in the absence and presence of oxygen, it is important to choose the optimum CO₂/O₂ ratios. At high CO₂ concentration (1,000 cm³ m^?3 and higher) oxygenation consumes less than 50% RuBP but the difference of concentrations of CO₂ at cell walls (C _w) and at the carboxylation centers (C _c) is 2?C5% and the influence of mesophyll resistance (r _md) is of minor importance. To accumulate large endogenous pool of RuBP, the leaves were preilluminated in the CO₂- and O₂-free gas environments for 8 to 10 s. Thereafter the light was switched off and the leaves were flushed with the gas containing different concentrations of ¹⁴CO₂ and O₂. The specificity factor of Rubisco was calculated from the amount of the tracer taken up under different ¹⁴CO₂/O₂ ratios by the exhaustion of the RuBP pool. Application of ¹⁴CO₂ allowed us to discriminate between the CO₂ uptake and the concurrent respiratory CO₂ release which proceeded at the expense of unlabelled intermediates.     

Wheat Response to CO2 Enrichment: Growth and CO2 Exchanges at Two Plant Densities

Du Cloux, H. C, André, M., Daguenet, A. and Massinuno,J. 1987. Wheat response to CO₂ enrichment: Growth and CO₂ exchangesat two plant densities.—J. exp. Bot. 38: 1421–1431. The vegetative growth of wheat (Triticum aestivum L., var. Capitole)was followed for almost 40 d after germination in controlledconditions. Four different treatments were carried out by combiningtwo air concentrations of CO₂, either normal (330 mm³ dm ³)or doubled (660 mm³ dm ³) with two plant densities, either 200plants m ² or 40 plants m ². Throughout the experiment the CO₂gas exchanges of each canopy were measured 24 h d¹. These provideda continuous growth curve for each treatment, which were comparedwith dry weights. After a small stimulation at the start (first13 d), no further effect of CO₂ enrichment was observed on relativegrowth rate (RGR). However, RGR was stimulated throughout theexperiment when plotted as a function of biomass. The finalstimulation ol dry weight at 660 mm³ dm ³ CO₂ was a factor of1·45 at high density and 1·50 at low density,contrary to other studies, no diminution of this CO₂ effecton dry weight was observed over time. Nevertheless, at low density,a transient additional enhancement of biomass (up to 1·70)was obtained at a leaf area index (LAI) below 1. This effectwas attributed to a different build up of the gain of carbonin the case of an isolated plant or a closed canopy. In theformer, the stimulation of leaf area and the net assimilationrate are both involved; in the latter the enhancement becomesindependent of the effect on leaf area because the canopy photosynthesisper unit ground area as a function of LAI reaches a plateau. Key words: Triticum aestuum, L. var. Capitole, Vegetative growth, Canopy     

Growth dynamics and transcriptional responses of a Red Sea Prochlorococcus strain to varying temperatures

Prochlorococcus play a crucial role in the ocean's biogeochemical cycling, but it remains controversial how they will respond to global warming. Here we assessed the response to temperature (22–30°C) of the growth dynamics and gene expression profiles of a Red Sea Prochlorococcus strain (RSP50) in a non-axenic culture. Both the specific growth rate (0.55–0.80 day⁻¹) and cell size (0.04–0.07 μm³) of Prochlorococcus increased significantly with temperature. The primary production released extracellularly ranged from 20% to 34%, with humic-like fluorescent compounds increasing up to fivefold as Prochlorococcus reached its maximum abundance. At 30°C, genes involved in carbon fixation such as CsoS2 and CsoS3 and photosynthetic electron transport including PTOX were downregulated, suggesting a cellular homeostasis and energy saving mechanism response. In contrast, PTOX was found upregulated at 22°C and 24°C. Similar results were found for transaldolase, related to carbon metabolism, and citrate synthase, an important enzyme in the TCA cycle. Our data suggest that in spite of the currently warm temperatures of the Red Sea, Prochlorococcus can modulate its gene expression profiles to permit growth at temperatures lower than its optimum temperature (28°C) but is unable to cope with temperatures exceeding 30°C.     

Effect of Mechanical Loading on Ventilatory Response to CO2 and CO2 Excretion

The ventilatory response to CO₂ (S) and respiratory exchange ratio have been measured in 10 healthy subjects breathing naturally and through added resistive loads. The changes in these values produced by the added loads were shown to be correlated with the unloaded CO₂ responsiveness. The results indicated that poorly responsive individuals had a greater depression of ventilatory response to CO₂ and were more liable to retain CO₂.These observations raise the possibility that the constitutional CO₂ responsiveness of an individual influences the alveolar ventilation achieved in the presence of airways obstruction. The propensity to develop respiratory failure may thus be conditioned by the premorbid CO₂ responsiveness.     

Response of young barley plants to CO2 enrichment

Barley (Hordeum vulgare L. cv. Digger) was grown for 22 d inenclosed chambers with a CO₂ enrichment of 35, 155, 400 or 675µmol CO₂ mol1. CO₂ enrichment increased photosyntheticcapacity in the plants grown at either of the two highest levelsof pCO₂. A CO₂ enrichment of 675µmol CO₂ caused a significantincrement of shoot dry weight, whereas no changes were observedin fresh weight, chlorophyll or protein levels. At a light intensityof 860µmol m^–2s^–1 CO₂ enrichment caused photosyntheticcapacity to increase by 250%, whereas no effect was observedat 80 µmol m^–2 s^–1. Over time, photosynthesisdecreased by 70% independent of CO₂. A time-dependent increasein the level of extractable fructose was observed whereas totalextractable carbohydrate only changed slightly. Key words: Carbohydrates, CO₂ enrichment, Hordeum vulgare, photosynthesis, respiration     

Response of Forest Trees to Increased Atmospheric CO 2

The CO ₂ fertilization hypothesis stipulates that rising atmospheric CO ₂ has a positive effect on tree growth due to increasing availability of carbon. The objective of this paper is to compare the recent literature related to both field CO ₂ -enriched experiments with trees and empirical dendrochronological studies detecting CO ₂ fertilization effects in tree-rings. This will allow evaluation of tree growth responses to atmospheric CO ₂ enrichment by combining evidence from both ecophysiology and tree-ring research. Based on considerable experimental evidence of direct CO ₂ fertilization effect (increased photosynthesis, water use efficiency, and above- and belowground biomass), and predications from the interactions of enriched CO ₂ with temperature, nitrogen and drought, we propose that warm, moderately drought-stressed ecosystems with an ample nitrogen supply might be the most CO ₂ responsive ecosystems. Empirical tree-ring studies took the following three viewpoints on detecting CO ₂ fertilization effect in tree-rings: 1) finding evidence of CO ₂ fertilization effect in tree-rings, 2) attributing growth enhancement to favorable climate rather than atmospheric CO ₂ enrichment, and 3) considering that tree growth enhancement might be caused by synergistic effects of several factors such as favorable climate change, CO ₂ fertilization, and anthropogenic atmospheric deposition (e.g., nitrogen). At temperature-limiting sites such as high elevations, nonfindings of CO ₂ fertilization evidence could be ascribed to the following possibilities: 1) cold temperatures, a short season of cambial division, and nitrogen deficiency that preclude a direct CO ₂ response, 2) old trees past half of their maximum life expectancy and consequently only a small increase in biomass increment due to CO ₂ fertilization effect might be diminished, 3) the elimination of age/size-related trends by statistical detrending of tree-ring series that might remove some long-term CO ₂ -related trends in tree-rings, and 4) carbon partitioning and growth within a plant that is species-specific. Our review supports the atmospheric CO ₂ fertilization effect hypothesis, at least in trees growing in semi-arid or arid conditions because the drought-stressed trees could benefit from increased water use efficiency to enhance growth.     

Human whole-blood O2 affinity: effect of CO2

The proton Bohr factor (phi H = alpha log PO2/alpha pH), the carbamate Bohr factor (phi C = alpha log PO2/alpha log PCO2), the total Bohr factor (phi HC = d log PO2/dpH[base excess) and the CO2 buffer factor (d log PCO2/dpH) were determined in the blood of 12 healthy donors over the whole O2 saturation (SO2) range. All three Bohr factors proved to be dependent on SO2, although to a lesser extent than reported in some of the recent literature. At SO2 = 50% and 37 degrees C, we found phi H = -0.428 +/- 0.010 (SE), phi C = 0.054 +/- 0.006, and phi HC = -0.488 +/- 0.007. The values obtained for phi H, phi C, and d log PCO2/dpH were used to calculate phi HC. Calculated and measured values of phi HC proved to be in good agreement. In an additional series of 12 specimens of human blood we determined the influence of PCO2 on phi H and the influence of pH on phi C. At SO2 = 50%, phi H varied from -0.49 +/- 0.009 at PCO2 = 15 Torr to -0.31 +/- 0.010 at PCO2 = 105 Torr and phi C from 0.157 +/- 0.015 at pH = 7.80 to 0.006 +/- 0.009 at pH = 7.00. When on the basis of these data a second-order term is taken into account, a still slightly better agreement between measured and calculated values of phi HC can be attained.     

Growth and tissue senescence inPrunus avium shoots grownin vitro at different CO3/O2 ratios

Summary The rate of metabolism and biosynthetic processes makein vitro cultures very sensitive to environmental changes, and therefore subject to physiological and morphological alterations leading to senescence in the short term. The effect of three different calibrated atmospheric compositions were studied duringin vitro culture ofPrunus avium shoots. At 0.034% CO₂-21% O₂ (vol/vol), which stimulate the natural atmosphere, the highest growth rate and chlorophyll content were recorded. When grown at 0.09% CO₂-8% O₂ (vol/vol), a favorable condition for photosynthesis and growth, cultures showed a higher percentage of dry matter and elevated ethylene production, but total chlorophyll was lower. These shoots were also highly lignified and fibrous with red pigmentation along the leaves and stems. At 0% CO₂-21% O₂ (vol/vol), in contrast, growth and ethylene formation were inhibited; chlorophyll content was lowest in comparison with the other two environmental conditions, but regreening of tissues was observed after the first half of the culture period. Senescence symptoms, as indicated by decreased chlorophyll, appeared after about 18 d of culture for tissues grown in CO₂-containing atmospheres. These experiments provided evidence that in CO₂-enriched cultures biomass production steadily increased even when chlorophyll decreased. A possible role of CO₂ in promoting tissue-senescence through activation of photooxidative events and ethylene synthesis is discussed.     

Response of plants to CO2 under water limited conditions

The influence of inefeased atmospheric CO₂ on the interaction between plant growth and water use is proving to be one of the most profound impacts of the anthropogenic Greenhouse Effect. This paper illustrates the interaction between CO₂ and water in plant growth at a range of scales. Most published work has concentrated on water use efficiency, especially at shorter time scales, and has shown large increases of leaf water use efficiency with increased CO₂. However, the magnitude of the effect is variable, and does not consistently agree with predictions from simple leaf gas exchange considerations. The longer the time scales considered, the less the information and the more the uncertainty in the response to CO₂, because of the additional factors that have to be considered, such as changes in leaf area, respiration of non-photosynthetic tissues and soil evaporation. The need for more detailed studies of the interactions between plant evaporation, water supply, water status and growth is stressed, as increased CO₂ can affect all of these either directly, or indirectly through feedbacks with leaf gas exchange, carbon partitioning, leaf growth, canopy development and root growth.     

辽东栎对大气CO2倍增的响应

 本文研究了CO2加浓对暖温带落叶阔叶混交林典型自然群落建群种辽东栎的影响,结果表明：在生理学方面,CO2倍增下气孔阻抗略增大,为对照的106％,蒸腾速率略下降,为对照的92％,暗呼吸速率与对照很接近,但略微下降为对照的98.9％。净光合速率、昼夜净光合量、水分利用效率都明显提高,分别为对照的155％,172％和179％。可以看出C02倍增对辽东栎的生理过程有促进作用,属正效应。其中以生长旺季6、7月增长更为明显。在生长方面,CO2倍增下生长各项指标增长也较明显,叶面积为对照的107％,叶干重为对照的140％,以植株高度增加最明显,为对照的331％,清楚的看出辽东栎的生长与生理过程的变化趋势是一致的、均属正效应。也就是说在其他环境资源满足植物要求时,CO2倍增对树木具有“施肥”作用,它可促进植物的生理过程和提高其生物生产力。     

Megakaryocytopoiesis in the mouse: Response to varying platelet demand

The response of murine megakaryocytopoiesis was studied under conditions of varying platelet demand. Twenty-four hours after mice were given a single injection of rabbit anti-platelet serum, megakaryocyte number and volume were increased, becoming maximal at 65 and 40 hr, respectively. Total body megakaryocytic colony-forming unit (CFU-M) numbers did not change until 90 hr, when a 35% increase in the experimental group was noted. The percentage of CFU-M in DNA synthesis in the experimental group was 38 ± 2% at 24 hr, 49 ± 1% at 40 hr, and returned to normal (11 ± 3%) at 90 hr. When mice were made thrombocytotic by platelet transfusions, both megakaryocyte number and volume were decreased compared to controls, while no difference was noted in the number and percentage of CFU-M in DNA synthesis. Finally, experiments were performed to examine the effect of platelet transfusions on regenerating marrow. Experimental mice were given platelet transfusions while control animals received platelet buffer solution. At sacrifice the number and volume of megakaryocytes in the experimental group (platelet count 2.568 × 10⁶/μl) were less than controls (platelet count 0.363 × 10⁶/μl), while the number and percentage of CFU-M in DNA synthesis were similar in both groups. These results demonstrate that CFU-M are not immediately responsive to acute changes in platelet demand. The data suggest that megakaryo-cytopoiesis is structured on at least two levels which are independently regulated.     

The Minimal CO2-Concentrating Mechanism of Prochlorococcus spp. MED4 Is Effective and Efficient

As an oligotrophic specialist, Prochlorococcus spp. has streamlined its genome and metabolism including the CO₂-concentrating mechanism (CCM), which serves to elevate the CO₂ concentration around Rubisco. The genomes of Prochlorococcus spp. indicate that they have a simple CCM composed of one or two HCO₃⁻ pumps and a carboxysome, but its functionality has not been examined. Here, we show that the CCM of Prochlorococcus spp. is effective and efficient, transporting only two molecules of HCO₃⁻ per molecule of CO₂ fixed. A mechanistic, numerical model with a structure based on the CCM components present in the genome is able to match data on photosynthesis, CO₂ efflux, and the intracellular inorganic carbon pool. The model requires the carboxysome shell to be a major barrier to CO₂ efflux and shows that excess Rubisco capacity is critical to attaining a high-affinity CCM without CO₂ recovery mechanisms or high-affinity HCO₃⁻ transporters. No differences in CCM physiology or gene expression were observed when Prochlorococcus spp. was fully acclimated to high-CO₂ (1,000 µL L⁻¹) or low-CO₂ (150 µL L⁻¹) conditions. Prochlorococcus spp. CCM components in the Global Ocean Survey metagenomes were very similar to those in the genomes of cultivated strains, indicating that the CCM in environmental populations is similar to that of cultured representatives.The marine picocyanobacteria genus Prochlorococcus along with its sister group the marine genus Synechococcus dominate primary production in oligotrophic marine environments (Partensky et al., 1999). Prochlorococcus spp. is an oligotrophic specialist with several key adaptations allowing it to outcompete other phytoplankton in the stable, low-nutrient regions where it thrives. These adaptations include small cell size (less than 1 μm), allowing it to effectively capture nutrients and light, and genome streamlining, which minimizes nutrient requirements (Partensky and Garczarek, 2010). At approximately 1,900 genes, the genomes of high-light-adapted Prochlorococcus spp. are the smallest known among photoautotrophs, suggesting that this is about the minimum number of genes needed to make a cell from inorganic constituents and light (Rocap et al., 2003). Genome reduction has been accomplished by both the loss of entire pathways and complexes, such as the phycobilisomes and many regulatory capabilities, and the paring down of systems to their minimal components, as is the case for the circadian clock and the photosynthetic complexes (Rocap et al., 2003; Kettler et al., 2007; Partensky and Garczarek, 2010).As part of this genome streamlining, the CO₂-concentrating mechanism (CCM), which enhances the efficiency of photosynthesis by elevating the concentration of CO₂ around Rubisco, has been reduced to what appears to be the minimal number of components necessary for a functional CCM (Badger and Price, 2003; Badger et al., 2006). In typical cyanobacteria, the CCM is composed of HCO₃⁻ transporters, CO₂ uptake systems, and the carboxysome, a protein microcompartment in which Rubisco and carbonic anhydrase (CA) are enclosed. HCO₃⁻ is accumulated in the cytoplasm by direct import from the environment and by the active conversion of CO₂ to HCO₃⁻ via an NADH-dependent process, which constitutes the CO₂ uptake mechanism (Shibata et al., 2001). The accumulated HCO₃⁻ then diffuses into the carboxysome, where CA converts it to CO₂, elevating the concentration of CO₂ around Rubisco (Reinhold et al., 1987; Price and Badger, 1989).Whereas some cyanobacteria have up to three different families of HCO₃⁻ transporters with differing affinities for use under different environmental conditions, Prochlorococcus spp. has only one or two families (Badger et al., 2006)_. Most cyanobacteria have low-affinity and high-affinity CO₂ uptake systems, but no CO₂ uptake systems are apparent in Prochlorococcus spp. genomes. The carboxysome of Prochlorococcus spp. and other α-cyanobacteria has apparently been laterally transferred from chemoautotrophs, but all of the required components of the carboxysome are present and it is functional (Badger et al., 2002; Roberts et al., 2012). Despite its simplicity, this CCM is likely functional. HCO₃⁻ can be accumulated in the cytoplasm by the HCO₃⁻ transporters and then diffuse into the carboxysome for conversion to CO₂ and subsequent fixation by Rubisco. However, the functionality of the CCM in Prochlorococcus spp. has not yet been tested. Prochlorococcus spp. is a representative of the α-cyanobacteria, a group with distinct CCMs, which have been much less well studied than the CCMs of β-cyanobacteria (Rae et al., 2011, 2013; Whitehead et al., 2014).We characterized inorganic carbon (C_i) acquisition and processing in Prochlorococcus spp. MED4, examined the effect of long-term acclimation to different CO₂ concentrations on CCM physiology and gene expression, and searched metagenomes for Prochlorococcus spp. CCM genes to determine if CCMs in the natural populations are similar to cultured strains.     

Ethylene: Response of Fruit Dehiscence to CO(2) and Reduced Pressure

These studies were conducted to determine whether ethylene serves as a natural regulator of fruit wall dehiscence, a major visible feature of ripening in some fruits. We employed treatments to inhibit ethylene action or remove ethylene and observed their effect on fruit dehiscence. CO₂ (13%), a competitive inhibitor of ethylene action in many systems, readily delayed dehiscence of detached fruits of cotton (Gossypium hirsutum L.), pecan (Carya illinoensis [Wang.] K. Koch), and okra (Hibiscus esculentus L.). The CO₂ effect was duplicated by placing fruits under reduced pressure (200 millimeters mercury), to promote the escape of ethylene from the tissue. Dehiscence of detached fruits of these species as well as attached cotton fruits was delayed. The delay of dehiscence of cotton and okra by both treatments was achieved with fruit harvested at intervals from shortly after anthesis until shortly before natural dehiscence. Pecan fruits would not dehisce until approximately 1 month before natural dehiscence, and during that time, CO₂ and reduced pressure delayed dehiscence. CO₂ and ethylene were competitive in their effects on cotton fruit dehiscence. All of the results are compatible with a hypothetical role of ethylene as a natural regulator of dehiscence, a dominant aspect of ripening of cotton, pecan, and some other fruits.