A New Screening Method for Algal Photosynthetic Mutants (CO2-Insensitive Mutants of the Green Alga Chlorella ellipsoidea)

A new method has been developed for screening algal photosynthetic mutants. This method uses autoradiography to assess gross photosynthetic 14C fixation by green algal colonies on agar plates and allows the identification of clones that differ in photosynthetic characteristics from wild-type cells. Three wild-type cells, high-CO2-grown Chlorella ellipsoidea, air-grown C. ellipsoidea, and air-grown Chlorella saccharophila, had K0.5 values for dissolved inorganic carbon (DIC) of 1083, 250, and 50 [mu]M, respectively, and as plaques on agar plates at Chl densities greater than 25 [mu]g cm-2 exhibited relative amounts of 14C fixation of 15, 55, and 100%, respectively. Cells of C. ellipsoidea were mutagenized with x-rays and screened by this method. Growth of C. ellipsoidea in high CO2 represses DIC transport and thus lowers its affinity for DIC. Five of the mutants detected by this method showed high-affinity photosynthesis similar to air-grown wild-type cells even when grown in high CO2. Seven other mutants when grown in high CO2 showed affinities for DIC intermediate between air-grown and high-CO2-grown wild-type cells. The affinities of high-CO2-grown mutants were reflected precisely in their capacities to accumulate DIC intracellularly. These results indicate that the mutants are fully or partially insensitive to the repressive effect of ambient CO2 concentration on DIC transport.     

Active transport of CO(2) and bicarbonate is induced in response to external CO(2) concentration in the green alga Chlorella kessleri

The time-course of induction of CO(2) and HCO(3)- transport has been investigated during the acclimation of high CO(2)-grown Chlorella kessleri cells to dissolved inorganic carbon (DIC)-limited conditions. The rate of photosynthesis of the cells in excess of the uncatalysed supply rate of CO(2) from HCO(3)- was taken as an indicator of HCO(3)- transport, while a stimulation of photosynthesis on the addition of bovine carbonic anhydrase was used as an indicator of CO(2) transport. The maximum rate of photosynthesis (Pmax) was similar for high CO(2)-grown and low CO(2)-grown cells, but the apparent whole cell affinity for DIC and CO(2) of high CO(2)-grown cells was found to be about 30-fold greater than in air-grown cells, which indicates a lower affinity for DIC and CO(2). It was found that HCO(3)- and CO(2) transport were induced in 5.5 h in cells acclimating to air in the light and in the presence and absence of 21% O(2), which indicates that a change in the CO(2)/O(2) ratio in the acclimating medium does not trigger induction of DIC transport. No active DIC transport was detected in high CO(2)-grown cells maintained on high CO(2) for 5.5 h in the presence of 5 mM aminooxyacetate, an aminotransferase inhibitor. These results indicate no involvement of photorespiration in triggering induction. Active DIC transport induction was inhibited in cells treated with 5 microgram ml(-1) cycloheximide, but was unaffected by chloramphenicol treatment, indicating that the induction process requires de novo cytoplasmic protein synthesis. The total DIC concentration eliciting the induction and repression of CO(2) and HCO(3)- transport was higher at pH 7.5 than at pH 6.6. The concentrations of external CO(2) required for the induction and repression of DIC transport were 0 and 120 microM, respectively, and was independent of the pH of the acclimation medium. Prolonged exposure to a critical external CO(2) concentration elicits the induction of DIC transport in C. kessleri.     

Active uptake of inorganic carbon by Chlorella saccharophila is not repressed by growth in high CO2

Regulation of transport of dissolved inorganic carbon (DIC)in response to CO₂ concentration in the external medium hasbeen compared in two closely-related green algae, Chlorellaellipsoidea and Chlorella saccharophila. C. ellipsoidea, whengrown in high CO₂, had reduced activities of both CO₂ and transport and DIC transport activitieswere increased after the cells had acclimated to air. However,high CO₂-grown C. saccharophila had a comparable level of photosyntheticaffinity for DIC to that of air-grown C. ellipsoidea and thiswas accompanied by a capacity to accumulate high internal concentrationsof DIC. The high photosynthetic affinity and the high intracellularDIC accumulation did not change in cells grown in air exceptthat the occurrence of external carbonic anhydrase (CA) in air-grownC. saccharophila stimulated the intracellular DIC accumulationin the absence of added CA. These data indicate that activeDIC transport is constitutively expressed in C. saccharophila,presumably because this alga is insensitive to the repressiveeffect of high CO₂ on DIC transport. This strongly supportsthe existence of a direct sensing mechanism for external CO₂in Chlorella species, but also indicates that external CA isregulated independently of DIC transport in Chlorella species. Key words: Carbonic anhydrase, Chlorella, CO₂-insensitive, DIC transport, wild type     

Long-term growth of soybean at elevated [CO2] does not cause acclimation of stomatal conductance under fully open-air conditions

Accurately predicting plant function and global biogeochemical cycles later in this century will be complicated if stomatal conductance (g(s)) acclimates to growth at elevated [CO(2)], in the sense of a long-term alteration of the response of g(s) to [CO(2)], humidity (h) and/or photosynthetic rate (A). If so, photosynthetic and stomatal models will require parameterization at each growth [CO(2)] of interest. Photosynthetic acclimation to long-term growth at elevated [CO(2)] occurs frequently. Acclimation of g(s) has rarely been examined, even though stomatal density commonly changes with growth [CO(2)]. Soybean was grown under field conditions at ambient [CO(2)] (378 micromol mol(-1)) and elevated [CO(2)] (552 micromol mol(-1)) using free-air [CO(2)] enrichment (FACE). This study tested for stomatal acclimation by parameterizing and validating the widely used Ball et al. model (1987, Progress in Photosynthesis Research, vol IV, 221-224) with measurements of leaf gas exchange. The dependence of g(s) on A, h and [CO(2)] at the leaf surface was unaltered by long-term growth at elevated [CO(2)]. This suggests that the commonly observed decrease in g(s) under elevated [CO(2)] is due entirely to the direct instantaneous effect of [CO(2)] on g(s) and that there is no longer-term acclimation of g(s) independent of photosynthetic acclimation. The model accurately predicted g(s) for soybean growing under ambient and elevated [CO(2)] in the field. Model parameters under ambient and elevated [CO(2)] were indistinguishable, demonstrating that stomatal function under ambient and elevated [CO(2)] could be modelled without the need for parameterization at each growth [CO(2)].     

Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to Free Air CO2 Enrichment (FACE)

The initial stimulation of photosynthesis observed on elevation of [CO2] in grasslands has been predicted to be a transient phenomenon constrained by the loss of photosynthetic capacity due to other limitations, notably nutrients and sinks for carbohydrates. Legumes might be expected partially to escape these feedbacks through symbiotic N2 fixation. The Free-Air Carbon dioxide Enrichment (FACE) experiment at Eschikon, Switzerland, has been the longest running investigation of the effects of open-air elevation of [CO2] on vegetation. The prediction of a long-term loss of photosynthetic capacity was tested by analysing photosynthesis in Trifolium repens L. (cv. Milkanova) in the spring and autumn of the eighth, ninth and tenth years of treatment. A high and low N treatment also allowed a test of the significance of exogenous N-supply in maintaining a stimulation of photosynthetic capacity in the long-term. Prior work in this Free Air CO2 Enrichment (FACE) experiment has revealed that elevated [CO2] increased both vegetative and reproductive growth of T. repens independent of N treatment. It is shown here that the photosynthetic response of T. repens was also independent of N fertilization under both current ambient and elevated (600 micro mol mol-1) [CO2]. There was a strong effect of season on photosynthesis, with light-saturated rates (Asat) 37% higher in spring than in autumn. Higher Asat in the spring was supported by higher maximum Rubisco carboxylation rates (Vc,max) and maximum rates of electron transport (Jmax) contributing to RuBP regeneration. Elevated [CO2] increased Asat by 37% when averaged across all measurement periods and both N fertilization levels, and decreased stomatal conductance by 25%. In spring, there was no effect of elevated [CO2] on photosynthetic capacity of leaves, but in autumn both Vc,max and Jmax were reduced by approximately 20% in elevated [CO2]. The results show that acclimation of photosynthetic capacity can occur in a nitrogen-fixing species, in the field where there are no artificial restrictions on sink capacity. However, even with acclimation there was a highly significant increase in photosynthesis at elevated [CO2].     

Interactive effects of elevated CO2, warming, and drought on photosynthesis of Deschampsia flexuosa in a temperate heath ecosystem

Global change factors affect plant carbon uptake in concert. In order to investigate the response directions and potential interactive effects, and to understand the underlying mechanisms, multifactor experiments are needed. The focus of this study was on the photosynthetic response to elevated CO(2) [CO2; free air CO(2) enrichment (FACE)], drought (D; water-excluding curtains), and night-time warming (T; infrared-reflective curtains) in a temperate heath. A/C(i) curves were measured, allowing analysis of light-saturated net photosynthesis (P(n)), light- and CO(2)-saturated net photosynthesis (P(max)), stomatal conductance (g(s)), the maximal rate of Rubisco carboxylation (V(cmax)), and the maximal rate of ribulose bisphosphate (RuBP) regeneration (J(max)) along with leaf δ(13)C, and carbon and nitrogen concentration on a monthly basis in the grass Deschampsia flexuosa. Seasonal drought reduced P(n) via g(s), but severe (experimental) drought decreased P(n) via a reduction in photosynthetic capacity (P(max), J(max), and V(cmax)). The effects were completely reversed by rewetting and stimulated P(n) via photosynthetic capacity stimulation. Warming increased early and late season P(n) via higher P(max) and J(max). Elevated CO(2) did not decrease g(s), but stimulated P(n) via increased C(i). The T×CO2 synergistically increased plant carbon uptake via photosynthetic capacity up-regulation in early season and by better access to water after rewetting. The effects of the combination of drought and elevated CO(2) depended on soil water availability, with additive effects when the soil water content was low and D×CO2 synergistic stimulation of P(n) after rewetting. The photosynthetic responses appeared to be highly influenced by growth pattern. The grass has opportunistic water consumption, and a biphasic growth pattern allowing for leaf dieback at low soil water availability followed by rapid re-growth of active leaves when rewetted and possibly a large resource allocation capability mediated by the rhizome. This growth characteristic allowed for the photosynthetic capacity up-regulations that mediated the T×CO2 and D×CO2 synergistic effects on photosynthesis. These are clearly advantageous characteristics when exposed to climate changes. In conclusion, after 1 year of experimentation, the limitations by low soil water availability and stimulation in early and late season by warming clearly structure and interact with the photosynthetic response to elevated CO(2) in this grassland species.     

Acclimation of rice photosynthesis to irradiance under field conditions

Acclimation to irradiance was measured in terms of light-saturated photosynthetic carbon assimilation rates (P(max)), Rubisco, and pigment content in mature field-grown rice (Oryza sativa) plants in tropical conditions. Measurements were made at different positions within the canopy alongside irradiance and daylight spectra. These data were compared with a second experiment in which acclimation to irradiance was assessed in uppermost leaves within whole-plant shading regimes (10% low light [LL], 40% medium light [ML], and 100% high light [HL] of full natural sunlight). Two varieties, japonica (tropical; new plant type [NPT]) and indica (IR72) were compared. Values for Rubisco amount, chlorophyll a/b, and P(max) all declined from the top to the base of the canopy. In the artificial shading experiment, acclimation of P(max) (measured at 350 microL L(-1) CO(2)) occurred between LL and ML for IR72 with no difference observed between ML and HL. The Rubisco amount increased between ML and HL in IR72. A different pattern was seen for NPT with higher P(max) (measured at 350 microL L(-1) CO(2)) at LL than IR72 and some acclimation of this parameter between ML and HL. Rubisco levels were higher in NPT than IR72 contrasting with P(max). Comparison of data from both experiments suggests a leaf aging effect between the uppermost two leaf positions, which was not a result of irradiance acclimation. Results are discussed in terms of: (a) acclimation of photosynthesis and radiation use efficiency at high irradiance in rice, and (b) factors controlling photosynthetic rates of leaves within the canopy.     

Inorganic carbon acquisition in red tide dinoflagellates

Carbon acquisition was investigated in three marine bloom-forming dinollagellates-Prorocentrum minimum, Heterocapsa triquetra and Ceratium lineatum. In vivo activities of extracellular and intracellular carbonic anhydrase (CA), photosynthetic O2 evolution, CO2 and HCO3- uptake rates were measured by membrane inlet mass spectrometry (MIMS) in cells acclimated to low pH (8.0) and high pH (8.5 or 9.1). A second approach used short-term 14C-disequilibrium incubations to estimate the carbon source utilized by the cells. All three species showed negligible extracellular CA (eCA) activity in cells acclimated to low pH and only slightly higher activity when acclimated to high pH. Intracellular CA (iCA) activity was present in all three species, but it increased only in P. minimum with increasing pH. Half-saturation concentrations (K1/2) for photosynthetic O2 evolution were low compared to ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics. Moreover, apparent affinities for inorganic carbon (Ci) increased with increasing pH in the acclimation, indicating the operation of an efficient CO2 concentration mechanism (CCM) in these dinoflagellates. Rates of CO2 uptake were comparably low and could not support the observed rates of photosynthesis. Consequently, rates of HCO3- uptake were high in the investigated species, contributing more than 80% of the photosynthetic carbon fixation. The affinity for HCO3- and maximum uptake rates increased under higher pH. The strong preference for HCO3- was also confirmed by the 14C-disequilibrium technique. Modes of carbon acquisition were consistent with the 13C-fractionation pattern observed and indicated a strong species-specific difference in leakage. These results suggest that photosynthesis in marine dinoflagellates is not limited by Ci even at high pH, which may occur during red tides in coastal waters.     

Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment

Four South African Leucadendron congenerics with divergent soil N and P preferences were grown as juveniles at contrasting nutrient concentrations at ambient (350 mol mol^-1) and elevated (700 mol mol^-1) atmospheric CO2 levels. Photosynthetic parameters were related to leaf nutrient and carbohydrate status to reveal controls of carbon uptake rate. In all species, elevated CO2 depressed both the maximum Rubisco catalytic activity (Vc,max, by 19-44%) and maximum electron transport rate (Jmax, by 13-39%), indicating significant photosynthetic acclimation of both measures. Even so, all species had increased maximum light-saturated rate of net CO2 uptake (Amax)) at the elevated growth CO2 level, due to higher intercellular CO2 concentration (ci). Leaf nitrogen concentration was central to photosynthetic performance, correlating with Amax, Vc,max and Jmax, Vc,max and Jmax were linearly co-correlated, revealing a relatively invariable Jmax:Vc,max ratio, probably due to N resource optimization between light harvesting (RuBP regeneration) and carboxylation. Leaf total non-structural carbohydrate concentration (primarily starch) increased in high CO2, and was correlated with the reduction in Vc,max and Jmax. Apparent feedback control of Vc,max and Jmax was thus surprisingly consistent across all species, and may regulate carbon exchange in response to end-product fluctuation. If so, elevated CO2 may have emulated an excess end-product condition, triggering both Vc,max and Jmax down-regulation. In Leucadendron, a general physiological mechanism seems to control excess carbohydrate formation, and photosynthetic responsiveness to elevated CO2, independently of genotype and nutrient concentration. This mechanism may underlie photosynthetic acclimation to source:sink imbalances resulting from such diverse conditions as elevated CO2, low sink strength, low carbohydrate export, and nutrient limitation.     

Inhibition of photosynthesis by intracellular carbonic anhydrase in microalgae under excess concentrations of CO2

When cells of Chlorococcum littorale that had been grown in air (air-grown cells) were transferred to extremely high CO₂ concentrations (>20%), active photosynthesis resumed after a lag period which lasted for 1–4 days. In contrast, C. littorale cells which had been grown in 5% CO₂ (5% CO₂-grown cells) could grow in 40% CO₂ without any lag period. When air-grown cells were transferred to 40% CO₂, the quantum efficiency of PS II (Φ_II) decreased greatly, while no decrease in Φ_II was apparent when the 5% CO₂-grown cells were transferred to 40% CO₂. In contrast to air-grown cells, 5% CO₂-grown cells showed neither extracellular nor intracellular carbonic anhydrase (CA) activity. Upon the acclimation of 5% CO₂-grown cells to air, photosynthetic susceptibility to 40% CO₂ was induced. This change was associated with the induction of CA. In addition, neither suppression of photosynthesis nor arrest of growth was apparent when ethoxyzolamide (EZA), a membrane-permeable inhibitor of CA, had been added before transferring air-grown cells of C. littorale to 40% CO₂. The intracellular pH value (pH_i) decreased from 7.0 to 6.4 when air-grown C. littorale cells were exposed to 40% CO₂ for 1–2 h, but no such decrease in pH_i was apparent in the presence of EZA. Both air- and 5% CO₂-grown cells of Chlorella sp. UK001, which was also resistant to extremely high CO₂ concentrations, grew in 40% CO₂ without any lag period. The activity of CA was much lower in air-grown cells of this alga than those in air-grown C. littorale cells. These results prompt us to conclude that intracellular CA caused intracellular acidification and hence inhibition of photosynthetic carbon fixation when air-grown C. littorale cells were exposed to excess concentrations of CO₂. No such harmful effect of intracellular CA was observed in Chlorella sp. UK001 cells. This revised version was published online in June 2006 with corrections to the Cover Date.     

柚树叶片CO2驯化的光合参数变化

柚树(Citrus grandis)幼树生长在砂和磋石的生长介质,每周供给0.05mmol P(正常P,P)和0．1mmol P(高磷,2P)的营养液．植株分别生长在空气CO2分压(约39Pa)和倍增CO2分压(81±5Pa)下45d,利用CI-301PS(CID,Inc)光合作用测定系统在较高光强(1150μmol·m^-2·s^-1)下测定叶片光合速率并得出的Pn-Pi关系曲线和在较高CO2分压(PCO2,56Pa)下得出Pn-PAR关系曲线计算有关光合参数。结果表明,大气CO2分压下2P植株最大光合速率较P植株高13．3％,倍增CO2分压下,无论P或2P植株最大光合速率较大气CO2分压下相应植株低,但在倍增CO2分压下2P植株较P植株高,且2P植株有较P植株高的表观量子产率和光能利用效率(P<0．05),但并不改变г^*、Rd和Rubisco羧化速率(Vc)和氧速率的比率(P>0．05)在大气CO2分压下2P植株的Vcmax和Jmax较P植株分别高83％和12．5％,在倍增CO2分压下2P植株的Vcmax和Jmax均较P植株高,柚树在高CO2驯化中改变叶N在Rubisco和捕光组分分配系数,但不改变叶N在光合电子传递链的分配系数,结果表明,增加P供给可以促进高CO2分压下光合碳循环中P的周转,提高倍增CO2分压下植株的光合速率,调节柚树叶片的CO2驯化的光合参数。     

Effect of dissolved inorganic carbon on oxygen evolution and uptake by Chlamydomonas reinhardtii suspensions adapted to ambient and CO2-enriched air

Mass spectrometric measurements of ¹⁶O₂ and ¹⁸O₂ isotopes were used to compare the rates of gross O₂ evolution (E₀), O₂ uptake (U₀) and net O₂ evolution (NET) in relation to different concentrations of dissolved inorganic carbon (DIC) by Chlamydomonas reinhardtii cells grown in air (air-grown), in air enriched with 5% CO₂ (CO₂-grown) and by cells grown in 5% CO₂ and then adapted to air for 6h (air-adapted).At a photon fluence rate (PFR) saturating for photosynthesis (700 mol photons m^-2 s^-1), pH=7.0 and 28°C, U₀ equalled E₀ at the DIC compensation point which was 10M DIC for CO₂-grown and zero for air-grown cells. Both E₀ and U₀ were strongly dependent on DIC and reached DIC saturation at 480 M and 70 M for CO₂-grown and air-grown algae respectively. U₀ increased from DIC compensation to DIC saturation. The U₀ values were about 40 (CO₂-grown), 165 (air-adapted) and 60 mol O₂ mg Chl^-1 h^-1 (air-grown). Above DIC compensation the U₀/E₀ ratios of air-adapted and air-grown algae were always higher than those of CO₂-grown cells. These differences in O₂ exchange between CO₂- and air-grown algae seem to be inducable since air-adapted algae respond similarly to air-grown cells.For all algae, the rates of dark respiratory O₂ uptake measured 5 min after darkening were considerably lower than the rates of O₂ uptake just before darkening. The contribution of dark respiration, photorespiration and the Mehler reaction to U₀ is discussed and the energy requirement of the inducable CO₂/HCO₃ ^- concentrating mechanism present in air-adapted and air-grown C. reinhardtii cells is considered.Abbreviations DIC dissolved inorganic carbon - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - E₀ rate of photosynthetic gross O₂ evolution - PCO photosynthetic carbon oxidation - PFR photon fluence rate - PS I photosystem I - PS II photosystem II - U₀ rate of O₂ uptake in the light - MS mass spectrometer     

Photosynthetic Acclimation in Pea and Soybean to High Atmospheric CO2 Partial Pressure

Nonnodulated pea (Pisum sativum L. cv Frosty) and soybean (Glycine max [L.] Merr. cv Wye) plants were grown under artificial lights from germination with ample nutrients, 600 [mu]mol photons m-2 s-1, and either 34 to 36 (control) or 64 to 68 Pa (enriched) CO2. For soybean, pod removal and whole-plant shading treatments were used to alter the source-sink balance and carbohydrate status of the plants. Growth of both species was substantially increased by CO2 enrichment despite some down-regulation of photosynthesis rate per unit leaf area ("acclimation"). Acclimation was observed in young pea leaves but not old and in old soybean leaves but not young. Acclimation was neither evident in quantum yield nor was it related to triose phosphate limitation of net photosynthesis. A correlation between levels of starch and sugars in the leaf and the amount of acclimation was apparent but was loose and only weakly related to the source-sink balance of the plant. A consistent feature of acclimation was reduced ribulose bisphosphate carboxylase (RuBPCase) content, although in vivo RuBPCase activity was not necessarily diminished by elevated growth CO2 owing to increased percentage of activation of the enzyme. A proposal is discussed that the complexity of photosynthetic acclimation responses to elevated CO2 is as an expression of re-optimization of deployment of within-plant resources at three levels of competition.     

厚壁毛竹光合作用对CO_2浓度倍增的短期响应

采用Li-6400P光合测定仪对比测定了大气CO2浓度和短期CO2浓度倍增下不同季节厚壁毛竹的光合特性,结果表明:CO2浓度加倍促使最大净光合速率、净光合速率、水分利用率、光合量子效率和光饱和点升高,年平均增幅分别为62.79%、48.74%、94.41%、8.70%和16.67%;CO2浓度加倍促使蒸腾速率、暗呼吸速率和光补偿点下降,年平均降幅分别为17.60%、37.25%和40.50%。不同季节厚壁毛竹光合生理特性参数在CO2浓度加倍后的增加幅度或降低幅度与叶片生理活性和气候变化密切相关。CO2浓度的倍增并未明显改变厚壁毛竹光合特性的季节变化规律,除光补偿点外,其它光合参数的季节大小顺序仍与大气CO2浓度下的相同。厚壁毛竹光合作用对短期CO2浓度升高的响应特征与C3植物光合作用对短期CO2浓度升高响应的普遍规律相符。     

Changes in carboxysome structure and grouping and in photosynthetic affinity for inorganic carbon in Anabaena strain PCC 7119 (Cyanophyta) in response to modification of CO2 and Na+ supply

In ANABAENA: PCC 7119 a 4-fold decrease in the value of the apparent photosynthetic affinity for external inorganic carbon [K1/2 (Ci)] occurred between 9 and 12 h after the transfer from high-CO2 (2% CO2-enriched air) to air-growing conditions. A slight increase in carboxysome frequency occurred, but during this transition their appearance and distribution remained unchanged. ANABAENA: PCC 7119 did not improve its K1/2 (Ci) beyond the above cited level of acclimation neither by culturing the cyanobacteria in Na+-deficient medium in air nor by aeration with CO2-depleted air. In air-grown cultures, Na+ deficiency induced a large increase in carboxysome frequency and an alteration of their appearance: the greatest proportion were electron-dense whereas this type constituted a minority in high-CO2 and in air, Na+-sufficient conditions. It also induced major changes in carboxysome distribution, whereby more than 60% were grouped, compared with only 10% in high-CO2 and in air, Na+-sufficient conditions. These changes in carboxysome expression were extremely rapid, occurring mainly during the first 2 h.     

Regulation of energy balance in photosystems in response to changes in CO2 concentrations and light intensities during growth in extremely-high-CO2-tolerant green microalgae

Regulation of energy balance in photosystems in response to extremely-high-CO2 (40%) and low-CO2 (0.04%) stress was studied in extremely-high-CO2-tolerant green microalgae, Chlorococcum littorale and Chlorella sp. UK001. To investigate the energy input process, we assessed an F714/F685-ratio in a 77K fluorescence emission spectrum induced by 440-nm excitation in intact cells, which represents a ratio of fluorescence intensities derived from light-harvesting chlorophyll complexes in PSI and PSII. The F714/F685-ratio increased in several days after transferring C. littorale cells from air to 40% CO2, from 3% to 40% CO2 and from 3% to air. In all cases, the increase in the F714/F685-ratio was observed in high cell density culture, but no or a little increase was apparent in sparse cell density culture, when these cultures were illuminated at 250 micromol photon m-2 s-1. Even in the sparse culture, however, a similar increase in the F714/F685-ratio was observed when C. littorale cells were transferred from 3% to 40% CO2 at 20 micromol photon m-2 s-1. The cell density did not affect the F714/F685-ratio when CO2 concentration was kept at 3%. The activity of PSI electron (e-) transport was much higher in 40% CO2-grown cells than in 3% CO2-grown cells irrespective of the cell density during the culture, whereas the difference in PSII activity between them was small. The PSI activity at high cell density was higher also in air-grown cells than that in 3% CO2-grown cells. In both dense and sparse culture, 40% CO2-grown cells and air-grown cells showed higher relative quantum yield of PSI in the presence of DCMU than 3% CO2-grown cells, suggesting an increase in cyclic electron flow around PSI. Likewise, the increase in the F714/F685-ratio in response to the transfer to 40% CO2 was observed also in another extremely-high-CO2-tolerant alga, Chlorella sp. UK001. The possible role of the increases in the F714/F685-ratio, PSI/PSII activity ratio and cyclic e- transport activity in extremely-high-CO2 acclimation is discussed in comparison with low-CO2 acclimation.     

The potential role of CO2 in initiation and maintenance of estivation in the land snail Helix lucorum

Elevated CO(2) levels are hypothesized to play a role in the initiation and maintenance of estivation in snails through disturbances of acid-base status. The aim of our study was to identify the ambient CO(2) threshold that induces disturbances in acid-base status in the air-breathing land snail Helix lucorum. Acid-base parameters were determined in the hemolymph of snails acclimated to 0.5%, 1%, 2%, 4%, and 8% CO(2) in air for 20 d. In addition, we evaluated the effects of long-term acclimation on metabolic rate and on levels of D-lactate dehydrogenase activity (D-LDH) and of D-lactate in snails after 20 d of exposure to increased CO(2) levels. Helix lucorum proved to be unable to compensate for a decrease in extracellular pH (pH(e)) when acclimated to levels higher than 1% CO(2) in air. The rate of oxygen consumption started to decrease when snails were acclimated to 0.5% CO(2) in air. However, there was no correlation between the drops in pH(e) and in metabolic rate. Long-term acclimation to elevated CO(2) levels induced an increase in the activity of D-LDH with a concomitant accumulation of D-lactate in tissues. This indicates that long-term acclimation to elevated ambient CO(2) levels could reduce the aerobic capacity of land snails and trigger expression of anaerobic pathways of ATP turnover. The threshold levels of ambient CO(2) that induce changes in acid-base status and elicit metabolic depression in adult land snails H. lucorum are higher than the future atmospheric levels that are expected to result from human use of fossil energy resources.     

Photosynthetic acclimation to elevated CO2 in a sunflower canopy

Sunflower canopies were grown in mesocosom gas exchange chambers at ambient and elevated CO2 concentrations (360 and 700 ppm) and leaf photosynthetic capacities measured at several depths within each canopy. Elevated [CO2] had little effect on whole-canopy photosynthetic capacity and total leaf area, but had marked effects on the distribution of photosynthetic capacity and leaf area within the canopy. Elevated [CO2] did not significantly reduce the photosynthetic capacities per unit leaf area of young leaves at the top of the canopy, but it did reduce the photosynthetic capacities of older leaves by as much as 40%. This effect was not dependent on the canopy light environment since elevated [CO2] also reduced the photosynthetic capacities of older leaves exposed to full sun on the south edge of the canopy. In addition to the effects on leaf photosynthetic capacity, elevated [CO2] shifted the distribution of leaf area within the canopy so that more leaf area was concentrated near the top of the canopy. This change resulted in as much as a 50% reduction in photon flux density in the upper portions of the elevated [CO2] canopy relative to the ambient [CO2] canopy, even though there was no significant difference in the total canopy leaf area. This reduction in PFD appeared to account for leaf carbohydrate contents that were actually lower for many of the shaded leaves in the elevated as opposed to the ambient [CO2] canopy. Photosynthetic capacities were not significantly correlated with any of the individual leaf carbohydrate contents. However, there was a strong negative correlation between photosynthetic capacity and the ratio of hexose sugars to sucrose, consistent with the hypothesis that sucrose cycling is a component of the biochemical signalling pathway controlling photosynthetic acclimation to elevated [CO2].     

Maintenance of C sinks sustains enhanced C assimilation during long-term exposure to elevated [CO2] in Mojave Desert shrubs

During the first few years of elevated atmospheric [CO(2)] treatment at the Nevada Desert FACE Facility, photosynthetic downregulation was observed in desert shrubs grown under elevated [CO(2)], especially under relatively wet environmental conditions. Nonetheless, those plants maintained increased A (sat) (photosynthetic performance at saturating light and treatment [CO(2)]) under wet conditions, but to a much lesser extent under dry conditions. To determine if plants continued to downregulate during long-term exposure to elevated [CO(2)], responses of photosynthesis to elevated [CO(2)] were examined in two dominant Mojave Desert shrubs, the evergreen Larrea tridentata and the drought-deciduous Ambrosia dumosa, during the eighth full growing season of elevated [CO(2)] treatment at the NDFF. A comprehensive suite of physiological processes were collected. Furthermore, we used C labeling of air to assess carbon allocation and partitioning as measures of C sink activity. Results show that elevated [CO(2)] enhanced photosynthetic performance and plant water status in Larrea, especially during periods of environmental stress, but not in Ambrosia. δ(13)C analyses indicate that Larrea under elevated [CO(2)] allocated a greater proportion of newly assimilated C to C sinks than Ambrosia. Maintenance by Larrea of C sinks during the dry season partially explained the reduced [CO(2)] effect on leaf carbohydrate content during summer, which in turn lessened carbohydrate build-up and feedback inhibition of photosynthesis. δ(13)C results also showed that in a year when plant growth reached the highest rates in 5 years, 4% (Larrea) and 7% (Ambrosia) of C in newly emerging organs were remobilized from C that was assimilated and stored for at least 2 years prior to the current study. Thus, after 8 years of continuous exposure to elevated [CO(2)], both desert perennials maintained their photosynthetic capacities under elevated [CO(2)]. We conclude that C storage, remobilization, and partitioning influence the responsiveness of these desert shrubs during long-term exposure to elevated [CO(2)].