C(4) Acid Metabolism and Dark CO(2) Fixation in a Submersed Aquatic Macrophyte (Hydrilla verticillata)

The CO₂ compensation point of the submersed aquatic macrophyte Hydrilla verticillata varied from high (above 50 microliters per liter) to low (10 to 25 microliters per liter) values, depending on the growth conditions. Plants from the lake in winter or after incubation in an 11 C/9-hour photoperiod had high values, whereas summer plants or those incubated in a 27 C/14-hour photoperiod had low values. The plants with low CO₂ compensation points exhibited dark ¹⁴CO₂ fixation rates that were up to 30% of the light fixation rates. This fixation reduced respiratory CO₂ loss, but did not result in a net uptake of CO₂ at night. The low compensation point plants also showed diurnal fluctuations in titratable acid, such as occur in Crassulacean acid metabolism plants. However, dark fixation and diurnal acid fluctuations were negligible in Hydrilla plants with high CO₂ compensation points.     

Seasonal Effects in the Hormonal Control of Growth in the Submerged Aquatic Macrophyte Ceratophyllum demersum

The season dependent changes in growth response to treatment with auxin or gibberellin were studied in the aquatic macrophyte Ceratophyllum demersum. Control plants show, under experimental conditions, a maximum growth in length in February. In the same period most of the lateral buds appear. Growth of the lateral buds occurs later. IAA causes a stimulation of growth in length from late November or December until February, in concentrations of 10^?9M and 10^?6M. There is almost no stimulation of lateral bud formation by IAA, only a slight increase from late November until December occurs by the lowest concentrations used. The highest concentration used, 10^?4M, is in most cases supraoptimal for lateral bud formation; only when plants become dormant (August), this high dose may stimulate the process. GA₃, in concentrations of 10^?9, 10^?6 or 10^?4M, exhibits a dose dependent increase of the response with respect to growth in length and lateral bud formation. The response occurs earlier than that for IAA: already early in November, or December, until February. Growth of the lateral buds may show only a slight stimulation by IAA as well as GA in winter. From February until April all GA concentrations used could cause a small increase of the growth of sprouts. In the case of IAA, however, only the lowest concentration could cause a small increase.     

Localized Tetrazolium Reduction in Relation to N2 Fixation, CO2 Fixation, and H2 Uptake in Aquatic Filamentous Cyanobacteria

The aquatic filamentous cyanobacteria Anabaena oscillarioides and Trichodesmium sp. reveal specific cellular regions of tetrazolium salt reduction. The effects of localized reduction of five tetrazolium salts on N₂ fixation (acetylene reduction), ¹⁴CO₂ fixation, and ³H₂ utilization were examined. During short-term (within 30 min) exposures in A. oscillarioides, salt reduction in heterocysts occurred simultaneously with inhibition of acetylene reduction. Conversely, when salts failed to either penetrate or be reduced in heterocysts, no inhibition of acetylene reduction occurred. When salts were rapidly reduced in vegetative cells, ¹⁴CO₂ fixation and ³H₂ utilization rates decreased, whereas salts exclusively reduced in heterocysts were not linked to blockage of these processes. In the nonheterocystous genus Trichodesmium, the deposition of reduced 2,3,5-triphenyl-2-tetrazolium chloride (TTC) in the internal cores of trichomes occurs simultaneously with a lowering of acetylene reduction rates. Since TTC deposition in heterocysts of A. oscillarioides occurs contemporaneously with inhibition of acetylene reduction, we conclude that the cellular reduction of this salt is of use in locating potential N₂-fixing sites in cyanobacteria. The possible applications and problems associated with interpreting localized reduction of tetrazolium salts in cyanobacteria are presented.     

C3 and CAM Photosynthetic Characteristics of the Submerged Aquatic Macrophyte Littorella uniflora: Regulation of Leaf Internal CO2 Supply in Response to Variation in Rooting Substrate Inorganic Carbon Concentration

The relationships between CO₂ concentrating mechanisms, photosyntheticefficiency and inorganic carbon supply have been investigatedfor the aquatic macrophyte Littorella uniflora. Plants wereobtained from Esthwaite Water or a local reservoir, with thelatter plants transplanted into a range of sediment types toalter CO₂ supply around the roots. Free CO₂ in sediment-interstitial-waterranged from 1–01 mol m^–3 (Esthwaite), 0.79 mol m^–3(peat), 0.32 mol m^–3 (silt) and 0–17 mol m^–3(sand), with plants maintained under PAR of 40 µmol m^–2s^–1. A comparison of gross morphology of plants maintained underthese conditions showed that the peat-grown plants with highsediment CO₂ had larger leaf fresh weight (0–69 g) andtotal surface area (223 cm² g^–1 fr. wt. including lacunalsurface area) than the sand-grown plants (0.21 g and 196 cm²g^–1 fr. wt. respectively). Root fresh weights were similarfor all treatments. In contrast, leaf internal CO₂ concentration[CO₂], was highest in the sand-grown plants (2–69 molm^–3, corresponding to 6.5% CO₂ in air) and lowest inthe Esthwaite plants (1–08 mol m^–3). Expressionof CAM in transplants was also greatest in the low CO₂ regime,with H⁺ (measured as dawn-dusk titratable acidity) of 50µmolg^– fr. wt., similar to Esthwaite plants in natural sediment.Assuming typical CAM stoichiometry, decarboxylation of malatecould account largely for the measured [CO₂]₁ and would makea major contribution to daytime CO₂ fixation in vivo. A range of leaf sections (0–2, 1–0, 5–0 and17–0 mm) was used to evaluate diffusion limitation andto select a suitable size for comparative studies of photosyntheticO₂ evolution. The longer leaf sections (17.0 mm), which weresealed and included the leaf tip, were diffusion-limited witha linear response to incremental addition of CO₂ and 1–0mol m^–3 exogenous CO₂ was required to saturate photosynthesis.Shorter leaf sections were less diffusion-limited, with thegreatest photosynthetic capacity (36 µmol O₂ g^–1 fr. wt. h^–1) obtainedfrom the 1.0 mm size and were not infiltrated by the incubatingmedium. Comparative studies with 1.0 mm sections from plants grown inthe different sediment types revealed that the photosyntheticcapacity of the sand-grown plants was greatest (45 µmolO₂ g^–1 fr. wt. h^–1) with a K_0.5 of 80 mmol m^–3.In terms of light response, saturation of photosynthesis intissue slices occurred at 850–1000 µmol m^–2s^–1 although light compensation points (6–11 µmolm^–2s^–1) and chlorophyll a: b ratios (1.3) were low.While CO₂ and PAR responses were obtained using varying numbersof sections with a constant fresh weight, the relationshipsbetween photosynthetic capacity and CO₂ supply or PAR were maintainedwhen the data were expressed on a chlorophyll basis. It is concludedthat under low PAR, CO₂ concentrating mechanisms interact inintact plants to maintain saturating CO₂ levels within leaflacunae, although the responses of the various components ofCO₂ supply to PAR require further investigation. Key words: Key words-Uttorella uniflora, internal CO₂ concentration, crassulacean acid metabolism, root inorganic carbon supply, CO₂ concentrating mechanism     

Short-Term Fixation of 14Carbon by the Submerged Aquatic Angiosperm Potamogeton pectinatus

In ¹⁴C fixation experiments, 3-phosphoglyceric acid was thefirst product of carbon assimilation in the light in Potamogetonpectinatus. The pattern of early ¹⁴C-labelled compounds wasthe same over a range of pH values of the medium from 3.5 to8.1. Rates of ¹⁴C incorporation declined with increasing pHof the medium indicating that free CO₂ is the major exogenoussource of carbon for photosynthesis in Potamogeton pectinatus.     

Localized Tetrazolium Reduction in Relation to N(2) Fixation, CO(2) Fixation, and H(2) Uptake in Aquatic Filamentous Cyanobacteria

The aquatic filamentous cyanobacteria Anabaena oscillarioides and Trichodesmium sp. reveal specific cellular regions of tetrazolium salt reduction. The effects of localized reduction of five tetrazolium salts on N(2) fixation (acetylene reduction), CO(2) fixation, and H(2) utilization were examined. During short-term (within 30 min) exposures in A. oscillarioides, salt reduction in heterocysts occurred simultaneously with inhibition of acetylene reduction. Conversely, when salts failed to either penetrate or be reduced in heterocysts, no inhibition of acetylene reduction occurred. When salts were rapidly reduced in vegetative cells, CO(2) fixation and H(2) utilization rates decreased, whereas salts exclusively reduced in heterocysts were not linked to blockage of these processes. In the nonheterocystous genus Trichodesmium, the deposition of reduced 2,3,5-triphenyl-2-tetrazolium chloride (TTC) in the internal cores of trichomes occurs simultaneously with a lowering of acetylene reduction rates. Since TTC deposition in heterocysts of A. oscillarioides occurs contemporaneously with inhibition of acetylene reduction, we conclude that the cellular reduction of this salt is of use in locating potential N(2)-fixing sites in cyanobacteria. The possible applications and problems associated with interpreting localized reduction of tetrazolium salts in cyanobacteria are presented.     

Heterotrophic Fixation of CO2 in Soil

The occurrence of heterotrophic CO₂ fixation by soil microorganisms was tested in several mineral soils differing in pH and two artificial soils (a mixture of silica sand, alfalfa powder, and nutrient medium inoculated with a soil suspension). Soils were incubated at ambient (∼0.05 vol%) and elevated (∼5 vol%) CO₂ concentrations under aerobic conditions for up to 21 days. CO₂ fixation was detected using either a technique for determining the natural abundance of ¹³C or by measuring the distribution of labeled ¹⁴C-CO₂ in soil and bacteria. The effects of elevated CO₂ on microbial biomass (direct counts, chloroform fumigation extraction method), composition of microbial community (phospholipid fatty acids), microbial activity (respiration, dehydrogenase activity), and turnover rate were also measured. Heterotrophic CO₂ fixation was proven in all soils under study, being higher in neutral soils. The main portion of the fixed CO₂ (98–99%) was found in extracellular metabolites while only ∼1% CO₂ was incorporated into microbial cells. High CO₂ concentration always induced an increase in microbial activity, changes in the composition of the microbial community, and a decrease in microbial turnover. The results suggest that heterotrophic CO₂ fixation could be a widespread process in soils.     

Resistance to Drying in Aquatic Mollusks

Morphological, behavioral, physiological, and biochemical adaptations of marine and freshwater mollusks favoring their survival in conditions of drying were reviewed.     

沉水植物川蔓藻的生态学特征及其对环境变化的响应

川蔓藻是近海生态系统中重要的初级生产者,了解它的生态学特征对于河口海岸带的生态修复具有重要意义。本文结合我们在天津滨海湿地生态修复工程中对于川蔓藻的研究成果,从以下几个方面探讨了川蔓藻的生态学特征及其与环境的关系：川蔓藻对极端环境因子的适应;川蔓藻在资源分配上的繁殖策略;川蔓藻的矿质营养代谢以及对栖息地生物地球化学循环的影响;川蔓藻种群衰退的原因;川蔓藻在环境修复中的应用。其中着重讨论了川蔓藻对环境变化的响应,并结合我们的研究工作展望了它在环境工程与近岸水体修复中的应用潜力。     

CO(2) Fixation in Opuntia Roots

Nonautotrophic CO₂ metabolism in Opuntia echinocarpa roots was studied with techniques of manometry and radiometry. The roots were grown in a one-quarter strength nutrient solution for several days; the distal 2 cm was used for physiological studies. The roots assimilated significant quantities of ¹⁴CO₂ and appeared to show a crassulacean-type acid metabolism with respect to quality and quantity. Most of the ¹⁴C activity was associated with the distal portion of the elongating root indicating correlation with metabolic activity. The ¹⁴CO₂ assimilation was comparable to a crassulacean leaf succulent, but 3 times greater than that found for stem tissue of the same Opuntia species.

The rates of O₂ and CO₂ exchange and estimated CO₂ fixation were 180, 123, and 57 μl/g per hour. A respiratory quotient of 0.66 was found.

The products of ¹⁴CO₂ fixation were similar in most respects to reported experiments with leaf succulents. Equilibration of the predominant malic acid with isocitric, succinic, and fumaric acids was not evident. The latter observation was interpreted as metabolic isolation of the fixation products rather than poor citric acid cycle activity.

A rapid turnover of the fixed ¹⁴CO₂ was measured by following decarboxlyation kinetics and by product analysis after a postincubation period. The first order rate constant for the steady state release was 4.4 × 10⁻³ min⁻¹ with a half-time of 157.5 minutes. Amino acids decayed at a more rapid rate than organic acids.

     

微藻固定CO2研究进展

空气中CO2浓度升高所导致的温室效应已成为重大的环境问题,受到人们普遍关注.概述了高效固定CO2微藻藻种的筛选和培养方法,分析了微藻固定CO2的无机碳利用形式和浓缩机制,讨论了高效光生物反应器设计和运行目标,简要介绍了微藻(酶)-膜生物反应器集成新技术.并认为今后的研究方向主要是在进一步探索微藻固定CO2有关机理的基础上,构建高效固定CO2的转基因微藻,开发高效膜生物反应集成系统.     

Decomposition and CO2 Evolution from Standing Litter of the Emergent Macrophyte Erianthus giganteus

Abstract Decomposition of standing litter of the emergent macrophyte Erianthus giganteus (plumegrass) was quantified in a small freshwater wetland in Alabama, USA. Living green shoots of E. giganteus were tagged and periodically retrieved for determination of leaf and culm mass loss, litter-associated fungal biomass (ergosterol), and nitrogen and phosphorus concentrations. Laboratory studies were also conducted to examine the effects of plant litter moisture content and temperature on rates of CO₂ evolution from plant litter. Culm and leaf material lost 25 and 32% AFDM, respectively, during plant senescence and early litter decay. Fungal biomass, as determined by ergosterol concentrations, increased significantly in both leaf and culm litter during decomposition, with maximum biomass accounting for 3.7 and 6.7% of the total detrital weight in culm and leaf litter, respectively. Spatial differences in fungal biomass were observed along the culm axis, with upper regions of the culm accumulating significantly greater amounts of fungal mass than basal regions (p < 0.01, ANOVA). Rates of CO₂ evolution from both leaf and culm litter increased rapidly after wetting (0 to 76 μg CO₂−C g⁻¹ AFDM h⁻¹ within 5 min). In addition, rates of CO₂ evolution from water saturated culms increased exponentially as the temperature was increased from 10 to 30°C. These results provide evidence that considerable microbial colonization and mineralization of standing emergent macrophyte litter can occur before collapse of senescent shoot material to the water and sediment surface. Received: 5 December 1998; Accepted: 31 March 1999     

Enzymology of the Acetyl-CoA Pathway of CO2 Fixation

Abstract

We know of three routes that organisms have evolved to synthesize complex organic molecules from CO₂: the Calvin cycle. the reverse tricarboxylic acid cycle, and the reductive acetyl-CoA pathway. This review describes the enzymatic steps involved in the acetyl-CoA pathway, also called the Wood pathway, which is the major mechanism of CO₂ fixation under anaerobic conditions. The acetyl-CoA pathway is also able to form acetyl-CoA from carbon monoxide.

There are two parts to the acetyl-CoA pathway: (1) reduction of CO₂ to methyltetrahydrofolate (methyl-H₄folate) and (2) synthesis of acetyl-CoA from methyl-H, folate, a carboxyl donor such as CO or CO₂, and CoA. This pathway is unique in that the major intermediates are enzyme-bound and are often organometallic complexes. Our current understanding of the pathway is based on radioactive and stable isotope tracer studies, purification of the component enzymes (some extremely oxygen sensitive), and identification of the enzyme-bound intcrmediates by chromatographic, spectroscopic. and electrochemical techniques. This review describes the remarkable series of enzymatic steps involved in acetyl-CoA formation by this pathway that is a key component of the global carbon cycle.     

Non-Autotrophic CO2 Fixation and Drought Tolerance in Mosses

Cratoneuron filicinum, a drought-sensitive moss, and Tortularuralis, a drought-tolerant moss, fix CO₂ non-autotrophicallyat a rate of about 1.2 and 2.2 µmol h^–1 g^–1dry wt. respectively. During drying, T. ruralis fixes CO₂ atan undiminished rate until the tissue loses about 60% of theinitial fresh weight. Thereafter, CO₂ fixation rapidly declinesto zero. Dark CO₂ fixation by C.filicinum declines steadilyduring the dehydration period. On rehydration, dark CO₂ fixationis resumed immediately in T. ruralis but not in C.filicinum.When dried T. ruralis is equilibrated with an atmosphere ofnearly 100% relative humidity, its weight increases to about40% of the original fresh weight and dark CO₂ fixation resumesat a rate about 60% of the fresh moss. In C.filicinum thereis only a small increase in weight and little CO₂ fixation inthe dark. The non-autotrophically fixed carbon, in both mossesstudied, is incorporated into amino acids (more than 60% ofthe total, mainly into aspartate, alanine and glutamate) andorganic acids (less than 40% of the total, mainly into malate).It is suggested that on rehydration immediate availability ofNADPH, known to be produced by transhydrogenation from NADHduring dark CO₂ fixation, may be an important factor in therepair of drought-induced cellular damage by reductive biosynthesisof membrane components and other cellular constituents. Key words: Mosses, Dehydration, Rehydration, Dark CO₂ fixation, Amino acids, Organic acids, NADPH, Drought tolerance.     

Resistance to CO2 diffusion in cuticular membranes of amphibious plants and the implication for CO2 acquisition

Cuticular membranes (CMs) were isolated from leaves of amphibious and submerged plants and their CO2 resistances were determined as a contribution to establish quantitatively the series of resistances met by CO2 diffusing from bulk water to the chloroplasts of submerged leaves. The isolation was performed enzymatically; permeabilities were determined and converted to resistances. The range of permeance values was 3 to 43 x 10(-6) m s(-1) corresponding to resistance values of 23 to 295 x 10(3) s m(-1), i.e. of the same order of magnitude as boundary layer resistances. The sum of boundary layer, CM, leaf cell and carboxylation resistances could be contained within the total diffusion resistance as determined from the photosynthetic CO2 affinity of the leaf. From the same species, the aerial leaf CM resistance was always higher than the aquatic leaf CM resistance. In a terrestrial plant, the CM resistance to CO2 diffusion was found lower in leaves developed submerged.     

Partitioning of Transfer and Carboxylation Components of Intracellular Resistance to Photosynthetic CO2 Fixation: A Critical Analysis of the Methods Used

The partitioning of intracellular resistance to CO₂ transferin two components: mesophyll (r_m.) and carboxylation (r_x) resistances,is based upon the combination of two sub-models: one encompassingtransfer processes and the second, the carboxylation system.All the determinations derived from this method yield a highr_m/r_x ratio. it is demonstrated in low oxygen conditions, whateverthe model used, that this conclusion is highly-dependent uponthe form of the equation used at the carboxylation level. Thepossible influence of O₂ concentration on the r_m/r_x ratio isdiscussed. Starting from a Rabinowitch model (rectangular hyperbola), someof the conditions necessary to yield lower r_m/r_x ratio are considered.It is shown that the most relevant factor when modelling theCalvin cycle is the number of limiting pools (enzymes or cofactors)rather than the number of limiting reactions.     

Heterophylly in some Species of Callitriche, with especial reference to Callitriche intermedia

Callitriche stagnalis, C. obtusangula, and C. intermedia, inthat order, were found to show a progressively greater variationin leaf form. Axes of C. intermedia bearing crowns of ovateleaves were submerged under various light and temperature conditionsand the growth rates studied in relation to the form of theleaves produced. Leaf movements were also studied in connexionwith the growth of such submerged shoots and with the generalquestion of heterophylly.     

Non-autotrophic CO2 Fixation during Shoot Formation in Tobacco Callus

Non-autotrophic carbon fixation has been studied during growthof tobacco callus cultured in dark under shoot-forming (SF)and non-shoot-forming (NSF) conditions. The enzymes involvedin malate metabolism—phosphoenolpyruvate carboxylase,malic dehydrogenase, glutamic-oxalacetic transaminase, and malicenzyme—increased sharply during the first 4 d of cultureparticularly in SF tissue. The activities of the enzymes studiedwere considerably greater in SF than in NSF tissue. There wasa dramatic increase in malate content in SF tissue during thefirst 4 d of culture. Subsequently malate was rapidly depletedduring the time of organogenesis. In NSF tissue there was acontinuous build-up of malate content throughout the cultureperiod. We suggest that malate derived from dark fixation ofCO₂ plays differing roles in NSF (callus) and SF tissues. Inthe former, malate acts primarily as an osmotic solute regulating,at least in part, cell expansion between successive cell divisions.In shoot-forming tissue, on the other hand, malate preferentiallyprovides NADPH for reductive biosynthesis.     

Green Light Drives CO2 Fixation Deep within Leaves

Maximal ^l4CO₂-fixation in spinach occurs in the middle of thepalisade mesophyll [Nishio et al. (1993) Plant Cell 5: 953],however, ninety percent of the blue and red light is attenuatedin the upper twenty percent of a spinach leaf [Cui et al. (1991)Plant Cell Environ. 14: 493]. In this report, we showed thatgreen light drives ¹⁴CO₂-fixation deep within spinach leavescompared to red and blue light. Blue light caused fixation mainlyin the palisade mesophyll of the leaf, whereas red light drovefixation slightly deeper into the leaf than did blue light.¹⁴CO₂-fixation measured under green light resulted in less fixationin the upper epidermal layer (guard cells) and upper most palisademesophyll compared to red and blue light, but led to more fixationdeeper in the leaf than that caused by either red or blue light.Saturating white, red, or green light resulted in similar maximal¹⁴CO₂-fixation rates, whereas under the highest irradiance ofblue light given, carbon fixation was not saturated, but itasymptotically approached the maximal ¹⁴CO₂-fixation rates attainedunder the other types of light. The importance of green lightin photosynthesis is discussed. ¹Supported in part by grants from Competitive Research GrantsOffice, U.S. Department of Agriculture (Nos. 91-37100-6672 and93-37100-8855).