Photosynthetic carbon dioxide assimilation by Rhodospirillum rubrum

Summary Although Rhodospirillum rubrum, grown photoheterotrophically on malate, assimilates carbon dioxide less rapidly than it does when grown autotrophically, the difference is less marked than previously suggested.The rate of photoassimilation of carbon dioxide varies during batch culture on malate, reaching a maximum at about mid-exponential phase. It also varies with density and growth rate in a turbidostat continuous-flow culture on malate and increases with decreasing growth rate in a chemostat continuous-flow culture growing with limiting malate concentrations.The changing rates of carbon dioxide photoassimilation during photoheterotrophic growth under the various conditions are paralleled by changing activities of ribulose diphosphate carboxylase.Under conditions of maximum carbon dioxide fixation the rate by photoheterotrophic cultures approaches that shown by the bacterium growing autotrophically and is assimilated eight to ten times more slowly than is malate in chemostat cultures.The rate of carbon dioxide fixation also increases to that shown by autotrophic cells when photoheterotrophic cultures are deprived of malate, but without subjecting them to the conditions required for autotrophic growth.     

Ferredoxin-dependent carbon assimilation in Rhodospirillum rubrum

Summary Evidence has been presented that a soluble fraction from R. rubrum cells contains two new primary carboxylation reactions which depend on the reducing power of ferredoxin: (a) pyruvate synthase which brings about a synthesis of pyruvate from acetyl-CoA and CO₂ and (b) -ketoglutarate synthase which brings about a synthesis of -ketoglutarate from succinyl-CoA and CO₂. The soluble fraction of R. rubrum cells contains also a series of other enzymes which, together with the ferredoxin-dependent enzymes, constitutes a reductive carboxylic acid cycle—a new cyclic pathway for CO₂ assimilation that was first found in the photosynthetic bacterium, Chlorobium thiosulfatophilum.Dedicated to C. B. van Niel on the occasion of his 70th birthday.Aided by grants from the National Institute of General Medical Sciences, Office of Naval Research and the National Science Foundation.     

A model of carbon dioxide assimilation in Chlamydomonas reinhardii

A simple model of photosynthetic CO₂ assimilation in Chlamydomonas has been developed in order to evaluate whether a CO₂-concentrating system could explain the photosynthetic characteristics of this alga (high apparent affinity for CO₂, low photorespiration, little O₂ inhibition of photosynthesis, and low CO₂ compensation concentration). Similarly, the model was developed to evaluate whether the proposed defects in the CO₂-concentrating system of two Chlamydomonas mutants were consistent with their observed photosynthetic characteristics. The model treats a Chlamydomonas cell as a single compartment with two carbon inputs: passive diffusion of CO₂, and active transport of HCO ₃ ^- . Internal inorganic carbon was considered to have two potential fates: assimilation to fixed carbon via ribulose 1,5-bisphosphate carboxylase-oxygenase or exiting the cell by either passive CO₂ diffusion or reversal of HCO ₃ ^- transport. Published values for kinetic parameters were used where possible. The model accurately reproduced the CO₂-response curves of photosynthesis for wild-type Chlamydomonas, the two mutants defective in the CO₂-concentrating system, and a double mutant constructed by crossing these two mutants. The model also predicts steady-state internal inorganic-carbon concentrations in reasonable agreement with measured values in all four cases. Carbon dioxide compensation concentrations for wild-type Chlamydomonas were accurately predicted by the model and those predicted for the mutants were in qualitative agreement with measured values. The model also allowed calculation of approximate energy costs of the CO₂-concentrating system. These calculations indicate that the system may be no more energy-costly than C₄ photosynthesis.Abbreviations Chl chlorophyll - RuBPC/O ribulose 1,5-bisphosphate carboxylase-oxygenase - CA carbonic anhydrase     

The pathway of carbon dioxide fixation in crassulacean plants

Combined gas chromatography-mass spectrometry of malic acid derivatives has been used to show unequivocally that malic acid, synthesized during active acid accumulation in the dark by Kalanchoë daigremontiana Hammet et Perrier in the presence of ¹³CO₂ is produced by a pathway involving a single carboxylation. The significance of the finding that crassulacean malate synthesized in the dark and in the presence of ¹⁴CO₂ often contains 66% of the total carboxyl label in carbon atom 4, which has previously been taken to indicate the operation of a double carboxylation pathway or has been dismissed as an artefact, is discussed.     

Spherical subunits in photosynthetic membranes of two cyanophyceae and the bacterium rhodospirillum rubrum

Summary The photosynthetic lamellae of Oscillatoria amoena and Pseudanabaena catenata appear as a two-dimensional packing of spherical units following permanganate fixation, ultrathin sectioning and high-resolution electron micrography. The usual three-layered appearance is explained as an optical artifact. Quite similar patterns were observed in chromatophore membranes of Rhodospirillum rubrum. Also, the cytoplasmic membrane of R. rubrum shows an identical pattern.Indication is presented that thylakoids of Cyanophyceae can arise as invaginations of the cytoplasmic membrane.The localization of chloroplast lipids in these organisms and the similarity of spherical units and quantasomes is discussed.     

Regulation of methanol oxidation and carbon dioxide fixation in Xanthobacter strain 25a grown in continuous culture

The regulation of C₁-metabolism in Xanthobacter strain 25a was studied during growth of the organism on acetate, formate and methanol in chemostat cultures. No activity of methanol dehydrogenase (MDH), formate dehydrogenase (FDS) or ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisC/O) could be detected in cells grown on acetate alone over a range of dilution rates tested. Addition of methanol or formate to the feed resulted in the immediate induction of MDH and FDH and complete utilization (D=0.10 h^-1) of acetate and the C ₁-substrates. The activities of these enzymes rapidly dropped at the higher growth rates, which suggests that their synthesis is further controlled via repression by heterotrophic substrates such as acetate. Synthesis of RuBisC/O already occurred at low methanol concentrations in the feed, resulting in additive growth yields on acetate/methanol mixtures. The energy generated in the oxidation of formate initially allowed an increased assimilation of acetate (and a decreased dissimilation), resulting in enhanced growth yields on the mixture. RuBisC/O activity could only be detected at the higher formate/acetate ratios in the feed. The data suggest that synthesis of RuBisC/O and CO₂ fixation via the Calvin cycle in Xanthobacter strain 25 a is controlled via a (de)repression mechanism, as is the case in other facultatively autotrophic bacteria. Autotrophic CO₂ fixation only occurs under conditions with a diminished supply of heterotrophic carbon sources and a sufficiently high availability of suitable energy sources. The latter point is further supported by the clearly more pronounced derepressing effect exerted by methanol compared to formate.Abbreviations FDH formate dehydrogenase - FBPase fructose-1,6-bisphosphatase - ICDH isocitrate dehydrogenase - MDH methanol dehydrogenase - PQQ pyrrolo quinoline quinone - PRK phosphoribulokinase - RuBisC/O ribulose-1,5-bisphosphate carboxylase/oxygenase - RuMP ribulose monophosphate - TCA tricarboxylic acid cycle     

Distribution of phosphatidylethanolamine in the lipid bilayer of chromatophores of the photosynthetic bacterium rhodospirillum rubrum

The distribution of phosphatidylethanolamine in the two lipid layers of chromatophores ofRhodospirillum rubrum has been analysed by chemical modification of phosphatidylethanolamine (PE) with trinitrobenzenesulfonic acid (TNBA) at low temperatures. Around 45±1% of the total phosphatidylethanolamine is labelled by this procedure independent on chromatophore purity, vesicle size, action of proteases and growth state of the cells. This demonstrates a complete modification of the accessible phosphatidylethanolamine and an asymmetric distribution of phosphatidylethanolamine, with 45% of the phosphatidylethanolamine in the outer part of the bilayer.Abbreviations TNBA 2,4,6 trinitrobenzenesulfonic acid - PE phosphatidylethanolamine - PMS phenazinmethosulfate     

自养微生物同化CO_2的分子生态研究及同化碳在土壤中的转化

大气中CO2浓度持续升高和全球气候变暖是亟待解决的重大环境问题。自养微生物在环境中广泛分布,能直接参与CO2的同化,因此研究自养微生物同化CO2的分子生态学机制具有重大的科学意义。以往对自养微生物的研究多针对基因组DNA,从DNA水平揭示了不同生态系统中碳同化自养微生物的种群结构和多样性,但这些微生物在生态系统中的具体功能有待进一步的研究。近年来,随着转录组学研究技术和稳定同位素探针技术(SIP)的发展,自养微生物同化CO2的生态机理研究不断深入,这些研究明确揭示了碳同化自养微生物是河流、湖泊和海洋生态系统中CO2固定作用的驱动者,并新发现了一些具有CO2同化功能的微生物群落。基于国内外有关研究进展,从DNA和RNA水平上对自养微生物同化CO2的分子机理以及稳定同位素探针技术(SIP)在碳同化微生物研究中的应用进行了分析和总结,初步展望了RNA-SIP技术在陆地生态系统碳同化微生物分子生态学研究中的前景。同时,探讨了陆地生态系统同化碳的转化和稳定性机理,以期为深入了解生态系统碳循环过程和应对气候变化提供理论依据。     

Effect of toxaphene on carbon dioxide assimilation and translocation in avena secale

In susceptible oat, toxaphene inhibits photosynthetic electron flow and concomitant ATP synthesis. Although the rate of ¹⁴CO₂ assimilation is apparently not affected markedly there is an increase in dry weight of leaves contacting the pesticide. The labelling patterns in leaf sections exposed to ¹⁴CO₂ are similar for both toxaphene-treated and untreated seedlings. However, if given a period in darkness before extraction it is evident that assimilation products in leaf sections from toxaphene-treated leaves remain as small M, materials, including substantial amounts of sugars, whereas in untreated controls these were converted to polymeric materials. In toxaphene-treated seedlings the translocation of assimilation products to the roots is decreased and sucrose accumulates in the leaves.