Photoelectrochemical Power,Chemical Energy and Catalytic Activity for Organic Evolution on Natural Pyrite Interfaces

Natural pyrite (FeS₂) has frequently been discussed as a material involved in CO₂ fixation in presence of H₂S and as a possible catalyst for the origin of life. A straightforward chemical fixation of carbon dioxide as proposed by Wächtershäuser could not be verified from thermo-chemical equilibrium calculations by minimizing Gibb's Free Energy in the system C, O, H, S, Fe and appears unlikely dueto the experimentally encountered large overpotentials involved in CO₂ fixation. However, the hypothesis, by W. R. Edwards, that pyrite in shallow coastal waters may have been involved, canbe sustained. In this case, daily available photoelectrochemical power from FeS₂/Fe^2+/3+ interfaces could have made thedifference in combination with electrochemical processes, such ashydrogen insertion, and the solubilization of pyrite by the aminoacid cysteine to yield dissolved chemical energy. Periodical changes in energy supply could also have entrained primitive self-organization processes for organic-biological evolution.Natural samples from thirteen ore deposits have been investigatedphotoelectrochemically. Efficient light-induced current generation has been found with several of these samples so thatphotoelectrochemical processes generated by pyrite have to be considered as naturally occurring phenomena, which could have been even more pronounced in oxygen deficient environments. Pyrite from the Murgul mine in Turkey of suboceanic volcanicorigin was closer examined as a model system to understand the morphology and chemistry of pyrite photoactivity.     

Soil carbon dioxide venting through rice roots

The growth of rice in submerged soils depends on its ability to form continuous gas channels—aerenchyma—through which oxygen (O₂) diffuses from the shoots to aerate the roots. Less well understood is the extent to which aerenchyma permits venting of respiratory carbon dioxide (CO₂) in the opposite direction. Large, potentially toxic concentrations of dissolved CO₂ develop in submerged rice soils. We show using X‐ray computed tomography and image‐based mathematical modelling that CO₂ venting through rice roots is far greater than thought hitherto. We found rates of venting equivalent to a third of the daily CO₂ fixation in photosynthesis. Without this venting through the roots, the concentrations of CO₂ and associated bicarbonate (HCO₃^?) in root cells would have been well above levels known to be toxic to roots. Removal of CO₂ and hence carbonic acid (H₂CO₃) from the soil was sufficient to increase the pH in the rhizosphere close to the roots by 0.7 units, which is sufficient to solubilize or immobilize various nutrients and toxicants. A sensitivity analysis of the model showed that such changes are expected for a wide range of plant and soil conditions.     

Rates of fixation by lightning of carbon and nitrogen in possible primitive atmospheres

A thermochemical-hydrodynamic model of the production of trace species by electrical discharges has been used to estimate the rates of fixation of C and N by lightning in the primitive atmosphere. Calculations for various possible mixtures of CH₄, CO₂, N₂, H₂, and H₂O reveal that the prime species produced were probably HCN and NO and that the key parameter determining the rates of fixation was the ratio of C atoms to O atoms in the atmosphere. Atmospheres with C more abundant than O have large HCN fixation rates, in excess of 10¹⁷ molecules J^–1, but small NO yields. However, when O is more abundant than C, the NO fixation rate approaches 10¹⁷ molecules J^–1 while the HCN yield is small. The implications for the evolution of life are discussed.     

Prebiotic Synthesis of Methionine and Other Sulfur-Containing Organic Compounds on the Primitive Earth: A Contemporary Reassessment Based on an Unpublished 1958 Stanley Miller Experiment

Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH₄), hydrogen sulfide (H₂S), ammonia (NH₃), and carbon dioxide (CO₂). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H₂S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery.     

Photosynthetic CO(2) Fixation at Air Levels of CO(2) by Isolated Spinach Chloroplasts

A system has been developed for the study of photosynthetic CO₂ fixation by isolated spinach chloroplasts at air levels of CO₂. Rates of CO₂ fixation were typically 20 to 60 micromoles/milligrams chlorophyll per hour. The rate of fixation was linear for 10 minutes but then declined to less than 10% of the initial value by 40 minutes. Ribulose 1,5-bisphosphate (RuBP) levels remained unchanged during this period, indicating that they were not the cause for the decline. The initial activity of the RuBP carboxylase in the chloroplast was high for 8 to 10 minutes and then declined similar to the rate of CO₂ fixation, suggesting that the decline in CO₂ fixation may have been caused by deactivation of the enzyme.     

The mechanism of the control of carbon fixation by the pH in the chloroplast stroma

The salts of several weak acids have been used to render the envelope permeable to protons. In order to investigate the role of stromal pH changes in the light regulation of CO₂ fixation, formate, octanoate, nitrite, and glyoxylate have been tried as tools to reverse the light-dependent alkalization of the stroma. For this purpose, the decrease of the stromal pH in illuminated spinach chloroplasts, as caused by the addition of these substances or by instantaneous lowering of the pH in the medium, has been compared with the corresponding decrease of CO₂ fixation and the change of stromal metabolite levels. It appears from out data that formate and octanoate are suited best to obtain a specific inhibition of CO₂ fixation by lowering the stromal pH. The measurement of the corresponding metabolite levels indicates that this inhibition is primarily due to an inhibition of fructose- and sedoheptulose bisphosphatase. It is concluded that these two enzymes are important regulatory steps for the light control of CO₂ fixation.     

Diffusional Contribution to Carbon Isotope Fractionation during Dark CO(2) Fixation in CAM Plants

A mathematical model is developed which can be used to predict in vivo carbon isotope fractionations associated with carbon fixation in plants in terms of diffusion, CO₂ hydration, and carboxylation components. This model also permits calculation of internal CO₂ concentration for comparison with results of gas-exchange experiments. The isotope fractionations associated with carbon fixation in Kalanchoë daigremontiana and Bryophyllum tubiflorum have been measured by isolation of malic acid following dark fixation and enzymic determination of the isotopic composition of carbon-4 of this material. Corrections are made for residual malic acid, fumarase activity, and respiration. Comparison of these data with calculations from the model indicates that the rate of carbon fixation is limited principally by diffusion, rather than by carboxylation. Processes subsequent to the initial carboxylation also contribute to the over-all isotopic composition of the plant.     

Nitrogen Fixation By Corona Discharge On The Early Precambrian Earth

We report the first experimental study of nitrogen fixation by corona discharge on the anoxic primitive Earth. The energy yields of nitric oxide (NO) and nitrous oxide (N₂O) were experimentally determined over a wide range of CO₂-N₂ mixtures simulating the evolution of the Earth's atmosphere during the Hadean and Archean eras (from 4.5 ba to 2.5 ba). NO, the principal form of fixed nitrogen in lightning and coronal discharge in early Earth, is produced ten times less efficiently in the latter type of electrical discharge with an estimated maximum annual production rate of the order of 10¹⁰ g yr⁻¹. For N₂O the maximum production rate was estimated to be ∼10⁹ g yr⁻¹. These low rates of syntheses indicate that corona discharges as point discharges on the clouds and ground did not play a significant role in the overall pool of reactive nitrogen needed for the emergence and sustainability of life.     

A cell-based model for the photoacclimation and CO2-acclimation of the photosynthetic apparatus

We have developed a mathematical model based on the underlying mechanisms concerning the responses of the photosynthetic apparatus of a microalga cell which grows under constant incident light intensity and ambient CO₂ concentration. Photosynthesis involves light and carbon-fixation reactions which are mutually dependent and affect each other, but existing models for photosynthesis don't account for both reactions at once. Our modeling approach allows us to derive distinct equations for the rates of oxygen production, NADPH production, carbon dioxide fixation, carbohydrate production, and rejected energy, which are generally different. The production rates of the photosynthesis products are hyperbolic functions of light and CO₂ concentration. The model predicts that in the absence of photoinhibition, CO₂-inhibition, photorespiration, and chlororespiration, a cell acclimated to high light and/or CO₂ concentration has higher photosynthetic capacity and lower photosynthetic efficiency than does a cell acclimated to low conditions. This results in crossing between the two curves which represent the oxygen production rates and carbon fixation rates in low and high conditions. Finally, in the absence of photoinhibition and CO₂-inhibition, the model predicts the carbohydrate production rate in terms of both light intensity and CO₂ concentration.     

An estimation of CO<Subscript>2</Subscript> fixation capacity in mangrove forest using two methods of CO<Subscript>2</Subscript> gas exchange and growth curve analysis

In many coastal areas of South-East Asia, attempts have been made to revive coastal ecosystem by initiating projects that encourage planting of mangrove trees. Compared to the terrestrial trees, mangrove trees possess a higher carbon fixation capacity. It becomes a very significant option for clean development mechanism (CDM) program. However, a reliable method to estimate CO₂ fixation capacity of mangrove trees has not been established. Acknowledging the above fact, we decided to set up an estimation method for the CDM program, using gas exchange analysis to estimate mangrove productivity, we put into consideration the net CO₂ fixation of reforested Kandelia candel (5-, 10-, and 15-year-old stand). This was estimated by gas exchange analysis and growth curve analysis. In growth curve analysis, we drew a growth curve of a single stand using data of above- and below-ground biomass. In the gas exchange analysis, we calculated CO₂ fixation capacity by (1) measuring respiration rate of each organ of stand and calculating respiratory CO₂ emission from above- to below-ground biomass. (2) Measuring the single-leaf photosynthetic rate in response to light intensity and calculating the photosynthetic CO₂ absorption. (3) We also developed a model for the diurnal changes in temperature, and monthly averages based on one-day estimation of CO₂ absorption and emission, which we corrected by this model in order to estimate the net CO₂ fixation capacity in response to temperature. Comparing the biomass accumulation of the two methods constructed for the same forest, the above-ground biomass accumulation of 10-year-old forest (34.3 ton ha⁻¹ yr⁻¹) estimated by gas exchange analysis was closely compared to those of growth curve analysis (26.6 ton ha⁻¹ yr⁻¹), suggesting that the gas exchange analysis was capable of estimating mangrove productivity. The validity of the estimated CO₂ fixation capacity by the gas exchange analysis and the growth curve analysis was also discussed.     

A model describing photosynthesis in terms of gas diffusion and enzyme kinetics

Summary A model predicting net photosynthesis of individual plant leaves for a variety of environmental conditions has been developed. It is based on an electrical analogue describing gas diffusion from the free atmosphere to the sites of CO₂ fixation and a Michaelis-Menten equation describing CO₂ fixation. The model is presented in two versions, a simplified form without respiration and a more complex form including respiration. Both versions include terms for light and temperature dependence of CO₂ fixation and light control of stomatal resistance. The second version also includes terms for temperature, light, and oxygen dependence of respiration and O₂ dependence of CO₂ fixation.The model is illustrated with curves based on representative values of the various environmental and biological parameters. These curves relate net photosynthesis to light intensity, [CO₂], [O₂], temperature, and resistances to CO₂ uptake. The shape of the [CO₂]-net photosynthesis curves depends on the total diffusion resistance to CO₂ uptake and the Michaelis constant for CO₂ uptake. The curves range from typical Michaelis-Menten to Blackman types.The model is combined with a model of leaf energy exchange permitting simultaneous estimation of net photosynthesis and transpiration. The combined model is illustrated with curves relating transpiration to photosynthesis under a wide variety of environmental conditions. Environmental regimes yielding maximum efficiency of water use are identified for the given assumptions and biological parameters.     

Origin and evolution of photosynthetic reaction centers

The prototype reaction center may have used protoporphyrin-IX associated with small peptides to transfer electrons or protons across the primitive cell membrane. The precursor of all contemporary reaction centers contained chlorophylla molecules as both primary electron donor and initial electron acceptor and an Fe-S center as secondary acceptor (RC-1 type). The biosynthetic pathway for chlorophylla evolved along with the evolution of a better organized reaction center associated with cytochromes and quinones in a primitive cyclic electron transport system. This reaction center probably functioned initially in photoassimilation, but was easily adapted to CO₂ fixation using H₂ and H₂S as reductants. During this phase bacteriochlorophyllg may have evolved from chlorophylla in response to competition for light, and thereby initiated the gram-positive line of eubacteria. A second reaction center (RC-2) evolved from RC-1 between 3.5 and 2.5 Ga ago in response to the competition for reductants for CO₂ fixation. The new organism containing RC-2 in series with RC-1 would have been able to use poor reducing agents such as the abundant aqueous ferrous ion in place of H₂ and H₂S. This new organism is proposed to be the common ancestor of all phototrophic eubacteria except those related to the gram-positive bacteria. All organisms containing bacteriochlorophylla lost either RC-1 or RC-2, while those organisms containing chlorophylla (ancestors of cyanobacteria) added a water-splitting enzyme to RC-2 between 3.0 and 2.5 Ga ago in order to use H₂O in place of hydrated ferrous ion as electron donor for autotrophic photosynthesis.     

Dark microbial CO2 fixation in temperate forest soils increases with CO2 concentration

Dark, that is, nonphototrophic, microbial CO₂ fixation occurs in a large range of soils. However, it is still not known whether dark microbial CO₂ fixation substantially contributes to the C balance of soils and what factors control this process. Therefore, the objective of this study was to quantitate dark microbial CO₂ fixation in temperate forest soils, to determine the relationship between the soil CO₂ concentration and dark microbial CO₂ fixation, and to estimate the relative contribution of different microbial groups to dark CO₂ fixation. For this purpose, we conducted a ¹³C‐CO₂ labeling experiment. We found that the rates of dark microbial CO₂ fixation were positively correlated with the CO₂ concentration in all soils. Dark microbial CO₂ fixation amounted to up to 320 µg C kg^?1 soil day^?1 in the Ah horizon. The fixation rates were 2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of dark microbial fixation were small compared to the respiration rate (1.2%–3.9% of the respiration rate), our findings suggest that organic matter formed by microorganisms from CO₂ contributes to the soil organic matter pool, especially given that microbial detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses indicated that CO₂ was mostly fixed by gram‐positive bacteria, and not by fungi. In conclusion, our study shows that the dark microbial CO₂ fixation rate in temperate forest soils increases in periods of high CO₂ concentrations, that dark microbial CO₂ fixation is mostly accomplished by gram‐positive bacteria, and that dark microbial CO₂ fixation contributes to the formation of soil organic matter.     

A Synergistic Stimulation of Avena sativa Coleoptile Elongation by Indoleacetic Acid and Carbon Dioxide

The ability of 0.03% CO₂ to stimulate growth has been investigated using etiolated Avena coleoptile sections maintained in buffered solution. This concentration of CO₂-stimulated growth after a lag period of 12 to 15 minutes, and a synergistic relationship between indoleacetic acid and CO₂ in stimulating growth has been demonstrated. The response to CO₂ is inhibited by cycloheximide and is lost approximately 10 minutes after exposure to CO₂-free air. Malate can replace CO₂ in stimulating growth. In the light of these data and recent literature on a growth response of coleoptile sections to CO₂-saturated solutions, the existence of two mechanisms of CO₂-simulated growth is proposed. In addition, it is suggested that growth promotion by 0.03% CO₂ is mediated by a process involving dark CO₂ fixation.     

The role of pH in the regulation of carbon fixation in the chloroplast stroma. Studies on CO2 fixation in the light and dark

1. The pH in the stroma and in the thylakoid space has been measured in a number of chloroplast preparations in the dark and in the light at 20 °C. Illumination causes a decrease of the pH in the thylakoid space by 1.5 and an increase of the pH in the stroma by almost 1 pH unit.2. CO₂ fixation is shown to be strongly dependent on the pH in the stroma. The pH optimum was 8.1, with almost zero activity below pH 7.3. Phosphoglycerate reduction, which is a partial reaction of CO₂ fixation, shows very little pH dependency.3. Low concentrations of the uncoupler m-chlorocarbonylcyanide phenylhydrazone (CCCP) inhibit CO₂ fixation without affecting phosophoglycerate reduction. This inhibition of CO₂ fixation appears to be caused by reversal of light induced alkalisation in the stroma by CCCP.4. Methylamine has a very different effect compared to CCCP. Increasing concentrations of methylamine inhibit CO₂ fixation and phosphoglycerate reduction to the same extent. The light induced alkalisation of the stroma appears not to be significantly inhibited by methylamine, but the protons in the thylakoid space are neutralized. The inhibition of CO₂ fixation by higher concentrations of methylamine is explained by an inhibition of photophosphorylation. It appears that methylamine does not abolish proton transport.5. It is shown that intact chloroplasts are able to fix CO₂ in the dark, yielding 3-phosphoglycerate. This requires the addition of dihydroxyacetone phosphate as precursor of ribulosemonophosphate and also to supply ATP, and the addition of oxaloacetate for reoxidation of the NADPH in the stroma.6. Dark CO₂ fixation in the presence of dihydroxyacetone phosphate and oxaloacetate has the same pH dependency as CO₂ fixation in the light. This demonstrates that CO₂ fixation in the dark is not possible, unless the pH in the medium is artificially raised to pH 8.8.7. It is shown that pH changes occurring in the stroma after illumination are sufficient to switch CO₂ fixation from zero to maximal activity. This offers a mechanism for light control of CO₂ fixation, avoiding wasteful CO₂ fixation in the dark.     

STUDIES ON THE METABOLISM OF AUTOTROPHIC BACTERIA : II. THE NATURE OF THE CHEMOSYNTHETIC REACTION

In a study of chemosynthesis (the fixation of CO₂ by autotrophic bacteria in the dark) in Thiobacillus thiooxidans, the data obtained support the following conclusions: 1. CO₂ can be fixed by "resting cells" of Thiobacillus thiooxidans; the fixation is not "growth bound." 2. The physiological condition of the cell is of considerable importance in determining CO₂ fixation. 3. CO₂ fixation can occur in the absence of oxidizable sulfur in "young" cells. The extent of this fixation appears to be dependent upon the pCO₂. 4. CO₂ fixation can also occur under anaerobic conditions and the presence of sulfur does not influence such fixation. 5. However, in the CO₂ fixation by cells in the absence of sulfur, only a limited amount of CO₂ can be fixed. This amount is approximately 40 µl. CO₂ per 100 micrograms bacterial nitrogen. After a culture has utilized this amount of CO₂ it no longer has the ability to fix CO₂ but releases it during its respiration. 6. Relatively short periods of sulfur oxidation can restore the ability of cells to fix CO₂ under conditions where sulfur oxidation is prevented. 7. It is possible to oxidize sulfur in the absence of CO₂ and to store the energy thus formed within the cell. It is then possible to use this energy at a later time for the fixation of CO₂ in the entire absence of sulfur oxidation. 8. Cultures of Thiobacillus thiooxidans respiring on sulfur utilize CO₂ in a reaction which proceeds to a zero concentration of CO₂ in the atmosphere. 9. CO₂ may act as an oxidizing agent for sulfur. 10. Hydrogen is not utilized by the organism. 11. It is possible to selectively inhibit sulfur oxidation and CO₂ fixation.     

Regulation of Soybean Net Photosynthetic CO(2) Fixation by the Interaction of CO(2), O(2), and Ribulose 1,5-Diphosphate Carboxylase

Kinetic properties of soybean net photosynthetic CO₂ fixation and of the carboxylase and oxygenase activities of purified soybean (Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO₂ concentration, and O₂ concentration. With leaves, O₂ inhibition of net photosynthetic CO₂ fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO₂ fixation at higher temperatures was caused by a reduced affinity of the leaf for CO₂ and an increased affinity of the leaf for O₂. With purified ribulose 1,5-diphosphate carboxylase, O₂ inhibition of CO₂ incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O₂ sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO₂ and a slightly increased affinity of the enzyme for O₂. The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO₂ and O₂ provides further evidence that the carboxylase regulates the O₂ response of photosynthetic CO₂ fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO₂ and O₂ fixation in vivo is proposed.     

Synthesis of putrescine under possible primitive earth conditions

The synthesis of putrescine was accomplished by decarboxylation of L-orithine when this amino acid was heated in aqueous solution and in the absence of oxygen. Chromatographic, radioisotopic, and enzymatic techniques were used to demonstrate that one mole of non-radioactive putrescine and one mole of¹⁴CO₂ was formed during the heating of L-(1-¹⁴C)-ornithine. This work indicates that the synthesis of putrescine can occur starting with ornithine and in conditions that are presumed could have existed on the primitive Earth. The possible significance of these results in the prebiotic molecular evolution is briefly discussed.