The origin of life on Earth

Terrestrial life can be schematically described as organic molecules organized in liquid water. According to Oparin's hypothesis, organic building blocks required for early life were produced from simple organic molecules formed in a primitive reducing atmosphere. Precursors of lipids, nucleic acids and enzymes obtained in the laboratory under simulating conditions are reviewed. Geochemists favor now a less reducing atmosphere dominated by carbon dioxide. In such an atmosphere, very few building blocks are formed under prebiotic conditions. Import of extraterrestrial organic molecules may represent an alternative supply. Experimental support for such an alternative scenario is examined in comets, cosmic dust, meteorites and micrometeorites. Even the prebiotic broth receives today severe criticism for being implausible. In contrast to the classical scenario, a chemoautotrophic origin of life is discussed. Finally, interesting information related to early terrestrial life may be gained from Mars exploration.     

Flowering time and elevated atmospheric CO2

Flowering is a critical milestone in the life cycle of plants, and changes in the timing of flowering may alter processes at the species, community and ecosystem levels. Therefore understanding flowering-time responses to global change drivers, such as elevated atmospheric carbon dioxide concentrations, [CO(2)], is necessary to predict the impacts of global change on natural and agricultural ecosystems. Here we summarize the results of 60 studies reporting flowering-time responses (defined as the time to first visible flower) of both crop and wild species at elevated [CO(2)]. These studies suggest that elevated [CO(2)] will influence flowering time in the future. In addition, interactions between elevated [CO(2)] and other global change factors may further complicate our ability to predict changes in flowering time. One approach to overcoming this problem is to elucidate the primary mechanisms that control flowering-time responses to elevated [CO(2)]. Unfortunately, the mechanisms controlling these responses are not known. However, past work has indicated that carbon metabolism exerts partial control on flowering time, and therefore may be involved in elevated [CO(2)]-induced changes in flowering time. This review also indicates the need for more studies addressing the effects of global change drivers on developmental processes in plants.     

The photochemical origins of life and photoreaction of ferrous ion in the archaean oceans

A general argument is made for the photochemical origins of life. A constant flux of free energy is required to maintain the organized state of matter called life. Solar photons are the unique source of the large amounts of energy probably require to initiate this organization and certainly required for the evolution of life to occur. The completion of this argument will require the experimental determination of suitable photochemical reactions. Our work shows that biogenetic porphyrins readily photooxidize substrates and emit hydrogen in the presence of a catalyst. These results are consistent with the Granick hypothesis, which relates a biosynthetic pathway to its evolutionary origin. We have shown that photoexcitation of ferrous ion at neutral pH with near ultraviolet light produces hydrogen with high quantum yield. This same simple system may reduce carbon dioxide to formaldehyde and further products. These reactions offer a solution to the dilemma confronting the Oparin-Urey-Miller model of the chemical origin of life. If carbon dioxide is the main form of carbon on the primitive earth, the ferrous photoreaction may provide the reduced carbon necessary for the formation of amino acids and other biogenic molecules. These results suggest that this progenitor of modern photosynthesis may have contributed to the chemical origins of life.     

Life on Mars: chemical arguments and clues from Martian meteorites

Primitive terrestrial life – defined as a chemical system able to transfer its molecular information via self-replication and to evolve – probably originated from the evolution of reduced organic molecules in liquid water. Several sources have been proposed for the prebiotic organic molecules: terrestrial primitive atmosphere (methane or carbon dioxide), deep-sea hydrothermal systems, and extraterrestrial meteoritic and cometary dust grains. The study of carbonaceous chondrites, which contain up to 5% by weight of organic matter, has allowed close examination of the delivery of extraterrestrial organic material. Eight proteinaceous amino acids have been identified in the Murchison meteorite among more than 70 amino acids. Engel reported that l-alanine was surprisingly more abundant than d-alanine in the Murchison meteorite. Cronin also found excesses of l-enantiomers for nonprotein amino acids. A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. In the 50- to 100-μm size range, carbonaceous micrometeorites represent 80% of the samples and contain 2% of carbon, on average. They might have brought more carbon than that involved in the present surficial biomass. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars, attesting the presence of an atmosphere capable of deccelerating C-rich micrometeorites. Therefore, primitive life may have developed on Mars as well and fossilized microorganisms may still be present in the near subsurface. The Viking missions to Mars in 1976 did not find evidence of either contemporary or past life, but the mass spectrometer on the lander aeroshell determined the atmospheric composition, which has allowed a family of meteorites to be identified as Martian. Although these samples are essentially volcanic in origin, it has been recognized that some of them contain carbonate inclusions and even veins that have a carbon isotopic composition indicative of an origin from Martian atmospheric carbon dioxide. The oxygen isotopic composition of these carbonate deposits allows calculation of the temperature regime existing during formation from a fluid that dissolved the carbon dioxide. As the composition of the fluid is unknown, only a temperature range can be estimated, but this falls between 0° and 90°C, which would seem entirely appropriate for life processes. It was such carbonate veins that were found to host putative microfossils. Irrespective of the existence of features that could be considered to be fossils, carbonate-rich portions of Martian meteorites tend to have material, at more than 1000 ppm, that combusts at a low temperature; i.e., it is an organic form of carbon. Unfortunately, this organic matter does not have a diagnostic isotopic signature so it cannot be unambiguously said to be indigenous to the samples. However, many circumstantial arguments can be made to the effect that it is cogenetic with the carbonate and hence Martian. If it could be proved that the organic matter was preterrestrial, then the isotopic fractionation between it and the carbon is in the right sense for a biological origin. Received: January 22, 1998 / Accepted: February 16, 1998     

Is there a common chemical model for life in the universe?

A review of organic chemistry suggests that life, a chemical system capable of Darwinian evolution, may exist in a wide range of environments. These include non-aqueous solvent systems at low temperatures, or even supercritical dihydrogen-helium mixtures. The only absolute requirements may be a thermodynamic disequilibrium and temperatures consistent with chemical bonding. A solvent system, availability of elements such as carbon, hydrogen, oxygen and nitrogen, certain thermodynamic features of metabolic pathways, and the opportunity for isolation, may also define habitable environments. If we constrain life to water, more specific criteria can be proposed, including soluble metabolites, genetic materials with repeating charges, and a well defined temperature range.     

Responses of respiration to increasing atmospheric carbon dioxide concentrations

It has been recently recognized that increases in carbon dioxide concentration such as are anticipated for the earth's atmosphere in the next century often reduce plant respiration. There can be both a short-term reversible effect of unknown cause, and long-term acclimation, which may reflect the synthesis and maintenance of less metabolically expensive materials in plants grown at elevated carbon dioxide concentrations. Because respiration provides energy and carbon intermediates for growth and maintenance, reductions in respiration by increasing carbon dioxide concentrations may have effects on physiology beyond an improvement in plant carbon balance. As atmospheric carbon dioxide concentration increases, reduced respiration could be as important as increased photosynthesis in improving the ability of terrestrial vegetation to act as a sink for carbon, but it could also have other consequences.     

The physics and ecology of mining carbon dioxide from the atmosphere by ecosystems

Reforesting and managing ecosystems have been proposed as ways to mitigate global warming and offset anthropogenic carbon emissions. The intent of our opinion piece is to provide a perspective on how well plants and ecosystems sequester carbon. The ability of individual plants and ecosystems to mine carbon dioxide from the atmosphere, as defined by rates and cumulative amounts, is limited by laws of physics and ecological principles. Consequently, the rates and amount of net carbon uptake are slow and low compared to the rates and amounts of carbon dioxide we release by fossil fuels combustion. Managing ecosystems to sequester carbon can also cause unintended consequences to arise. In this paper, we articulate a series of key take‐home points. First, the potential amount of carbon an ecosystem can assimilate on an annual basis scales with absorbed sunlight, which varies with latitude, leaf area index and available water. Second, efforts to improve photosynthesis will come with the cost of more respiration. Third, the rates and amount of net carbon uptake are relatively slow and low, compared to the rates and amounts and rates of carbon dioxide we release by fossil fuels combustion. Fourth, huge amounts of land area for ecosystems will be needed to be an effective carbon sink to mitigate anthropogenic carbon emissions. Fifth, the effectiveness of using this land as a carbon sink will depend on its ability to remain as a permanent carbon sink. Sixth, converting land to forests or wetlands may have unintended costs that warm the local climate, such as changing albedo, increasing surface roughness or releasing other greenhouse gases. We based our analysis on 1,163 site‐years of direct eddy covariance measurements of gross and net carbon fluxes from 155 sites across the globe.     

二氧化碳生物转化制甲烷技术研究进展

二氧化碳减量化与转化是当前业界关注及着手解决的重要问题,将二氧化碳作为资源转化为甲烷,有利于环境与社会的可持续发展。本文在分析二氧化碳转化为甲烷技术的基础上,重点介绍了国内外二氧化碳生物转化的研究与进展;总结了二氧化碳生物转化途径及其影响因素,分析了氢营养型、甲基营养型生物转化甲烷机理和生物转化能量来源;探讨了不同产甲烷菌微生物电合成产甲烷和氢气研究进展,总结了微生物电合成法、光合作用法和厌氧消化法等二氧化碳生物转化技术在反应器设计、电极材料选择、工艺条件优化及试验结果评估等方面取得的进展及存在的问题。重点就微生物电合成法的未来研究提出了增强微生物活性、提升氢气利用率、加快高效电极开发、提高能量效率、加强工业废气试验研究和强化光能转化等研究重点和发展方向,同时加强计算机模拟等交叉学科协同创新是促进二氧化碳生物转化技术进步的新方向。     

Underwater photosynthesis in flooded terrestrial plants: a matter of leaf plasticity

BACKGROUND: Flooding causes substantial stress for terrestrial plants, particularly if the floodwater completely submerges the shoot. The main problems during submergence are shortage of oxygen due to the slow diffusion rates of gases in water, and depletion of carbohydrates, which is the substrate for respiration. These two factors together lead to loss of biomass and eventually death of the submerged plants. Although conditions under water are unfavourable with respect to light and carbon dioxide supply, photosynthesis may provide both oxygen and carbohydrates, resulting in continuation of aerobic respiration. SCOPE: This review focuses on evidence in the literature that photosynthesis contributes to survival of terrestrial plants during complete submergence. Furthermore, we discuss relevant morphological and physiological responses of the shoot of terrestrial plant species that enable the positive effects of light on underwater plant performance. CONCLUSIONS: Light increases the survival of terrestrial plants under water, indicating that photosynthesis commonly occurs under these submerged conditions. Such underwater photosynthesis increases both internal oxygen concentrations and carbohydrate contents, compared with plants submerged in the dark, and thereby alleviates the adverse effects of flooding. Additionally, several terrestrial species show high plasticity with respect to their leaf development. In a number of species, leaf morphology changes in response to submergence, probably to facilitate underwater gas exchange. Such increased gas exchange may result in higher assimilation rates, and lower carbon dioxide compensation points under water, which is particularly important at the low carbon dioxide concentrations observed in the field. As a result of higher internal carbon dioxide concentrations in submergence-acclimated plants, underwater photorespiration rates are expected to be lower than in non-acclimated plants. Furthermore, the regulatory mechanisms that induce the switch from terrestrial to submergence-acclimated leaves may be controlled by the same pathways as described for heterophyllous aquatic plants.     

The mode of action of insecticidal controlled atmospheres

Arthropods cope with reduced oxygen and elevated carbon dioxide atmospheres with a reduction in metabolic rate, also called metabolic arrest. The reduction in metabolism lessens the pressure on the organism to initiate anaerobic metabolism, but also leads to a reduction in ATP production. The natural permeability of cellular membranes appears to be important for the survival of the arthropod under low oxygen or high carbon dioxide atmospheres. Despite the similarities in response, arthropod mortality is generally greater in response to high carbon dioxide as apposed to low oxygen atmospheres. There appears to be a greater decrease in ATP and energy charge in arthropods exposed to high carbon dioxide as compared with low oxygen atmospheres, and this may be due to greater membrane permeability under carbon dioxide leading to an inefficient production of ATP. Reduced oxygen and elevated carbon dioxide atmospheres can have an additive effect in some cases, depending on the concentrations used. The effect of these atmospheres on arthropods depends also on temperature, species and life stage. Additional work is needed to fully understand the mode of action of controlled atmospheres on arthropod pests.     

How plants changed the world: Using qualitative reasoning to explain plant macroevolution's effect on the long-term carbon cycle

We present a qualitative reasoning model of how plant colonization of land during the mid Paleozoic era (450–300 million years ago) altered the long-term carbon cycle resulting in a dramatic decrease in global atmospheric carbon dioxide levels. This model is aimed at facilitating learning and communication about how interactions between biological and geological processes drove system behavior. The model is developed in three submodels of the main system components, namely how competition for limited land habitat drove natural selection for increasing adaptations to life on land; how these adaptations resulted in increased formation of organic-rich sedimentary rocks (coal); and how these adaptations altered weathering of calcium and magnesium silicate rocks, resulting in increased deposition of inorganic carbonates in oceans. These separate submodels are then assembled to derive the full dynamic model of plant macroevolution, colonization of land, and plummeting carbon dioxide levels that occurred during the mid Paleozoic. The qualitative reasoning framework supports explicit representation of causal feedbacks — as with previously developed systems analysis models — but also supports simulation of system dynamics arising from the configuration of entities in the system. The ability of qualitative reasoning to provide causal accounts (explanations) of why certain phenomena occurred and when, is a powerful advantage over numerical simulation such as the complex GEOCARB models, where explanation must be left to interpretation by experts.     

Tempol diverts peroxynitrite/carbon dioxide reactivity toward albumin and cells from protein-tyrosine nitration to protein-cysteine nitrosation

Tempol has been shown to protect experimental animals from injuries associated with excessive nitric oxide production. In parallel, tempol decreased the levels of protein-3-nitrotyrosine in the injured tissues, suggesting that it interacted with nitric oxide-derived oxidants such as nitrogen dioxide and peroxynitrite. Relevantly, a few recent studies have shown that tempol catalytically diverts peroxynitrite/carbon dioxide reactivity toward phenol from nitration to nitrosation. To examine whether this shift occurs in biological environments, we studied the effects of tempol (10-100 microM) on peroxynitrite/carbon dioxide (1 mM/2 mM) reactivity toward proteins, native bovine serum albumin (BSA) (0.5-0.7 cys/mol) and reductively denatured BSA (7-19 cys/mol), and cells (J774 macrophages). Although not a true catalyst, tempol strongly inhibited protein-tyrosine nitration (70-90%) and protein-cysteine oxidation (20-50%) caused by peroxynitrite/carbon dioxide in BSA, denatured BSA, and cells while increasing protein-cysteine nitrosation (200-400%). Tempol consumption was attributed mainly to its reaction with protein-cysteinyl radicals. Most of the tempol, however, reacted with the radicals produced from peroxynitrite/carbon dioxide, that is, nitrogen dioxide and carbonate radical anion. Accordingly, tempol decreased the yields of BSA-cysteinyl and BSA-tyrosyl/tryptophanyl radicals, as well their decay products such as protein-3-nitrotyrosine. The parallel increase in protein-nitrosocysteine yields demonstrated that part of the peroxynitrite is oxidized to nitric oxide by the oxammonium cation produced from tempol oxidation by peroxynitrite/carbon dioxide-derived radicals. Protein-nitrosocysteine formation was shown to occur by radical and nonradical mechanisms in studies with a protein-cysteinyl radical trapper. These studies may contribute to the understanding of the protective effects of tempol in animal models of inflammation.     

Acetate,methanol and carbon dioxide as substrates for growth of Methanosarcina barkeri

Methanosarcina barkeri grows in defined media with acetate, methanol or carbon dioxide as carbon sources. Methanol is used for methanogenesis at a 5 times higher rate as compared with the other substrates. M. barkeri can use the substrates simultaneously, but due to acidification or alkalification of the medium during growth on methanol or acetate, respectively, growth and methanogenesis may stop before the substrates are exhausted. Growth and methanogenesis on methanol or acetate are inhibited by the presence of an excess of H₂; the inhibition is abolished by the addition of carbon dioxide, which probably serves as an essential source of cell carbon, in the absence of which methano-genesis ceases.     

Plant oils as feedstock alternatives to petroleum – A short survey of potential oil crop platforms

Our society is highly depending on petroleum for its activities. About 90% is used as an energy source for transportation and for generation of heat and electricity and the remaining as feedstocks in the chemical industry. However, petroleum is a finite source as well as causing several environmental problems such as rising carbon dioxide levels in the atmosphere. Petroleum therefore needs to be replaced by alternative and sustainable sources. Plant oils and oleochemicals derived from them represent such alternative sources, which can deliver a substantial part of what is needed to replace the petroleum used as feedstocks.     

Ethyl acetate: a possible alternative for anaesthetizing insects

In order to anaesthetize insects in a laboratory, chilling and application of diethyl ether and carbon dioxide are commonly used. However none of the above methods is problem free. In particular, the use of diethyl ether, despite its simplicity, is now limited due to its poor safety. In our research, we evaluated ethyl acetate as an alternative anaesthetic substance. The effects of ethyl acetate anaesthesia were compared with those produced by carbon dioxide on adult green lacewings (Neuroptera: Chrysopidae). The biological parameters measured were longevity and fecundity. No significant differences appeared between the two treatments and the control. Although further research is necessary, the use of ethyl acetate proves to be very promising and presents a valid alternative to the use of diethyl ether and, in many cases, also to carbon dioxide and chilling.     

Biochemical aspects of renal ammonia formation in metabolic acidosis.

In omnivorous creatures, the diet is acidogenic, especially as a result of the meat content, which gives rise to phosphoric and sulphuric acids, i.e., to metabolic acidosis. In the short term, metabolic acids are buffered by tissue proteins and bicarbonate (the 'alkali reserve'). In the longer term, acid must be excreted, or neutralized with base which is also generated from the diet, by conversion of dietary amino-nitrogen to ammonia. The final steps of this process occur in the kidney, which converts circulating glutamine to ammonia, and to carbon products such as glucose and carbon dioxide, by metabolic reactions which adapt during acidosis to generate more ammonia and maintain an increased renal ammonia content. The complex mechanisms which govern the formation of ammonia, glucose and carbon dioxide from glutamine, involving the reactions of amino acids, the tricarboxylic acid cycle, and gluconeogenesis, are reviewed.     

CATALYTIC EFFECTS OF CARBON DIOXIDE IN CARBON DIOXIDE ASSIMILATING CELLS

It has been confirmed that absence of carbon dioxide may decreasethe rate of oxygen production which accompanies the photochemicalreduction of p-benzoquinone in algae and chloroplasts. Thisinfluence of carbon dioxide partial pressure does not applyto the overall oxygen yield. In the blue-green alga Anacystisnidulans the initially small carbon dioxide deficiency effectincreases with time spent in the dark. The deterioration ofreaction rates is counteracted by light. There seems to be nodirect connection or interdependence between the photosyntheticreduction of carbon dioxide and the sensitivity of some partof the photochemical mechanism to loss of carbon dioxide. Notonly does addition of quinone to living cells in these experimentsdestroy their capacity for photosynthesis, but mutant cellsthat never had this capacity still retain the sensitivity towardslack of carbon dioxide when tested for their ability to reducequinone. Many different metabolic reactions have been seen topossess such dependency on traces of carbon dioxide, also innon-photosynthetic cells and tissues. The explanation for "catalytic"effects of carbon dioxide ought to be a general one–suchas an influence on the efficiency of certain phosphorylationswhich occur everywhere in the living world. ¹ Dedicated to Prof. H. TAMIYA on the occasion of his 60th birthday.These studies were aided by contract NONR 988 (10) between theOffice of Naval Research, Department of the Navy, and the FloridaState University, respectively. ² Present address: Charles F. KETTERING Research Laboratories,Yellow Springs, Ohio. (Received December 7, 1962; )     

Development of respirometry methods to assess the microbial activity of thermophilic bioleaching archaea

Respirometry methods have been used for many years to assess the microbial activity of mainly heterotrophic bacteria. Using this technique, the consumption of oxygen and evolution of carbon dioxide for heterotrophic carbon catabolism can be used to assess microbial activity. In the case of autotrophic bioleaching bacteria, carbon dioxide is used as a carbon source resulting in the consumption of both oxygen and carbon dioxide. The use of such respirometry techniques at high temperatures (up to 80 degrees C) for the investigation of bioleaching Archaea, however, poses particular difficulties. At these elevated temperatures, the solubility of oxygen into the liquid phase is particularly poor. This work details specific methods by which high temperature constraints are overcome while monitoring the activity of thermophilic Archaea using a Micro-Oxymax respirometer (Columbus Instruments). The use of elevated headspace oxygen concentrations, in order to overcome low oxygen solubility, is demonstrated as well as the effect of such elevated oxygen concentrations on microbial oxygen consumption rates. The relative rates of oxygen and carbon dioxide consumption are also illustrated during the oxidation of a chalcopyrite concentrate. In addition, this paper details generic methods by which respirometry data can be used to quantify inhibitory effects of a compound such as Na(2)SO(4). The further use of such data in predicting minimum hydraulic reactor retention times for continuous culture bioleaching reactors, as a function of concentration of potentially inhibitory compounds, is also demonstrated.     

Combined heat and controlled atmosphere quarantine treatments for control of western cherry fruit fly in sweet cherries

Nonchemical quarantine treatments, using a combination of short duration high temperatures under low oxygen, elevated carbon dioxide atmospheric environment were developed to control western cherry fruit fly, Rhagoletis indifferens Curran, in sweet cherries, Prunus avium (L.). The two treatments developed use a chamber temperature of 45 degrees C for 45 min and a chamber temperature of 47 degreesd C for 25 min, both under a 1% oxygen, 15% carbon dioxide, -2 degrees C dew point environment. Both these treatments have been shown to provide control of all life stages of western cherry fruit fly while preserving commodity market quality. There was no definitive egg or larval stage, which was demonstrated to be the most tolerant to either controlled atmosphere temperature treatment system treatment. Efficacy tests for both treatments resulted in 100% mortality of >5000 western cherry fruit flies in each treatment. These treatments may provide, with further study, quarantine security in exported sweet cherries where western cherry fruit fly is a quarantine concern and fumigation with methyl bromide is not desired.     

ACID-BASE RELATIONSHIPS WITHIN THE AVIAN EGG

1. In the newly laid egg of the domestic fowl the pH values of the albumen and yolk are about 7.6 and 6.0 respectively. 2. When the egg is stored in air there is a loss of carbon dioxide from the albumen and the pH of this fluid rises to a maximum value of about 9.5. A large proportion of the carbon dioxide which remains in the albumen is in the form of carbonate. 3. In the fertile incubated egg the pH of the albumen attains a maximum value within a period of about 2 days; the albumen then becomes less alkaline and it is nearly neutral by the end of the second week. The increasing acidity of the albumen can be attributed to (a) the secretion of hydrogen ions by the blastoderm and (b) the output of carbon dioxide by developing tissues. 4. During the first 2 weeks of incubation the pH of the yolk progressively increases to a maximum value of about 7.5: there is then a tendency for the pH of this fluid to fall and the yolk that is retained within the body of the hatched chick is slightly acidic. 5. The embryo may never come into direct contact with either the albumen or the yolk when the pH of these fluids are high and low respectively. At the beginning of embryonic development the blastoderm is separated from the albumen by the vitelline membrane and from the yolk by a layer of subgerminal fluid with a maximum pH of about 7.8. The vitelline membrane ruptures on day 4 but by this time the embryo is bathed in amniotic fluid with a pH of about 7.5. 6. The pH of amniotic fluid falls from a maximum value of about 7.5 during week I to a minimum value of about 6.5 during week 2. Amniotic fluid is a simple solution of salts until day 12; albumen then begins to flow into the amniotic cavity and the buffering capacity of amniotic fluid increases. 7. The principal end-product of nitrogenous metabolism in the chick embryo is uric acid and about 100 mg of this substance are deposited within the allantoic cavity. The pH of allantoic fluid may exceed 7.5 during week 1 but falls to 6.0 or below after day 13. 8. The tension of carbon dioxide within the egg is determined by the ratio of the rate of carbon dioxide production by the embryo to the permeability of the shell towards carbon dioxide. For the greater part of the period of incubation the permeability of the shell towards carbon dioxide is constant. Thus, as the carbon dioxide output of the embryo increases, the carbon dioxide tension within the egg rises. 9. The pH of the blood can be defined in terms of the ratio of the bicarbonate concentration to the carbon dioxide tension. There is a progressive increase in the carbon dioxide tension of the blood during the period of incubation but the pH is maintained at about 7.4 by an increase in bicarbonate concentration. 10. Part of the increase in bicarbonate is due to the removal of hydrogen ions from carbonic acid by haemoglobin. There is also a large influx of bicarbonate into the blood, but the source of this bicarbonate is not known; the evidence that renal mechanisms are involved is inconclusive and it is probable that the embryo utilizes the enormous potential store of bicarbonate in the egg shell.