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Geochemical Connections to Primitive Metabolism Deep hydrothermal “black smokers” may promote chemical reactions that were important in life’s origins. George D. Cody 1 M any microorganisms make extensive use of transition metal sulfide clusters in their metabolic chemistry. Similarly, transition metal sulfide minerals, e.g., pyrrhotite and pyrite, have the potential to provide the essential catalytic chemistry for Earth’s earliest life. Experiments reveal that transition metal sulfides have the capacity to both catalyze and, in some cases, participate in organosynthetic reactions that bear similarity to modern biosynthetic pathways. These experiments are buttressed by recognition of natural cases of extensive abiotic organosynthesis in the Earth’s crust—reactions that could have provided the first life with a large complement of functionally useful protobiological organic compounds. KEYWORDS: metabolism, biosynthesis, biocatalysis, origin of life, biogenesis INTRODUCTION At present there is no completely satisfactory theory for the origin of life. Several origins hypotheses focus on the identifiable qualities of life, e.g., information and replication, as in the case of the RNA world hypothesis, and early biological energy conversion, as in the case of the “metabolic” theories of life. The metabolic perspective rests on the fact that all life requires a source of energy. In the absence of light energy, life must use the natural chemical potential derived from chemical disequilibrium in the environment. Kinetic barriers that inhibit thermodynamic equilibrium endow the natural environment with ample sources of chemical potential energy that could be exploited for organosynthesis, provided that catalysts exist to promote such reactions. It is intriguing to consider whether certain minerals may have provided catalytic function for Earth’s earliest life. TRANSITION METAL SULFIDES AND METABOLISM In the metabolism-first scenario of life’s origins, the first cells were “chemoautotrophs” (see Table 1). It is worthwhile to consider how these microorganisms survive based on simple chemical disequilibria alone. In hydrogen-based subsurface microbial ecosystems, certain anaerobic microorganisms, e.g., the methanogens, extract energy and synthesize biomolecules by exploiting the natural thermodynamic disequilibrium of coexisting CO2 and H2 (Gottschalk 1986). Methanogenic microorganisms utilize a complex array of metallo-enzymes to catalyze the reduction of CO2 selectively towards the formation of useful biochemical products. The primary carbon-fixing pathway involves the production of the energy-rich molecule acetylcoA (Lengeler et al. 1999). TABLE 1 SOME TERMS RELATED TO THE METABOLISM OF MICROBES Metabolism The network of chemical reactions by which cells process matter and energy from their environment Autotroph A cell that manufactures its own biomolecules from small molecules Chemoautotroph An autotroph that harvests the chemical energy of rocks and other chemicals in disequilibrium in the environment Methanogen A cell that generates methane as a byproduct of metabolism Enzyme A chemical (usually a protein) that catalyzes a biochemical reaction Synthase An enzyme that helps to assemble two smaller molecules into a larger molecule Co-enzyme An organic molecule (not a protein) that helps a protein enzyme Acetyl-coA An important metabolic coenzyme (gram formula weight ~ 800) Organosynthesis Natural reactions that produce organic molecules Biosynthesis Biological reactions that produce bio-organic molecules Prokaryote A generic name for single-celled organisms that do not have a nucleus Mitochondria The energy generating element in eukaryotic cells 1 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd NW, Washington, DC 20015, USA E-mail: [email protected] ELEMENTS, VOL. 1, PP 139–143 139 J UNE 2005 According to these hypotheses, Earth’s earliest life was derived from abiotic reactions catalyzed by transition metal sulfide minerals. The vestiges of this non-enzymatic stage of biology are preserved today in multitudes of critical metabolic metallo-enzymes. Two notable hypotheses include Wächtershäuser’s iron-sulfur world (Wächtershäuser 1988, 1990, 1992) and Russell and Hall’s iron-sulfur membrane model (Russell et al. 1988, Russell and Hall 1997). Both proposals predict that life emerged as a direct consequence of chemical reactions derived from the interaction of reduced fluids (crustal and/or mantle-derived) with reactive and catalytic transition metal sulfides. One of the essential differences between these hypotheses and those that preceded them is the idea that Earth’s first organism was an autotroph, i.e., a life form capable of synthesizing all of its biomolecular constituents from simple inorganic compounds such as CO2, NH3, H2S, and PO4-3. In 1988 German chemist and patent lawyer Günter Wächtershäuser proposed that pyrite formation provided a viable energy source for Earth’s first life, via the reaction of iron monosulfide and hydrogen sulfide to produce pyrite and hydrogen: FeS + H2S à FeS2 + H2 A simplified representation of catalytic pathways of the acetyl-coA synthase complex. The left reaction pathway sequentially reduces CO2 to a tetrahydrofolate (THF) bound methyl group. This methyl group is transferred to a cobalt cobalamin cofactor and then to a Ni-X-Fe4S4 cluster (where X = sulfur and possibly a second transition metal). The right reaction pathway first promotes the water-gas shift reaction reducing CO2 to CO by utilizing a second NiX-Fe4S4 cluster. Carbonyl migration and insertion between the Ni-CH3 bond yields the acetyl group. Transfer of the acetyl group first to the thiol end of cofactor A yields acetyl-coA. FIGURE 1 This exergonic (energy-releasing) reaction has the potential to drive otherwise endergonic (energy-consuming) reactions, such as the biologically important reduction of CO2 to form formic acid: CO2(aq) + FeS + H2S à HCOOH + FeS2 + H2O Catalytic transition metal sulfide clusters such as Fe4S4 are essential to the chemistry required for acetyl-coA synthesis. One key reaction pathway towards acetyl-coA synthesis (FIG. 1) leads to the progressive reduction of CO2, ultimately to a transferable methyl group (CH3), while the second pathway promotes the reduction of CO2 to CO. The two reaction pathways join at the point where a methyl group is transferred, first to a cobalt atom and ultimately to a nickel atom in a Ni-X-Fe4S4 cluster, where X indicates sulfur and possibly a second transition metal (e.g., Qiu et al. 1994; Doukov et al. 2002). One reason why transition metals play such a prominent role in acetyl-coA synthase is the high bonding affinity of CO for transition metals (Fe, Co, and Ni in particular). The electronic configuration of CO and transition metals allows for considerable sharing of charge, hence the formation of a weak metal–carbon bond. Vibrational spectroscopy reveals that the bond of CO to Fe is stronger than to that of Co or Ni. That extant life chooses to use Fe, Co, and Ni (and Cu) in acetyl-coA may thus reflect a fine tuning of the catalytic function of the enzyme complex. Transition metals such as Fe, Co, and Ni play a critical role in numerous metabolic strategies in all chemoautotrophic microbes. Other examples of enzymes that rely on transition metals and sulfur include (1) nitrogenases, which use Mo, Fe, and S to reduce N2 to NH3; (2) hydrogenases, which employ Fe, Ni, and S to derive electrons from H2 to H+; and (3) aldehyde oxidoreductase enzymes, which use W and Mo to interconvert organic aldehydes and acids via electron transfer. TRANSITION METAL SULFIDES AND THE ORIGIN OF LIFE The ubiquitous role of transition metals and sulfur in key microbial enzymes has led to hypotheses proposing that sulfide minerals act as catalysts in organosynthesis. ELEMENTS (1) (2) Wächtershäuser postulated that the formation of pyrite would provide a catalyst to drive a broad range of essential protobiochemical reactions (Wächtershäuser 1992). Perhaps the most imaginative aspect of Wächtershäuser’s iron-sulfur world hypothesis was the proposal that Earth’s first organism existed not encapsulated in a roughly spherical cellular membrane, but rather protected from the environment by a half membrane on the surface of pyrite. He proposed that such an organism—the progenitor of all modern cells—eventually detached from the pyrite surface, taking with it the metabolic functionality it “learned” on the pyrite surface. British geochemists Michael Russell and Allan Hall (1997) have developed an alternative hypothesis regarding the role of transition metal sulfides in the origin of life. Their theory postulates that the early Hadean oceans were warm, mildly acidic, and relatively rich in dissolved Fe2+ and Ni2+. They propose that hydrothermal fluids generated within the ancient Hadean oceanic crust would have been highly alkaline, hot, rich in bisulfide, and reduced. Where these hydrothermal exhalations mixed with ocean water at the sea floor, reaction of the bisulfide with dissolved base metals would have resulted in the rapid precipitation of transition metal sulfides. Russell and Hall propose that iron sulfide bubbles formed at the sites of mixing, and that these bubbles might have served as primitive membranes. The purported advantage afforded by the FeS-membrane theory is that the membrane naturally separates a low pH exterior fluid from a high pH reduced interior fluid. Appealing to the comparison of cell membrane function in mitochondria and certain prokaryotes, Russell and Hall note that the presence of both pH gradients and Eh gradients would have allowed these membranes to support processes akin to electron-transport mediated chemistry in modern cells. One significant difference between Wächtershäuser’s model and that of Russell and Hall is that the latter does not utilize oxidation of FeS as an energy source, nor does it invoke pyrite as a catalytic agent for organosynthetic reaction. Rather, Russell and Hall propose that the FeS 140 J UNE 2005 membrane served a function similar to the hydrogenase and acetyl-coA enzymes, by catalytically converting the chemical potential of H2 + CO2 into biological energy generation and biochemical synthesis. MINERAL-CATALYZED ORGANIC SYNTHESIS Does mineral-catalyzed organic synthesis occur in nature? Barbara Sherwood Lollar and coworkers (2002) presented perhaps the most convincing evidence for locally extensive abiotic organosynthesis with their observations of hydrocarbon-rich gases, including methane, ethane, propane, and butane. These gases, collected from hard-rock terrains of the Canadian Shield in close proximity to the ancient Kidd Creek volcanogenic massive sulfide deposits, were unambiguously identified as abiotic based on their distinctive distribution of stable carbon and hydrogen isotopes. Deep-sea hydrothermal vents also are known to emit large quantities of methane derived from abiotic reduction of CO2 during the aqueous alteration of basalt and related rocks (Kelley et al. 2001). This so-called serpentinization reaction, wherein anhydrous olivine [(Mg,Fe)2SiO4] is converted to the hydrous mineral serpentine [Mg3Si2O5(OH)4] and magnetite (Fe3O4), has the capacity to generate copious quantities of hydrogen as a by-product (Berndt et al. 1996). The reactions most likely responsible for these natural hydrocarbon syntheses are Fischer-Tropsch Type reactions (FTT), by which carbon atoms in CO groups sequentially bind to a catalytic surface, are reduced by hydrogen, and are added to an elongating hydrocarbon chain (FIG. 2). The concentrations of hydrocarbons measured naturally by Sherwood Lollar et al. (2002) and generated experimentally by Berndt et al. (1996) are plotted against carbon number (1 = methane, 2 = ethane, 3 = propane, etc.) in FIGURE 3. The power law relationships exhibited in both cases are characteristic of FTT synthesis, suggesting that surface catalysis is responsible for the observed hydrocarbons. The log of the concentration of chain-like aliphatic hydrocarbons and alkane thiols as a function of the number and carbon atoms. Note that the data of Berndt et al. (1996) and Heinen and Lauwers (1996) are in log nanomoles, whereas the data of Sherwood-Lollar et al. (2002) are in log %. FIGURE 3 Also plotted in Figure 3 is an intriguing series of chain-like organic molecules called alkane thiols, which were generated in a very different experiment. To test one of Wächtershäuser’s central postulates, Heinen and Lauwers (1996) performed a series of reactions using FeS, H2S, and water, blanketed by an atmosphere of N2/CO2 or pure CO2, and demonstrated the apparently facile reduction of CO2 to form alkane thiols coincident with the formation of pyrite. The predominant product was methane thiol (CH3SH), followed by a series of alkane thiols up to the 5-carbon pentane thiol. Heinen and Lauwers reported that, following the reaction, a “silvery” floating layer, predominantly pyrite, formed. It is intriguing that the relationship between yield of alkane thiol and carbon number shown in Figure 3 is also consistent with a surface-catalyzed FTT process. The most likely site of the reaction was on the newly formed pyrite. Thus, iron sulfides are apparently also capable of promoting FTT synthesis reactions. The FTT reaction, however, is not immediately obvious as a useful starting point for chemistry that may have benefited the origins of life. Indeed, the facile conversion of CO2 and H2 to form methane leaves one in a difficult predicament for subsequent organosynthetic reactions because methane is not particularly useful in jump-starting metabolism. The formation of partially oxidized carbon species like alkane thiols, on the other hand, leaves open the possibility for much more interesting prebiotic chemistry. EXPERIMENTAL EXPLORATION OF METAL SULFIDE-PROMOTED REACTIONS A schematic representation of the catalytic, FischerTropsch Type (FTT) reduction of CO on a hypothetical transition metal solid, where the transition metal atoms are shown in purple and non-metal atoms are shown in green. A sequence of carbonyl insertion followed by reduction by H2 leads to progressive hydrocarbon chain growth. FIGURE 2 ELEMENTS Wächtershäuser’s groundbreaking hypothesis has inspired numerous experiments. Blöchl et al. (1992) and Kaschke et al. (1994), for example, tested the reducing power of the pyrite-forming reaction on a number of oxidized organic molecules and convincingly demonstrated the capacity of iron sulfide minerals to promote interesting organic reduction reactions. In order for transition metal sulfide-mediated chemistry to have true significance to prebiotic chemistry, however, it remained to demonstrate that these minerals 141 J UNE 2005 could promote reactions that link functionalized molecules, i.e., to form macromolecules, not just aid chemical reductions. Keller et al. (1994) tested this aspect of Wächtershäuser’s hypothesis by demonstrating the formation of nitrogencontaining amide bonds, which are critical to life’s biochemistry. What made Keller et al.’s reaction especially interesting is that they relied on a reactive intermediate molecule called a thioacid; but one cannot start with a thioacid because such compounds are unstable in hot water and rapidly react to form other compounds. Consequently, thioacids must be formed “on the fly,” as reaction intermediates. To accomplish this goal, they linked reactions wherein FeS was sulfidized with H2S in the presence of other reactants and produced a satisfying 2% yield of acetanilide in a matter of days. Notwithstanding the incremental progress demonstrated by these experimental results, the abiotic reaction that has received the most attention was that reported by Huber and Wächtershäuser (1997). They conceived a series of experiments designed to test whether transition metal sulfides are capable of mimicking the CO-insertion reaction that is promoted in cells by the acetyl-coA synthase enzyme complex (FIG. 4). Remarkably in one case, using an unidentified mixed (Ni,Fe)S compound synthesized during the experiment, they measured a 41% conversion of methane thiol to acetic acid, where the synthesis of acetic acid in particular was important in that it served to demonstrate a carbonyl inserting reaction. Huber and Wächtershäuser interpreted this unusual catalytic enhancement by the mixed (Fe,Ni)S phase as a consequence of CO having a greater affinity for iron, while CH3 has a greater affinity for nickel—an interpretation similar to that proposed for the acetyl-coA synthase enzyme, itself (FIG. 4). Other intriguing reactions are also attributed to this (Ni,Fe)S phase, notably the formation of so-called “peptide” bonds, by which amino acids link together to form proteins (Huber and Wächtershäuser 1998). This formation of peptide bonds is obviously important, but is surprising because the spontaneous condensation of amino acids to form pep- A schematic representation of the reaction between methane thiol (CH3SH) and carbon monoxide on a highly ordered surface of (Ni,Fe)S as proposed by Huber and Wächtershäuser (1997). It has been proposed that this (Ni,Fe)S phase catalyzes a carbonyl insertion reaction as follows: first, a methyl group derived from methane thiol is transferred to a nickel atom (green). An adjacent iron atom (blue) is carbonylated with carbon monoxide. Carbonyl insertion leads to the formation of the nickel-bound acetyl group. Nucleophilic attack by either hydoxyl, bisulfide, or methane thiol yields acetic acid, thioacetic acid, or methyl thioacetate, respectively. FIGURE 4 ELEMENTS tides is both thermodynamically and kinetically inhibited. Consequently, the reaction must have been coupled to another exergonic reaction. The likely mechanism was discovered recently by Leman et al. (2004), who found that carbonyl sulfide (COS) readily promotes the formation of peptides from free amino acids in the absence of any transition metal sulfide species. These results appear to suggest that the addition of CO to an aqueous solution containing the (Ni,Fe)S phase yields COS and some carbonylated Ni-Fe phase. As we will see, carbonylated transition metal complexes derived from metal sulfide minerals may have other prebiotic utility. Organometallic transition metal sulfide complexes are readily synthesized from monosulfides, e.g., NiS, CoS, and FeS. This fact, coupled with the successful demonstration of (Ni,Fe)S-catalyzed carbonyl insertion reactions (Huber and Wächtershäuser 1997), suggests another route to the formation of key metabolic molecules, such as pyruvic acid and other so-called “alpha ketoacids”. Thus, Cody et al. (2000) explored whether, under high pressure, reactions of alkane thiols, carbon monoxide, and FeS might provide a source of alpha ketoacids. Their experiments were run with pure FeS, aqueous formic acid (HCOOH), and the volatile 9-carbon molecule, nonane thiol, at 250°C and pressures up to 2000 atmospheres for six hours. After reaction, the product solutions had turned a brilliant red color, and UVvisible light and Raman spectroscopy of the solutions revealed the presence of carbonylated iron–sulfur species. The intensity of absorption, furthermore, increased substantially with increased pressure. Analysis of the products revealed substantial quantities of sulfur-containing organic molecules, as well as the ten-carbon molecule, decanoic acid. The synthesis of decanoic acid from a nine-carbon reactant is analogous to the formation of two-carbon acetate from one-carbon precursors by Huber and Wächtershäuser (1997). However, in the earlier experiments, FeS did not promote acetate synthesis. In the Cody et al. experiment, it is likely that the site of reaction was the carbonylated iron–sulfur clusters. Of particular interest to early, protometabolic chemistry was the detection of traces of the alpha ketoacids, pyruvic acid, and 2-oxo undecanoic acid. Pyruvic acid must have formed from methane thiol derived from CO reduction. Given the severe conditions of Cody et al.’s experiment, it is possible that the alpha ketoacids formed via double carbonylation followed by hydrolysis. These results taken in context with those of Huber et al. (2003) and Leman et al. (2004) point to the possibility that at least some transition metal sulfides react in the presence of CO to produce new phases that either directly or indirectly afford extremely useful protometabolic reactions. These results naturally also raise the question as to whether any of the transition metal sulfides are actually acting as surface catalysts. Certainly this appears to be the case for the FTT synthesis of alkane thiols by Heinen and Lauwers (1996). However, in the case of the carbonyl insertion reactions, the evidence is not as robust. This question has been addressed in a pair of papers that focused on targeted carbonyl insertion reactions for the purpose of (1) identifying a plausible protometabolic pathway to formation of familiar biological metabolic intermediates, e.g., citric acid, and (2) assaying a broader range of transition metal sulfide minerals for their capacity to promote carbonyl insertion reactions in general. In 2001, Cody et al. set out to test whether the transition metal sulfides have the capacity to convert simple olefins like propene (as are formed in Heinen and Lauwers’ 1996 experiment) to biologically important mono-, di-, and tricarboxylic acids. At each step, reactions run in the presence 142 J UNE 2005 of NiS yielded the targeted product. Particularly interesting, however, was the fact that while numerous different structural modifications, or isomers, of these products would be expected, a high degree of isomeric selectivity was observed. For example, while one would predict that two different 5-carbon dicarboxylic acids would be formed, reactions in the presence of NiS produced only one. This high degree of selectivity is consistent with these reactions being surface catalyzed. Another surprising result of these experiments was that in addition to the product carboxylic acids, partially oxidized compound products (thiol derivatives of methyl succinic acid) were also formed. The identity of the electron acceptor that promoted this reaction has not been determined, but it is certainly possible that a redox coupling reaction, in which NiS was reduced to Ni3S2, was involved. The formation of these organothiols provides support for the idea that a protometabolic path from CO2 up to citric acid catalyzed by transition metal sulfides is plausible. Expanding on these results, Cody et al. (2004) recently assayed the catalytic qualities of a broad range of transition metal sulfides for organosynthesis. This study revealed that many transition metal sulfides, including such common copper sulfides as chalcopyrite, bornite, and chalcocite, provide some catalytic function. Only FeS appears to suffer significant CO-derived dissolution; all the other transition metal sulfides are apparently stable under the reaction REFERENCES Berndt ME, Allen DE, Seyfried Jr WE (1996) Reduction of CO2 during serpentinization of olivine at 300°C and 500 bars. Geology 24: 351-354 Blöchl E, Keller M, Wächtershäuser G, Stetter KO (1992) Reactions depending on iron sulfide and linking geochemistry with biochemistry. Proceedings of the National Academy of Sciences 89: 81178120 Cody GD, Boctor NZ, Filley TR, Hazen RM, Scott JH, Sharma A, Yoder HS Jr (2000) Primordial carbonylated iron-sulfur compounds and the synthesis of pyruvate. Science 289: 1337-1340 Cody GD, Boctor NZ, Hazen RM, Brandes JA, Harold J, Morowitz HJ, Yoder HS Jr (2001) Geochemical roots of autotrophic carbon fixation: hydrothermal experiments in the system citric acid, H2O(±FeS)-(±NiS). Geochimica et Cosmochimica Acta 65: 3557-3576 Cody GD, Boctor NZ, Brandes JA, Filley TR, Hazen RM, Yoder HS Jr (2004) Assaying the catalytic potential of transition metal sulfides for abiotic carbon fixation. Geochimica et Cosmochimica Acta 68: 2185-2196 Doukov TI, Iverson TM, Seravalli J, Ragsdale SW, Drennan CL (2002) A NiFe-Cu center in a bifunctional carbon monoxide dehydrogenase/acetyl-coA synthase. Science 298: 567-571 Gottshcalk G (1986) Bacterial Metabolism. Springer, New York Heinen W, Lauwers AM (1996) Organic sulfur compounds resulting from the interaction of iron sulfide, hydrogen ELEMENTS conditions. It was further shown that there exists a strong correlation between reaction yield and mineral surface area, which strongly supports surface catalysis as a mechanism. It is clear from these results that many common transition metal sulfides could have provided catalytic function to jump-start metabolism on the prebiotic Earth. CONCLUSIONS Natural transition metal sulfide minerals can promote a broad range of organic reactions, either catalytically or as reaction participants. Whether and how this chemistry may have aided the emergence of life remains a mystery. It is clear, however, that organosynthetic reactions would have occurred independently of biological carbon fixation on the early Earth and that the first life would have had access to a broad range of molecular building blocks by virtue of the natural world’s capacity for abiotic organosynthesis. ACKNOWLEDGMENTS I gratefully acknowledge my collaborators in this area of research, Drs. N. Boctor, J. Brandes, T. Filley, M. Fogel, R. Hazen, J. Scott, and the late H. Yoder. Some of the research discussed in this paper was supported by a grant from NASA Exobiology (NAG5-13445) and the Carnegie-NASA Astrobiology Institute. . sulfide, and carbon dioxide in an anaerobic aqueous environment. Origins of Life and Evolution of the Biosphere 26: 131-150 Huber C, Wächtershäuser G (1997) Activated acetic acid by carbon fixation on (Fe,Ni)S under primordial conditions. Science 276: 245-247 Huber C, Wächtershäuser G (1998) Peptides by activation of amino acids with CO on (Ni,Fe)S surfaces: implications for the origin of life. Science 281: 670-672 Huber C, Eisenreich W, Hecht S, Wächtershäuser G (2003) A possible primordial peptide cycle. Science 301: 938-940 Kaschke M, Russell MJ, Cole WJ (1994) [FeS/FeS2]. A REDOX system for the origin of life. Origins of Life and Evolution of the Biosphere 24: 43-56 Keller M, Blöchl E, Wächtershäuser G, Stetter KO (1994) Formation of amide bonds with a condensation agent and implications for the origin of life. Nature 368: 836-838 Kelley DS, Karson JA, Blackman DK, FrühGreen GL, Butterfield DA, Lilley MD, Olson EJ, Schrenk MO, Roe KK, Lebon GT, Rivizzigno P, AT3-60 Shipboard Party (2001) An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30° N. Nature 412, 145-149 Qiu D, Kumar M, Ragsdale SW, Spiro TG (1994) Nature’s carbonylation catalyst: Raman spectroscopic evidence that carbon monoxide binds to iron, not nickel, in CO dehydrogenase. Science 264: 817-819 Russell MJ, Hall AJ, Cairns-Smith AG, Braterman PS (1988) Submarine hot springs and the origin of life. Nature 336: 117 Russell MJ, Hall AJ (1997) The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front. Journal of the Geological Society, London 154: 377-402 Sherwood Lollar B, Westgate TD, Ward JA, Slater GF, Lacrampe-Couloume G (2002) Abiogenic formation of alkanes in the Earth’s crust as a minor source for global hydrocarbon reservoirs. Nature 416: 522524 Wächtershaüser G (1988) Before enzymes and templates: theory of surface metabolism. Microbiological Reviews 52: 452-484 Wächtershaüser G (1990) Evolution of the first metabolic cycles. Proceedings of the National Academy of Sciences 87: 200204 Wächtershäuser G (1992) Groundworks for an evolutionary biochemistry: the ironsulfur world. Progress in Biophysics and Molecular Biology 58: 85-201 . Leman L, Orgel L, Ghadiri MR (2004) Carbonyl sulfide-mediated prebiotic formation of peptides. Science 306: 283286 Lengeler JW, Drews G, Schlegel HG, editors (1999) Biology of the Prokaryotes. Blackwell Science, New York, 955 pp 143 J UNE 2005