Unravelling Earth’s Sulfur Cycle

Sulfur fractionated by enzymatic catalysis has been harvested in the laboratory from living cultures of sulfur-metabolizing microbes and analyzed for ³²S-³³S-³⁴S. The species Archhaeoglobus fulgidus, Desulfovibrio gigas, Thermodesulfovibrio yellowstonii, Desulfobacter hydrogenophilius, D. jorgensenii, D. autotrophicum, D. thiozymogenes, D. sulfodismutans, and a culture of sulfur disproportionators from Gulfo Dulce, Costa Rica, all have mass-dependent sulfur isotope fractionations. A model that describes the mass-dependent sulfur isotope fractionations for sulfate reducers and sulfur disproportionators has been developed. In that model, as has been demonstrated in studies of oxygen isotope fractionation, mass dependent reactions transfer non-mass dependent signatures from reactants to products, albeit diminished in magnitude by dilution. Microbes may have played an important dual role in both preserving but also diluting anomalous sulfur isotope fractionations. As an agent of preservation, microbial enzymatic catalysis precipitates pyrite, a secure storage site for sulfur isotopes, but as an inhibitor, the full magnitude of isotope anomalies may not be preserved because of dilution.

Applying these ideas to sulfur isotope measurements on single pyrite grains from a single bed of 3.5-Ga chert from the North Pole district, Western Australia (Figure 1), indicates behavior expected from the microbial metabolism of sulfur. The North Pole microbes accepted ambient sulfur with a non-mass dependent fractionation of ³³S, metabolized it mass dependently, and left behind a linear array of δ³³S versus δ³⁴S values offset from bulk Earth by -0.3 per mil in Δ³³S.

Figure 1. Sulfur isotope composition of pyrite grains from a single sedimentary bed of chert from the North Pole District, Pilbara, Western Australia. The dashed line gives compositions for bulk Earth.

The Critical Role of Sulfur in Prebiotic (Protometabolic) Organic Chemistry

Over the past year Co-Investigator Cody and his colleagues have been focusing on a potentially important set of prebiotic reactions. Following up on their earlier work with hydrothermal reactions with citric acid they began exploring similar chemistry in the presence of NH₃-NH₄⁺ and in the presence of transition metal sulfides. They previously demonstrated that metal sulfide catalyzed reactions could provide a source of citric acid under reduced hydrothermal conditions. Citric acid is a useful source of alpha-keto acids (oxalacete and pyruvate) via reaction path α (Figure 2). The natural question arises as to whether one can convert these alpha-keto acids to their respective amino acids (aspartate and alanine) via a meta-sulfide-catalyzed reductive amination. The use of heterocatalysis (i.e., surface catalysis) offers the tantalizing possibility that regioselective reduction of the imine intermediate might be topologically controlled on the mineral surface and lead to enantiomeric excesses.

Figure 2. The principal degradative pathways of citric acid in hot water. Reaction pathway α mimics the branch point in the reverse citrate cycle, i.e., the retro Aldol reaction that leads to the formation of oxaloacetate and acetate. Oxaloacetic acid undergoes a facile decarboxylation to yield pyruvic acid and CO₂. Pyruvic acid undergoes an oxidative decarboxylation yielding acetic acid, CO₂ and H₂. A separate degradation pathway, ß , involves citric acid eliminating water to form aconitic acid, which undergoes a decarboxylation to form itaconic acid and CO₂ (note that itaconic acid exists in equilibrium with two other unsaturated isomers as well as the hydroxylated dibasic acid, citramalic acid). Itaconic acid decarboxylates to yield methacrylic acid and CO₂, which in turn can decarboxylate to form propene. Abiotic reversal of the α pathway does not appear probable. However, under reducing conditions and in the presence of effective catalysts (such as NiS ) a pathway, ß ', exists that can plausibly lead from propene up to citric acid.

The team has been extensively exploring a range of reaction conditions that might be favorable for the transformation of citric acid to aspartate and alanine. They have found that alanine synthesis is highly favorable, but aspartate synthesis is not. Evidently the rate of decomposition of oxalacetate to pyruvate occurs too rapidly to favor reductive amination. They have found only one set of conditions that produce trace quantities of aspartate. These difficulties aside, they discovered other reactions that were unanticipated and may be of importance to prebiotic chemistry. Toward this end they are now refining their understanding of the reaction space through numerous (hundreds to date) experiments of varying T, P, and composition.

In addition to these experiments they completed the synthesis of ³⁴S-labeled metal sulfides to explore the extent of sulfur exchange accompanying hydrothermal reactions with alkyl thiols. In particular they are focusing on the metal-sulfide-promoted synthesis of citric acid, wherein a number of thiol intermediates form via a redox reaction with the metal sulfide. Cody and colleagues intend to explore further metal-sulfide-catalyzed reductive amination reactions, and they are particularly interested in identifying metal sulfide phases that will promote this reaction at lower temperatures. If they can obtain reasonable yields of amino acids at temperatures less than 100 °C, there is a chance that any enantiomeric excesses formed via regioselective reductions on the catalytic surfaces could be preserved against the long-term probability of racemization.

Finally, the group has moved and re-installed the high-pressure flow reactor into their hydrothermal laboratory. This summer they will replumb the system and begin long-term studies using the flow reactor and fluorescent molecular probes to detect products at extreme dilution.