Sulfur BiogeochemistryâPast and Present

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$Carbon isotope fractionation during calcium carbonate precipitation ...$

Physiological and ecological aspects of sulfur isotope fractionation 9pyruvate yields more than three times as much energy per moleof sulfate oxidized compared to the complete oxidation of acetateor benzoate. Although not a direct relationship, ultimately theamount of free energy conserved by the oxidation of an electrondonor will translate into the amount of ATP that can be formed forbiomass synthesis and cellular maintenance.Substrate Limitation as Regulator for Isotope FractionationIn continuous culture experiments with Desulfovibrio desulfuricans,fractionations decreased from 35‰ to 18‰ as sulfatereduction rates increased from 0.7 femtomoles cell −1 day −1 to 8.1femtomoles cell −1 day −1 (Chambers et al., 1975). Canfield (2001b)determined isotope fractionations on bulk sediment with naturalcommunities of sulfate-reducing bacteria, which were suppliedcontinuously with electron donors and acceptors. He observedlower fractionations at high sulfate reduction rates when lactatewas not limiting, and high fractionations at lower rates of sulfatereduction when lactate was supposedly limiting. Interestingly,at the bulk community level, changes in electron donor affectedwhole-sediment sulfate reduction rates, but the overall isotopefractionation varied little. Both studies rationalized their resultsin terms of the rate of cell-specific sulfate reduction and variableexchange of sulfur intermediates across the cell membrane usingthe rationale of Rees (1973). At high rates, a greater fraction of theintracellular sulfate pool is consumed, with the consequence thatthe isotope composition of sulfide approaches that of the importedsulfate (Fig. 3). At low rates, only a fraction of the intracellularsulfate is consumed. Consequently, the potential enzymatic isotopefractionation during the reductive process is expressed. Whilethe results of Chambers et al. (1975) indicate that cell-specificsulfate reduction rates influence isotope fractionation, the resultsof Canfield (2001b) allow multiple explanations. As indicatedabove, different sulfate-reducing bacteria likely consume lactateand acetate. Canfield (2001b) did not determine the end productsof carbon oxidation or investigate shifts in microbial communitycomposition despite the extended duration of his experiment. Pureculture investigations have demonstrated that the fractionationsproduced in the two metabolic pathways—lactate versus acetateoxidation—can differ by up to 40‰ (Detmers et al., 2001a),which raises the possibility that changes in isotope fractionationreflect changes in sulfate-reducing community composition.New Experimental Results of Isotope Fractionations byPure Cultures Grown on Multiple and Highly RefractorySubstratesTo further test the relationship between substrate variabilityand isotope fractionation, I present results from experiments withpure cultures capable of growth on multiple substrates, someof which are extremely refractory organic compounds. Isotopefractionations were determined for four different strains capableof growth on multiple substrates, following the same proceduresas described in Brüchert et al. (2001). The isotope fractionationFigure 3. Regulation of kinetic isotope effects at the adenosine phosphosulfatereductase and the dissimilatory sulfite reductase in a steadystateopen system. The relative percentages of reactant and product forthe two enzymatically catalyzed reductions depend on the availabilityof electrons carried by the electron transport chain to the DSR (dissimilatorysulfite reductase) and APSR (adenosine phosphosulfatereductase). APS does not accumulate in the cell and concentrations inthe cell are likely only nanomolar (Rabus et al., 2000). Therefore, theequilibrium for the reaction of adenosine phosphosulfate (APS)/sulfiteis far on the side of sulfite (gray shaded area in Fig. 3A). Even if fractionationoccurs during APS reduction, the near-complete consumptionof APS prevents a strong isotope effect. Isotope effects can occurat the DSR during reduction of sulfite to hydrogen sulfide. The largerthe intracellular sulfite pool, the larger the isotope effect, irrespectiveof whether sulfite reduction occurs via the trithionate pathway or thesingle 6-electron transfer. The size of the sulfite pool must be regulatedby the availability of electron donors. At low electron donor availability,the isotope effect is large, at high electron donor availability theisotope effect decreases to zero (Fig. 3B). The dashed line indicates theisotope composition of external sulfate for reference.was determined for Desulfobacterium autotrophicum for growthon hydrogen/CO 2and butyrate, and for the cold-adapted strainDesulfofrigus oceanensis and the Desulfobacter strain ASv 20 forgrowth on lactate and acetate. In addition, the thermophilic strainTD-3 (Rueter et al., 1994) was used to investigate fractionationof refractory organic substrates such as n-alkanes, longer-chainfatty acids, aromatics, and crude oil. Briefly, experiments wereperformed with growing batch cultures with an excess of organicsubstrate. Time points were taken in the lag, exponential, andstationary phase of growth. At each time point, the abundance oflive cells was counted. Sulfate and sulfide concentrations weredetermined by ion chromatography and spectrophotometry asdescribed in Brüchert et al. (2001). The isotope compositionof dissolved sulfate and sulfide was determined for each timepoint by EA IRMS using a ThermoFinnigan delta Plus Massspectrometer coupled to a Euro 3000 elemental analyzer via aConflo 2 interface. Accuracy and analytical precision were determinedusing the secondary standard BaSO 4standard NBS 127(δ 34 S versus Vienna Canyon Diablo Troilite [VCDT] = 20.0‰ ±0.3). Since the experiments were performed as closed systems,
10 V. Brüchertfractionation factor epsilon (ε) could be calculated using the Rayleighrelationship as described in Brüchert et al. (2001).Results are summarized in Table 3. In all experiments,fractionations exceeded 18‰ whenever the terminal product ofsubstrate oxidation was CO 2. Fractionations were
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Page 8: Preface viisulfurization on the pre
Page 11 and 12: 2 V. BrüchertThe resulting sulfur
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Page 15 and 16: 6 V. Brüchertnot yet been resolved
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Page 25 and 26: 16 V. BrüchertRudnicki, M.D., Elde
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60 A. SchippersGeochimica et Cosmoc
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62 A. SchippersSasaki, K., Tsunekaw
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Formation and degradation of seafl
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Distribution and fate of sulfur int
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- HSO 4SO 2-4H 2 SCO 32-HCO 3-HCO 3
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Sedimentary pyrite formation 121mar
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Sedimentary pyrite formation 123The
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Sedimentary pyrite formation 125Ben
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Sedimentary pyrite formation 127TAB
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Sedimentary pyrite formation 129The
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Sedimentary pyrite formation 131Dek
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Recent advances and future research
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Using sulfur isotopes to elucidate
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Sulfur isotope composition of carbo
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4 Ga of seawater evolution 197brine
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