Sulfur BiogeochemistryâPast and Present

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Physiological and ecological aspects of sulfur isotope fractionation 7Differences in isotope fractionation may be deduced fromthe structural and compositional properties of the dissimilatorysulfite reductase enzymes. DSR enzymes are distinguished onthe basis of their spectroscopic properties. There are four typesof dissimilatory sulfite reductases known for sulfate-reducingbacteria: Desulfiviridin, Desulforubidin, Desulfofuscidin, andP-582 (Rabus et al., 2000). These enzymes share the presenceof a siroheme complex responsible for the transfer of electronsto sulfite, which is exchange-coupled to a 4Fe-4S reactive center(Rabus et al., 2000; Steuber and Kroneck, 1998). Sulfite is ligatedto the iron atom of the siroheme. The specific configuration ofthe siroheme complex and the exchange coupling of sulfite withthe 4Fe-4S cluster are responsible for the size of the isotopefractionation. There is some indirect evidence for structural differencesbetween DSR enzymes, which may be deduced fromthe phylogenetic distance of dissimilatory sulfite reductase genesof different sulfate reducers (Hipp et al., 1997). Gene sequencedifferences of DSR and APSR also affect the tertiary structureof the enzymes (Hipp et al., 1997). These sequence differencesinfluence the three-dimensional structure of the docking sites forthe APS and sulfite, and may modify the kinetic property of theenzymes (Steuber and Kroneck, 1998).However, sequence analyses of the dissimilatory sulfitereductase genes of different sulfate reducers suggest that thesequences encoding the active center of the enzyme are highlyconserved (M. Bauer, 2002, personal commun.), which pointsto a similar structure of the active center and suggests a generalsimilarity of fractionation. A comparison of isotope fractionationand type of DSR enzyme present in 31 species of sulfate-reducingbacteria also suggested no relationship between isotopefractionation and enzyme type (Klein et al., 2001; Detmers et al.,2001a). Nevertheless, it is important to point out that sequencehomology does not equal functional equality. Protein- or DNAbasedamino acid sequences of different APSR or DSR enzymesmay be statistically homologous, but the kinetic properties ofhomologous enzymes may not be identical. Multiple lateral genetransfers have occurred for the DSR and APSR (Klein et al.,2001; Friedrich, 2002). Lateral gene transfer results in sequencesimilarities of enzymes in distant lineages of sulfate-reducingbacteria. For this reason and from the arguments presentedabove, it will be difficult to link differences in isotope fractionationto gene sequence dissimilarity. Kinetic experiments withpurified enzymes of different types and computational modelingof the binding configuration of the electron acceptor may providea better understanding of the differences in isotope fractionationassociated with a particular enzyme type.EFFECT OF TEMPERATURE AND SULFATECONCENTRATION ON ISOTOPE FRACTIONATIONTemperature has been considered one of the cardinal regulatorsfor isotope fractionation. In general, temperature affectsrates of sulfate reduction because the specific activity of theenzymes slows down (Rabus et al., 2002). Early experimentsby Kaplan and Rittenberg (1964) suggested that the sulfur isotopefractionation increased as rates decreased with decreasingtemperature. However, these experiments were performed withresting cells of cultures that were incubated at temperatures atwhich the bacteria could not grow and more likely exhibitedstress responses rather than healthy physiological characteristics.Experiments with growing psychrophilic and mesophilicstrains over temperatures at which the bacteria were viablehave shown no effect of temperature on isotope fractionation(Brüchert et al., 2001).These findings have implications for the regulation ofisotope fractionation. The underlying assumption of the modelby Rees (1973) was that a less stringent sulfate demand at lowrates of sulfate reduction would enhance isotope fractionation.According to Equation 1, at low temperatures and high concentrationsof sulfate and electron donor, the rate of reduction is thedominating regulator for isotope fractionation, since there wouldbe no limitation in sulfate uptake. In fact, the constant isotopefractionation in the experiments cited above showed that overtheir viable temperature ranges these sulfate reducers neverexperienced uptake limitation, although the cell-specific sulfatereduction rates varied more than 10-fold (Brüchert et al., 2001).Even at the highest sulfate reduction rates, the isotope fractionationswere the same as those at the lowest rates.These experiments provide further evidence that the Rees(1973) model of reverse 34 S-enriched sulfate transport out of thecell requires modification. According to the model, the lowerthe rates of sulfate reduction, the higher the fractionation, withthe consequence that more 34 S-enriched sulfate or other sulfurcompounds have to be transported out of the cell. However,since the experiments by Brüchert et al. (2001) demonstratedconstant fractionations independent of the rate of sulfatereduction, sulfur compounds that were transported out of thecell must have had the same isotope composition at all rates.These experiments clearly demonstrate that, at seawater sulfateconcentration, the cardinal regulator for isotope fractionation isthe rate of electron donor transport, which regulates the isotopecomposition of sulfate taken up. At all experimental rates, thecells were not sulfate-limited, with the consequence that isotopefractionations did not vary.Similar considerations apply to the role of sulfate concentrationon isotope fractionation. Recently, Habicht et al. (2002)demonstrated for Archaeoglobus fulgidus reduced isotopefractionation at sulfate concentrations below 200 µM. Whensulfate concentrations exceeded 1 mM, there was no effect onisotope fractionation. To summarize, the effect of sulfate uptakeon isotope fractionation for many environments is likely minorgiven the high concentrations of sulfate in the world ocean andin surface sediment pore waters. Fractionations will decreaseonly when sulfate concentrations of the ambient environmentare extremely low. This may be the case in recent sedimentaryenvironments with concentrations of sulfate less than 1%, themodern seawater concentrations of 28 mM, and in the earlyPrecambrian ocean.
8 V. BrüchertEFFECT OF ELECTRON DONORS ON ISOTOPEFRACTIONATIONThe early studies on regulation of sulfur isotope fractionationby Kaplan and Rittenberg (1964) and Kemp and Thode(1968) suggested an effect of the electron donor on isotopefractionation. The smallest fractionations were observed forhydrogen, followed by lactate, acetate, and ethanol (Kaplan andRittenberg, 1964). In the experimental design used by Kaplanand Rittenberg (1964), cell-specific sulfate reduction rates wereslower with ethanol and acetate than with lactate, which led theauthors to conclude that different substrates could result in differentisotope fractionations insofar as cell-specific sulfate reductionrates varied. By contrast, recent studies with a large numberof pure cultures capable of growth on a variety of electron donorshave indicated no unique relationship between cell-specific sulfatereduction rate and electron donor (Detmers et al., 2001a).The new finding of this study was that the particular substrateoxidation pathways used by sulfate reducers are reflected in thesulfur isotope fractionation.Substrate Oxidation Pathways as Regulators for IsotopeFractionationElectron donor flow during bacterial sulfate reduction canbe simplified into two categories: (1) the oxidation of hydrogenand the incomplete oxidation of lactate, propionate, or pyruvate toacetate involving a periplasmic hydrogenase, and (2) the completeoxidation of organic electron donors to CO 2(Rabus et al., 2000).Desulfovibrio species have a periplasmic hydrogenase, butthey do not have a carbon monoxide dehydrogenase that can oxidizecoenzyme A-bound acetate to CO 2. Acetate must thereforebe released as the terminal oxidation product. The schematicsof sulfate reduction coupled to hydrogen oxidation shown inFigure 2 illustrate the potential effect on isotope fractionation.The periplasmic location of the hydrogenase has the principaladvantage that electrons are translocated through the cytoplasmicmembrane by electron-carrying cytochromes, while the protonsremain in the periplasm to generate a proton motive force thatcan be used for ATP synthesis (Fig. 2). Cytochrome C3 and anadditional unknown electron carrier transport the electrons to theAPSR and the DSR (Steuber and Kroneck, 1998). It is conceivablethat this mechanism provides a direct electron shuttle to theAPSR and DSR. The consequence is an efficient consumption ofAPS with little buildup of intermediate sulfite, which would minimizeisotope fractionation. In agreement with this hypothesis arethe higher growth rates of sulfate-reducing bacteria when growingon hydrogen, pyruvate, or lactate compared to acetate (Rabus etal., 2000). In contrast, there are two known pathways for completeelectron donor oxidation. One, used by Desulfonema and Desulfobacteriumspecies, involves the oxidation of acetate, bound asacetyl CoA, to CO 2using a carbon monoxide dehydrogenase(Schauder et al., 1986). The other pathway is used by Desulfobacterspecies and employs a modified version of the citric acidcycle (Brysch et al., 1987). Both pathways involve significantlymore intermediates and enzymes to catalyze the carbon transformationwhen compared to the oxidation of lactate-hydrogenpyruvate.It is therefore conceivable, but difficult to demonstratedirectly, that the type of electron donor pathway affects isotopefractionation because the rate of transfer of electrons along theelectron transport chain to the APSR and DSR affects the residencetimes of the intermediate APS and sulfite pools. In essence,the hypothesis is that transport of electron donors and acceptorsmay be less tuned in complete-oxidizing than in incomplete-oxidizingsulfate-reducing bacteria. Some support for this hypothesismay be drawn from the free energy yield for the oxidationof different electron donors with sulfate (Detmers et al. 2001a)(Table 2). The incomplete oxidation of lactate and propionate and
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Recent advances and future research
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Using sulfur isotopes to elucidate
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Sulfur BiogeochemistryâPast and Present