Concurrent low- and high-affinity sulfate reduction kinetics in marine sediment

118

117

The authors note that on page E1484, right column, first full paragraph, line 8, "the anaerobic oxidation of methane with sulfate (AOM) (1)" should instead appear as "the anaerobic oxidation of methane (AOM) with sulfate (1)."Also on page E1484, right column, Eq. 2 and its explanation appeared incorrectly. The corrected equation and its corrected explanation appear below.[2](Index e indicates activity ratio applying for equilibrium, viz. ΔG = 0; {H 2 O} is taken as 1) and thus one of the least exergonic processes sustaining life (ΔG in situ is often between −20 and −40 kJ mol −1 ) (21-23). On page E1485, right column, third full paragraph, lines 4-9, "If the subsequent reactions occur with reversibility and are viewed from the molecular perspective (that is, regarding microstates stochastically) at a given moment, the larger fraction of the enzyme molecules of each reaction performs the forward reaction, whereas a smaller fraction simultaneously performs the reverse reaction" should instead appear as "If subsequent reactions occur with reversibility and if the involved population of enzyme molecules is viewed from the molecular perspective (that is, regarding microstates stochastically) within a time interval in the range of an enzymatic turnover, the forward reaction is catalyzed more frequently than the reverse reaction."On page E1487, right column, Eq. 11 within the footnote appeared incorrectly. The corrected equation appears below. ∂Last, Fig. 3 appeared incorrectly. The corrected figure and its corresponding legend appear below. These errors do not affect the conclusions of the article.www.pnas.org/cgi/doi/10.1073/pnas.1218683109 Fig. 3. Depiction of forward and back flux during the net reaction A → P in a steady state catalytic system with reversible but otherwise unknown internal reactions. Such a system can consist of an entire pathway (A) or a single enzymatic reaction (B; E + A → EA → P + E) (Figs. S4 and S5). All reactions, including uptake or binding and release, are reversible. Arrows indicate rates (velocities). The forward (v + ) and back (v − ) rates correspond to individual steps in a catalytic system. The index n refers to the number of forward and backward fluxes (B; n = 2 for a single enzymatic reaction) (Fig. S5). A description of abbreviations is given in Table S3. (A) Catalytic system: the fate of substrate and product is followed by the different labels (A (Fig. S5) in a steady state (pools of A and P are essentially constant). The sizes of the rates are indicated by lengths of arrows. The same net rate (v) is the difference of uptake (binding) and release of substrate (v +1 , v −1 ) and product (v +2 , v −2 ) as well as the difference between forward and back flux (f + , f − ). The rate constants for the reaction steps (k +1 , k −1 , k +2 , k −2 ) and the actual concentrations of E, A, and P determine v +1 , v −1 , v +2 , v −2 , respectively, as indicated, and the resulting f + , and f − , which can only be revealed by labeling (Eqs. S35 and S36). The vector model was calculated for a...

Section: Pool Ofsupporting

confidence: 63%

Carbon and sulfur back flux during anaerobic microbial oxidation of methane and coupled sulfate reduction

Holler

Wegener

Niemann

et al. 2011

118

117

“…Microbial sulfate reduction can occur over a wide span of sulfate and sulfide concentrations. It is sustained at sulfate concentrations from hundreds of millimolar, as found in some hypersaline soda lakes (58), down to tens of micromolar, as shown by precise measurements of the sulfate affinity of actively growing sulfate reducers (59). Sulfide concentrations much higher than tens of millimolar, however, appear to inhibit microbial sulfate reduction (60).…”

Section: Resultsmentioning

confidence: 99%

Intracellular metabolite levels shape sulfur isotope fractionation during microbial sulfate respiration

Wing

Halevy

2014

214

266

We present a quantitative model for sulfur isotope fractionation accompanying bacterial and archaeal dissimilatory sulfate respiration. By incorporating independently available biochemical data, the model can reproduce a large number of recent experimental fractionation measurements with only three free parameters: (i) the sulfur isotope selectivity of sulfate uptake into the cytoplasm, (ii) the ratio of reduced to oxidized electron carriers supporting the respiration pathway, and (iii) the ratio of in vitro to in vivo levels of respiratory enzyme activity. Fractionation is influenced by all steps in the dissimilatory pathway, which means that environmental sulfate and sulfide levels control sulfur isotope fractionation through the proximate influence of intracellular metabolites. Although sulfur isotope fractionation is a phenotypic trait that appears to be strain specific, we show that it converges on near-thermodynamic behavior, even at micromolar sulfate levels, as long as intracellular sulfate reduction rates are low enough (<<1 fmol H 2 S·cellissimilatory sulfate reduction is a respiratory process used by some bacteria and archaea to generate energy under anaerobic conditions. Aqueous sulfate serves as the terminal electron acceptor in this process, leading to the oxidation of organic carbon compounds and sometimes hydrogen and to the production of aqueous sulfide (1). Dissimilatory sulfate respiration was one of the first microbial metabolisms to be isotopically characterized through culture experiments (2), with 32 S-bearing sulfate shown to be consumed preferentially to 34 S-bearing sulfate. Early experiments identified two critical features of this dissimilatory sulfur isotope fractionation: Its magnitude correlates inversely with the sulfate reduction rate of an individual cell but correlates directly with extracellular sulfate concentrations (3-5).Through careful regulation of the environmental controls on respiration, more recent experiments have precisely calibrated these relationships and suggest that their particular form may be strain specific (6-11). All experiments, however, show a nonlinear response, where sulfur isotope fractionation increases rapidly with decreasing rate. At the low-rate limit, sulfur isotope fractionation appears to approach levels defined by thermodynamic equilibrium between aqueous sulfate and sulfide (8, 12), the initial reactant and final waste product in the respiratory processing chain.In parallel with experimental studies, theoretical work has built a broad foundation for understanding the net sulfur isotope fractionation expressed during sulfate respiration (13-17). These efforts initially dealt with sulfur flow through simplified metabolic networks (13) (Fig. 1A) and have expanded to incorporate, for example, electron supply to the reaction cycles of individual respiratory enzymes (17). The reversibility of an individual enzymatic reaction is a central theoretical concept behind these approaches, as it carries the isotopic memory of downstream steps in the pathway...

“…Strain DvH is among the most well-studied sulfate reducers and is genetically tractable (36,37). Although DvH is nominally a nonmarine strain, the concentration of sulfate in the chemostat was always near modern marine levels (28 mM) and well above known sulfate affinity constants (38). The sole electron donor (lactate) was always the limiting nutrient in these experiments and was provided at a stoichiometric 1:2 or 1:20 ratio with sulfate (Materials and Methods and SI Text).…”

Section: Resultsmentioning

confidence: 99%

Influence of sulfate reduction rates on the Phanerozoic sulfur isotope record

Leavitt

Halevy

Bradley

et al. 2013

298

276

Phanerozoic levels of atmospheric oxygen relate to the burial histories of organic carbon and pyrite sulfur. The sulfur cycle remains poorly constrained, however, leading to concomitant uncertainties in O 2 budgets. Here we present experiments linking the magnitude of fractionations of the multiple sulfur isotopes to the rate of microbial sulfate reduction. The data demonstrate that such fractionations are controlled by the availability of electron donor (organic matter), rather than by the concentration of electron acceptor (sulfate), an environmental constraint that varies among sedimentary burial environments. By coupling these results with a sediment biogeochemical model of pyrite burial, we find a strong relationship between observed sulfur isotope fractionations over the last 200 Ma and the areal extent of shallow seafloor environments. We interpret this as a global dependency of the rate of microbial sulfate reduction on the availability of organic-rich sea-floor settings. However, fractionation during the early/mid-Paleozoic fails to correlate with shelf area. We suggest that this decoupling reflects a shallower paleoredox boundary, primarily confined to the water column in the early Phanerozoic. The transition between these two states begins during the Carboniferous and concludes approximately around the Triassic-Jurassic boundary, indicating a prolonged response to a Carboniferous rise in O 2 . Together, these results lay the foundation for decoupling changes in sulfate reduction rates from the global average record of pyrite burial, highlighting how the local nature of sedimentary processes affects global records. This distinction greatly refines our understanding of the S cycle and its relationship to the history of atmospheric oxygen.Phanerozoic oxygen | sulfate-reducing bacteria T he marine sedimentary sulfur isotope record encodes information on the chemical and biological composition of Earth's ancient oceans and atmosphere (1, 2). However, our interpretation of the isotopic composition of S-bearing minerals is only as robust as our understanding of the mechanisms that impart a fractionation. Fortunately, decades of research identify microbial sulfate reduction (MSR) as the key catalyst of the marine S cycle, both setting the S cycle in motion and dominating the massdependent fractionation preserved within the geological record (1, 3, 4). Despite the large range of S-isotope variability observed in biological studies (4-6), attempts to calibrate the fractionations associated with MSR are less mechanistically definitive (7, 8) than analogous processes influencing the carbon cycle (9, 10). What is required is a means to predict S isotope signatures as a function of the physiological response to environmental conditions (e.g., reduction-oxidation potential).Microbial sulfate reduction couples the oxidation of organic matter or molecular hydrogen to the production of sulfide, setting in motion a cascade of reactions that come to define the biogeochemical S cycle. In modern marine sediments, sulfide i...