Sulfate reduction in sediments

Naturally occurring snlfides in sediments and euxinic waters are commonly depleted in by np to 70%c (Jprgensen et al. 2004), far beyond the apparent capabilities of sulfate reducing bacteria. As has been shown above, most of the sulfide produced by sulfate reduction in sediments is reoxidized, often via compounds in which snlfm has intermediate oxidation states that do not accnmnlate, bnt... 

Existing data lend mixed support to the hypothesis that sulfate reduction is limited by availability of electron donors. Laboratory studies have shown that sulfate reduction in sediments can be stimulated by addition of carbon substrates or hydrogen (e.g., 85, 86). Increases in storage of reduced sulfur in sediments caused by or associated with addition of organic matter (108, 109) also have been interpreted as an indication that sulfate reduction is carbon-limited. Addition of nutrients to Lake 227 in the Experimental Lakes Area resulted in increased primary production and increased storage of sulfur in sediments (110, 111). Natural eutrophication has been observed to cause the same effect (23, 24, 112). Small or negligible decreases in sulfate concentrations in pore waters of ultra-oligotrophic lakes have been interpreted... 

All measured profiles of sulfate reduction in sediments indicate that much sulfide production and, by inference, oxidation occurs in permanently anaerobic sediments (78, 73, 90,101). The two most likely electron acceptors for anaerobic sulfide oxidation are manganese and iron oxides. Burdige and Nealson (151) demonstrated rapid chemical as well as microbially catalyzed oxidation of sulfide by crystalline manganese oxide (8-Mn02), although elemental S was the inferred end product. Aller and Rude (146) documented microbial oxidation of sulfide to sulfate accompanied by reductive dissolution... 

Nauhaus K., Boetius A., Kruger M., and Widdel E. (2002) In vitro demonstration of anaerobic oxidation of methane coupled to sulfate reduction in sediment from a marine gas hydrate area. Environ. Microbiol. 4, 296—305. 

Fossing H. and Jprgensen B. B. (1989) Measurement of bacterial sulfate reduction in sediments evaluation of a single step chromium reduction. Biogeochem. 8, 205-222. 

Adler, M., Hensen, C., Kasten, S. and Schulz, H.D., 2000. Computer simulation of deep-sulfate reduction in sediments off the Amazon Fan. International Journal of Earth Sciences (Geol. Rdsch.), 88 619-629. 

Joye, S.B., Boetius, A., Orcutt, B.N., Montoya, J.P., Schulz, H.N., Erickson, M.J., and Lugo, S.K. 2004. The anaerobic oxidation of methane and sulfate reduction in sediments from Gulf of Mexico cold seeps. Chemical Geology 205 219-238... 

Depletion of dissolved SOq in marine sediments is displayed over widely varying depth ranges, depending mainly on the interplay of rates of microbial metabolism, sediment accumulation and diffusive sulfate replenishment. Sulfate concentration gradients can be used with sedimentation rates, diffusion coefficients and model assumptions to estimate rates or rate constants for microbial sulfate reduction in sediments (Toth and Lerman, 1977 Berner, 1978 Canfield, 1991). There is a general proportionality between rates of sediment accumulation and sulfate reduction, but this relation is only weakly predictive for specific sites because of the influence of other factors on sulfate reduction rates (e.g. organic matter composition, microbial population density, etc.). 

There is evidence for the anaerobic degradation of alkanes to COj, plausibly under conditions of sulfate reduction. In experiments with sediment slurries from contaminated marine areas, was recovered from " C-hexadecane (Coates et al. 1997), and was inhibited by molybdate that is consistent with the involvement of sulfate reduction. Under sulfate-reducing conditions was produced from C[14,15]octacosane (CagHjg) (Caldwell et al. 1998). Different mechanisms have been elucidated for the anaerobic degradation of higher alkanes, and both occurred simultaneously in a sulfate-reducing consortium (Callaghan et al. 2006) ... 

Pallud, C. and P. Van Cappellen, 2006, Kinetics of microbial sulfate reduction in estuarine sediments. Geochimica et Cosmochimica Acta 70, 1148-1162. 

Bruchert V, Knoblauch C, Jorgensen BB (2001) Controls on stable sulfur isotope fractionation during bacterial sulfate reduction in Arctic sediments. Geochim Cosmochim Acta 65 763-776 Bryan BA, Shearer G, Skeeters JL, Kohl DH (1983) Variable expression of the nitrogen isotope effect associated with denitrification of nitrate. J Biol Chem 258 8613-8617 Canfield DE (2001) Biogeochemistry of sulfur isotopes. Rev Mineral Geochem 43 607-636 Chau YK, Riley JP (1965) The determination of selenium in sea water, silicates, and marine organisms. Anal Chim Acta 33 36-49... 

Habicht KS, Canfield DE (1997) Sulfur isotope fractionation during bacterial sulfate reduction in organic-rich sediments. Geochim CosmochimActa61(24) 5351-5361 Habicht KS, Gade M, Thamdrup B, Berg P, Canfield DE (2002) Calibration of sulfate levels in the Archean ocean. Science 298 2372-2374... 

Rouxel O, Galy A, Elderfield H (2006) Germanium isotope variations in igneous rocks and marine sediments. Geochim Cosmochim Acta 70 3387-3400 Rouxel O, Ono S, Alt J, Rumble D, Ludden J (2008) Sulfur isotope evidence for microbial sulfate reduction in altered oceanic basalts at ODP Site 801. Earth Planet Sd Lett 268 110-123 Rozanski K, Sonntag C (1982) Vertical distribution of deuterium in atmospheric water vapour. Tellus 34 135-141... 

Rudnicki MD, Elderfield H, Spiro B (2001) Fractionation of sulfur isotopes during bacterial sulfate reduction in deep ocean sediments at elevated temperatures. Geochim Cosmochim Acta 65 777-789... 

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Figure 2. Rates of sulfate reduction in lake sediments reported in the literature range over 3 orders of magnitude and are not correlated with lake sulfate concentrations. All measurements were made with l5S in intact cores or core sections. References are given in Table I.

Much confusion exists over the effects of sulfate concentration and carbon availability on rates of sulfate reduction (cf. 1, 4, 5, 72, 85, 106). Sulfate reducers in lake sediments exhibit low half-saturation constants for sulfate (10-70 xmol/L 4, 72, 78, 85) as well as for acetate and hydrogen (4, 13, 87). The low half-saturation constants allow them to outcompete methano-gens for these substrates until sulfate is largely consumed within pore waters (4, 90). Low concentrations of sulfate in lakes confine the zone of sulfate reduction to within a few centimeters of the sediment surface (e.g., 4, 90, 98). The comparability of rates of sulfate reduction in freshwater and marine... 

Relative rates of sulfate reduction and methanogenesis in lakes of varying trophic status are claimed to indicate that sulfate reduction rates are limited by the supply of sulfate (4, 5, 13). According to this hypothesis, at high rates of carbon sedimentation, rates of sulfate reduction are limited by rates of sulfate diffusion into sediments, and methanogenesis exceeds sulfate reduction. In less productive lakes, rates of sulfate diffusion should more nearly equal rates of formation of low-molecular-weight substrates, and sulfate reduction should account for a larger proportion of anaerobic carbon oxidation. Field data do not support this hypothesis (Table II). There is no relationship between trophic status, an index of carbon availability, and rates of anaerobic... 

Hydrolysis of sulfate esters also cannot supply the quantity of sulfate required for sulfate reduction. Hydrolysis of sulfate esters has not been measured directly in any lakes (cf. 73, 83), but the annual supply of sulfate esters is less than annual rates of sulfate reduction. In Wintergreen Lake the annual supply of ester sulfate to the sediments is only 4% of annual sulfate reduction (73). Similarly, in Little Rock Lake the supply of ester sulfate is less than 1% of the rate of sulfate reduction (72). In both lakes, hydrolysis of sulfate esters is estimated to be less than half of the rate of supply to the sediments. 

The studies cited do not clarify what factors determine rates of sulfate reduction in lake sediments. The absence of seasonal trends in reduction rates suggests that temperature is not a limiting factor. Rates of sulfate reduction are not proportional to such crude estimates of carbon availability as sediment carbon content or carbon sedimentation rate, although net reduction and storage of reduced sulfur in sediments often does increase with increasing sediment carbon content. Measured rates of sulfate reduction are not proportional to lake sulfate concentrations, and the relative rates of sulfate reduction and methanogenesis in a variety of lakes do not indicate that sulfate diffusion becomes limiting in eutrophic lakes. Direct comparison of diffusion and reduction rates indicates that diffusion of sulfate into sediments cannot supply sulfate at the rates at which it is reduced. Neither hydrolysis of sulfate... 

Sulfate Reduction in Lake Sediments A Biofilm Model... 

Mass balance calculations clearly show that sulfate is removed from the water column by in-lake processes. Three processes are potentially important 1) diffusion of sulfate into sediments and subsequent reduction, 2) sedimentation of seston, and 3) dissimilatory sulfate reduction in the hypolimnion. 

Finally, there is a potential for inhibition of sulfate reduction by sediment acidification in highly impacted sites. In the first two years of experimental acidification of Little Rock Lake there is no evidence of decreased pH in porewater 1 cm below the interface. It is not clear, however, whether sediment acidification will occur with further increases in acid loadings to the lake. Rudd et al. (fi) showed that porewaters from lakes Hovattn and little Hovattn were acidic at fall turnover and postulated that this may occur by oxidation of reduced sulfur compounds. Although sediments from 223 showed no evidence of acidification after 10 years of experimental lake acidification, the pH of porewater from Lake 114 declined by > 0.5 units after just three years of experimental acidification (fi). 

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