Sulfate reduction rates

The temperature dependency of the sulfate reduction rate for single sulfate-reducing bacteria is high, corresponding to a temperature coefficient, a, of about 1.13, i.e., a change in the rate with a factor Q10 = 3.4 per 10°C of temperature increase. Because diffusion of substrate into biofilms or sediments is typically limiting sulfide formation, the temperature coefficient is reduced to about... 

The cycle of iron solubilization will continue as long as bacteria and/or plants produce organic ligands.The cycle will stop when sulfate reduction rates are high and organic ligand production is low. At this point soluble hydrogen sulfide reacts with Fe(II) to form sulfide minerals. The iron cycle shown in Fig. 10.15 for salt marsh sediments may also occur in other marine sedimentary systems. 

Table I. Measurements of Sulfate Reduction Rates in Lake Sediments...

Factors Controlling Rates of Sulfate Reduction. Factors typically cited as controlling sulfate reduction include temperature, sulfate concentration, and availability of carbon substrates. Although sulfate-reducing bacteria typically exhibit steep responses to temperature (rates increase 2.4- to 3.7-fold per increase of 10 °C 85, 101, 105), neither differences between deep and shallow lakes (Table I) nor seasonal variation have been observed in rates of sulfate reduction (78, 85, 101). This apparent lack of response of sulfate reduction rates to changes in temperature may indicate that rates are limited by other factors. 

Figure 3. Rates of sulfate reduction (all measured with 35S) reported in the literature (references in Table I) show no obvious relationship to either sediment carbon content or carbon sedimentation rates (measured with sediment traps). The lowest reported rate of sulfate reduction occurs in the lake with the lowest carbon sedimentation rate, but there is no evidence of carbon limitation among the other lakes. Error bars indicate the range of reported sulfate reduction rates.

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... 

Possible sources of sulfate include diffusion from the water column, hydrolysis of sulfate esters, and oxidation of reduced sulfur. Diffusion of S042" into sediments cannot supply sulfate at the measured rates of sulfate reduction. Rates of sulfate diffusion into sediments generally are 2 orders of... 

Figure 4. A, Pore-water profiles (October 15, 1990) in Lake Sempach typically indicate that sulfate is consumed within the upper 3 cm. B, Diffusion rates for sulfate calculated from the profile in panel A (T = 5°C) indicate that the rate is maximal just below the interface, but all rates are less than 2 nmol/cm2 per hour. C, Sulfate reduction rates measured with 15S in intact cores on the same date are 2 orders of magnitude greater and do not exhibit the same depth profile as diffusion rates. Error bars indicate the standard deviation among 10-15 replicates. The sulfate profile in panel A was measured by centrifuging pore water from cores identical to those in which sulfate reduction was measured. D, The 35S measurements indicate that 50% of the areal sulfate reduction occurs below 2-cm depth and 25% occurs below 5-cm depth. The pore-water profile indicates that negligible...

Oxidation of Reduced S. Indirect evidence suggests that microbial oxidation of sulfide is important in sediments. If it is assumed that loss of organic S from sediments occurs via formation of H2S and subsequent oxidation of sulfide to sulfate (with the exception of pyrite, no intermediate oxidation states accumulate in sediments cf. 120, 121), the stated estimates of organic S mineralization suggest that sulfide production and oxidation rates of 3.6-124 mmol/m2 per year occur in lake sediments. Similar estimates were made by Cook and Schindler (1.5 mmol/m2 per year 122) and Nriagu (11 mmol/m2 per year 25). A comparison of sulfate reduction rates (Table I) and rates of reduced S accumulation in sediments (Table III) indicates that most sulfide produced by sulfate reduction also must be reoxidized but at rates of 716-8700 mmol/m2 per year. Comparison of abiotic and microbial oxidation rates suggests that such high rates of sulfide oxidation are possible only via microbial mediation. 

The observed sulfate reduction rates in freshwater sediments cannot be explained by diffusion of sulfate from the lake water into the sediment, because much steeper sulfate concentration gradients should then be observed. Assuming diffusive supply alone, Urban (28) calculated that the change of sulfate concentration with depth should take place within 1 mm instead of several centimeters, which are usually measured. This assumption, however, also means that the sulfate recycling process and the sulfate reduction rate should not be limited by the vertical transport of sulfide to (frequently solid) oxidants or to the oxic boundary layer. 

Figure 6. An idealized scheme for a sedimentary porous medium with pore walls covered by a biofilm. High sulfate reduction rates are maintained even in depths to which sulfate cannot diffuse because of recycling of sulfate within the biofilm. Numbered points (in black circles) denote the following processes I, Respiration consumes oxygen. 2, Microbial reduction of reactive metal Oxides. Reduction of reactive ferric oxides is in equilibrium with reoxidation of ferrous iron by Os. Thus, no net loss of reactive iron takes place in these layers. 3, Microbial reduction of ferric oxides. 4, Sulfate reduction rate (denoted as SRR). 5, Sulfide oxidation, either microbiologically or chemically. 6, Sulfide builds up within the hiofilm, sulfate consumption increases, reactive iron pool decreases. 7, Formation of iron sulfides.

The bulk solution becomes impoverished with respect to oxygen. However, the sulfate concentration remains constant as long as the recycling rate of reduced sulfur to sulfate is higher than the sulfate reduction rate (denoted SRR in Figure 6). In the opposite case the sulfate concentration of the bulk solution also decreases, and H2S is slowly enriched (point 6). The requirements for FeS precipitation and subsequent pyrite formation are then fulfilled (point 7). 

IV. Table II summarizes sulfate reduction rate data at both Third Sister and Frains Lakes and indicates a range of 0.7 to 3.2 mg S L 1 d 1 and a mean of 1.7 at both stations for 23 determinations. Table III includes data from putrefactive hydrogen sulfide production at the two lakes showing that rates varied from 0.13 at Frains Lake to 1.51 mg S L1 d 1 at Third Sister with a mean of approximately 0.4 mg S L 1 d 1. Hydrogen sulfide contributions from putrefaction, summarized in Table IV, were found to range from 5.1 to 53.0 percent with means of 27.6, 18.3 and 11.1 percent for Thira Sister West, East and Frains sites respectively. Although the sulfate reduction values are similar to those obtained by others (0.01-15 mg S L 1 d-1 14), there are no other putrefactive data with which to compare the values of Table III. Microbial enumeration data averaged 5 x 102 and 2 x 104 cells mL 1 for sulfate reducers and proteolytic bacteria respectively. These values are similar to those obtained by others (14). 

Sulfate retention in softwater lakes appears to increase in proportion to sulfate loadings (IQ). This raises the issue of factors that limit the magnitude of sulfate retention. At least three potential conditions might limit sulfate reduction rates 1) supply of Fe2+ to sequester reduced S, 2) supply of carbon to support microbial reduction, and 3) inhibition of sulfate reauction by acidification. 

Table 12.2 Comparison of depth-integrated sulfate reduction rates (upper 10-15 cm) in shallow-water subtidal and intertidal coastal sediments.

Crill., P.M., and Martens, C.S. (1987) Biogeochemical cycling in an organic-rich coastal marine basin. 6. Temporal and spatial variation in sulfate reduction rates. Geochim. Cosmochim. Acta 51, 1175-1186. 

Tg year x. Lein and Ivanov [71] have estimated the total sulfide burial in the Black Sea of 2.4 Tgyear 1 including about 1 Tgyear-1 that is buried in the anoxic zone. Using these data and integrated over the upper 20 cm of sediment sulfate reduction rates, Neretin and co-authors [75] concluded that the annual sulfide flux into the water column from sediments of the anoxic zone is between 3 and 5 Tgyear x. The value is likely to be overestimated due to spatial differences in pyrite burial rates and possible sulfide diffusion downward into the deeper sediment layers. 

Figure 3 Fractionation of SO4 by SRB versus specific sulfate reduction rate (SRR). The shaded area is the range of values observed in laboratory experiments. The points are from field data assuming two different densities of cells for comparison with lab data (Habicht and Canfield, 1997) (reproduced by permission of Elsevier from Geochim. Cosmochim. Acta, 1997, 61, 5351-5361).

S content of total solid-phase sulfur. Spy pyrite sulfur content. 6Spy isotopic composition of pyiite. Sorg organic sulfur. S elemental sulfur. SO4 pore-water sulfate concentration. 5SO4 isotopic composition of pore-water sulfate. H2S pore-water sulfide concentration. 5SH2S isotopic composition of pore-water sulfide. SRR sulfate reduction rate. 

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