Sulfate reduction and methane oxidation

The coincidence of maxima in the methane oxidation rate and the sulfate reduction rate in Saanich Inlet strongly suggests that the methane oxidizing agent was sulfate, either via direct reaction, or coupled indirectly through reactions with other substrates (Devol, 1983). A methane-sulfate coupled reaction diffusion model was developed to describe the inverse relationship commonly observed between methane and sulfate concentrations in the pore waters of anoxic marine sediments. When fit to data from Saanich Inlet (B.C., Canada) and Skan Bay (Alaska), the model not only reproduces the observed methane and sulfate pore water concentration profiles but also accurately predicts the methane oxidation and sulfate reduction rates. In Saanich Inlet sediments, from 23 to 40% of the downward sulfate flux is consumed in methane oxidation while in Skan Bay this value is only about 12%. 

Methane oxidation rates were proportional to the concentrations of methane, and also increased with increasing methane concentrations in the absence of sulfate or the presence of molybdate. When sulfate was added to sulfate-depleted incubation bags, methane oxidation rates decreased immediately to less than half the rate measured prior to the addition, while sulfate reduction was stimulated. When molybdate (a specific inhibitor of sulfate-reducing bacteria) was added to a sulfate-free incubation bag, methane oxidation responded after a lag period of approximately 3 days by uncoupling methane oxidation rates from methane concentrations. Methane production was not affected. From the outcome of their incubation bag experiments, we conclude that methane is not, as previously proposed, oxidized by sulfate reducers alone. 

Rates of methane production via both acetate fermentation and C02 reduction were directly measured with radiotracer techniques in the sulfate-depleted sediments of Saanich and Princess Louisa Inlets (Kuivila etal., 1990). Comparison of measured and modeled rates suggests that these two pathways account for the majority of methane produced below the sulfate reduction zone in the sediments of both the basins. Prior aerobic degradation of the organic matter has little influence on the pathways of methane production. 

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Devol, A.H., Anderson, J.J., Kuivila, K., and Murray, J.W. (1984) A model for coupled sulfate reduction and methane oxidation in the sediments of Saanich Inlet. Geochim. Cosmochim. Acta 48, 993-1004. 

Fig. 8.5 Profiles of pore-water sulfate and methane concentrations and of rates of sulfate reduction and methane oxidation for a sediment core recovered from the Kattegat (Station B 65 m water depth). The broken horizontal line denotes the depth where sulfate and methane were at equimolar concentrations - indicating the peak of the sulfate/methane transition. From Iversen and Jorgensen (1985).

Fossing, H., Ferdelman, T.G., and Berg, P., 2000. Sulfate reduction and methane oxidation in continental margin sediments influenced by irrigation (South-East Atlantic off Namibia). Geochimica et Cosmochimica Acta, 64 897-910. 

Anoxic systems are divided into those with or without measureable sulfide, which Berner termed sulfidic and nonsulfidic. Nonsulfidic environments themselves are described as postoxic if too oxidized to permit sulfate reduction and methanic if strongly reduced with sulfate reduction and methane formation. Berner suggests that the presence or absence of specific iron and manganese minerals in Table 11.5 can be used to distinguish these different redox environments. 

Dissolved arsenic is correlated with ammonia (Fig. 4), consistent with a release mechanism associated with the oxidation of organic carbon. Other chemical data not shown here provide clear evidence of iron, manganese and sulfate reduction and abundant methane in some samples indicates that methanogenesis is also occurring. It is not clear however if arsenic is released primarily by a desorption process associated with reduction of sorbed arsenic or by release after the reductive dissolution of the iron oxide sorbent. Phreeqc analysis shows PC02 between 10"12 and 10"° bars and that high arsenic waters are supersaturated with both siderite and vivianite. 

A few examples of chemoautolithotrophic processes have been mentioned in this chapter, namely anaerobic methane oxidation coupled to sulfate reduction and the ones listed in Table 12.2 involving manganese, iron, and nitrogen. Another example are the microbial metabolisms that rely on sulfide oxidation. Since sulfide oxidation is a source of electrons, it is a likely source of energy that could be driving denitrification, and manganese and iron reduction where organic matter is scarce. 

Fig. 8.8 Schematic profiles of sulfate and methane in marine sediments. A) In sediments with low methane flux, sulfate reduction based on oxidation of sediment organic matter predominates throughout the sulfate zone. B) In sediments with high methane flux, sulfate reduction based on anaerobic oxidation of methane tends to straighten out the sulfate profile.

Jorgensen, B.B., Weber, A., and Zopfi, J., 2001. Sulfate reduction and anaerobic methane oxidation in Black Sea sediments. Deep-Sea Research, 48 2097-2120. 

Hydrogen sulfide and methane can be removed by aeration, although the largest reduction in hydrogen sulfide may result from oxidation by the dissolved oxygen introduced during the aeration. At low pH values, the product is sulfate, whereas at high pH values, the product is free sulfur. 

The oxidation of methane was very slow under the experimental conditions employed The slowest rates are those with anhydrite as oxidant. Because the ratio of the rate constants, a, is dependent upon the oxidant, it is difficult to estimate the carbon isotope selectivity during sulfate reduction at temperatures relevant to TSR in sour gas occurrences. However, the effects are substantial with the cupric oxide-manganese dioxide and hematite-anhydrite trends in Figure 2 giving extrapolated a-values of about 1.02 and 1.04 respectively at 200°C. 

There are two fermentative processes that at first appear to be quite similar to oxygen and nitrate-dependent respirations the reduction of C02 to methane and of sulfate to sulfide. However, on closer examination, it is clear that they bear little resemblance to the process of denitrification. In the first place, the reduction of C02 and of sulfate is carried out by strict anaerobes, whereas nitrate reduction is carried out by aerobes only if oxygen is unavailable. Equally important, nitrate respirers contain a true respiratory chain sulfate and C02 reducers do not. Furthermore, the energetics of these processes are very different. Whereas the free energy changes of 02 and nitrate reduction are about the same, the values are much lower for C02 and sulfate reduction. In fact, the values are so low that the formation of one ATP per H2 or NADH oxidized cannot be expected. Consequently, not all the reduction steps in methane and sulfide formation can be coupled to ATP synthesis. Only the reduction of one or two intermediates may yield ATP by electron transport phosphorylation, and the ATP gain is therefore small, as is typical of fermentative reactions. 

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. 

Niewohner C., Heasen C., Kasten S., Zabel M., and Schulz H. D. (1998) Deep sulfate reduction completely mediated by anaerobic methane oxidation in sediments of the upwelling area off Namibia Geochim. Cosmochim. Acta 62, 455-464. 

Because of its abundance in anoxic aquatic environments and its importance as a greenhouse gas, methane transformation by anaerobic oxidation has been the subject of numerous studies. The rates of anaerobic methane oxidation and the environments where it has been found were reviewed by Spormann and Widdel (2000). In marine systems, sulfate reduction has been shown to be an important part of the methane oxidation process. Landhlls, however, not hydrocarbon contaminations per se, are the main source of anthropogenic methane emissions in the US and, therefore, methane degradation processes are not discussed further in this chapter (see Chapter 9.16 for a discussion of methane generation from landfills). 

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