Isotopic Effects Associated with Methane Production Mechanisms

Variation in the two predominant mechanisms of methane production, CO2 reduction and acetate fermentation, has been hypothesized to be associated with inverse or antipathetic shifts in <5D and <513C of CH4 (Fig. 6.1;

Table 6.1 Examples of Relative Changes in <SD of CH4 Relative to <513C of CH4 Caused by Methanotrophic Activity

Environment

A<5D/A<51SC

References

Landfill soil

2.5-3.7

Liptay etal. (1998)

Landfill soil

4

Lubina etal. (1996a,b)

Laboratory incubations

8.5-13.5

Coleman etal. (1981)

Wetlands

5.5

Happell et al. (1994)

Wetlands

2.5

Burke etal. (1988b)

Whiticar et al, 1986; Schoell, 1988; Burke et al, 1988a,b; Whiticar, 1993, 1999; Hornibrook et al, 1997). Carbon isotope separation between CO2 and ch4 ranges from 60 to 90%o for CO2 reduction and 40 to 60%o for acetate fermentation (Whiticar, 1999; Conrad et al, 2002). Hydrogen isotope effects lead to ch4-h2o separation of 170 to 250%o for CO2 reduction and 250 to 400%o for acetate fermentation. The hypothesis is that if all things are relatively equal, fermentation of acetate will result in ch4 which is 13C-enriched and D-depleted relative to ch4 produced via the CO2 reduction pathway. There are two key words in the pronouncement above, 'relatively equal.'

Above we referred to understanding the changes in ch4 isotopic composition along a gradient. The scale of this gradient must be sufficiently small that the isotopic composition of methane precursors does not change significantly in such a way that interpretation will be compromised. For example, Waldron et al. (1999) demonstrated that on a grand spatial gradient from the tropics to the poles, methane <5D composition is controlled by the <5D of water present. However, at smaller spatial scales, where the <5L) of water is fairly constant, 51) variation of ch4 is most likely due to variation in the oxidation or production mechanism, as discussed below. Similarly, the <513C of methane can be affected by the isotopic composition of the parent organic matter. Methane produced from the decay of c3 vegetation has been observed to be I3C-depleted relative to methane produced from the decay of c4 vegetation (Chanton and Smith, 1993).

Across what kind of spatial and temporal gradients can one expect to find transitions in the methane production mechanism? Methane is produced primarily in flooded basins and sedimentary environments; in these environments, the production of acetate and its subsequent utilization are generally greater in the upper portion of the peat or sediment column than in deeper layers (Whiticar, 1999 and references therein). This acetate production in the surface layer of peat or sediment may be associated with the breakdown of more labile organic matter, which is derived from plant root exudates in the rhizosphere (Whiting and Chanton, 1993; Shannon and White, 1994; Chanton et al, 1993, 1995; Shannon et al., 1996; Chasar et al, 2000a,b).

Because marine systems contain abundant sulfate, which is an energy-yielding electron acceptor, bacterial respiration in marine and salt-marsh surface sediments is dominated by sulfate reduction. In these systems sulfate-reducing bacteria are able to out-compete methanogens for acetate because sulfate respiration is a more energy-yielding process than methane production. Consequently, in marine systems sulfate and acetate are consumed in surficial layers and methanogenesis is confined to deeper layers where sulfate is depleted and where there is much less acetate. Methane production in marine systems is thus formed primarily by CO2 reduction (see Whiticar, 1999).

In contrast, freshwater systems have much lower concentrations of sulfate and it is possible for methane production to occur in surface layers of sediment or peat, i.e., more acetate is available to methanogens. This supports the idea that in marine systems methane production proceeds primarily via CO2 reduction while in freshwater systems acetate fermentation is the dominant processes (Whiticar et al, 1986; Whiticar, 1999). However, isotopic studies in freshwater wetlands indicate that with increasing depth in the peat there is a transition from acetate fermentation to C02 reduction (Hornibrook et al., 1997, 2000a,b; Chasar et al, 2000a,b). There is also evidence that vegetation affects the relative importance of methane production pathways in freshwater wetlands, although how this occurs is not clear. For example, ch4 in Sph(igiiimi-domiivd\cx\ bogs is produced by C02 reduction predominantly, while acetate fermentation is of more relative importance in Carex-dominated fens (Kelley et al, 1992; Lansdown et al, 1992; Shannon and White, 1994; Chasar et al, 2000a,b).

Blair (1998) described the relative importance of methane production mechanisms in marine sediments as being controlled by the balance between the rate of deposition of organic matter and the relative importance of electron acceptors other than methane produced in the respiration of that organic matter. As more organic matter passes through the gauntlet of the higher-energy respiration modes, more methane production results and a greater percentage of that methane is produced from acetate fermentation. Measurements of fractionation factors for acetate production during anaerobic fermentation are scarce (Conrad et al, 2002). Blair et al (1985) reported that the carboxyl group of acetate produced in an anaerobic culture was 13C enriched relative to the parent organic matter (glucose) by about 12%o. However, the carbon of acetate transferred to methane comes from the methyl group. Values for the methyl carbon are related to the 13C of the parent organic matter but are '''Ode pie ted relative to the carboxyl carbon (Blair et al, 1987; Blair and Carter, 1992; Sugimoto and Wada, 1993; Hornibrook etal., 2000a). Methyl carbon <513C values vary from —40 to —15%o for anaerobic incubation of C-3 plant material, and —14 to +4%o for acetate extracted from a marine sediment (references above). Kruger et al. (2002) measured the <513C of acetate extracted from the soil pore water of an Italian rice field and obtained values ranging from —16 to —21 %o for the average of both carbons.

An isotopic fractionation of — 21 %o has been reported for the transformation of acetate methyl carbon into methane from microbial culture experiments (Gelwicks et al, 1994). Depending upon the fraction of acetate that is funneled into methane production in natural systems, that fractionation may not be fully expressed due to closed system isotope effects.

Conrad et al (2002), in an elegant paper calculating the relative amounts of methane derived from the two production mechanisms, assumed that the methyl group was enriched similar to the carboxyl group and then applied the fractionation factor of—21 %o to calculate the 513C of methane produced from acetate. Clearly more research needs to be performed in this area.

The validity of considering 3D variation to be a result of changes in production mechanism has recently been questioned (Waldron etal, 1998a,b, 1999; Sugimoto and Wada, 1995). In incubation studies where substrate lability or changes in acetate production have been manipulated to vary production mechanism, 13C shifts have been observed but concurrent shifts in ¿1) have not (Waldron et al, 1998a; Sugimoto and Wada, 1995). While the H of ch4 produced by CO2 reduction clearly comes from environmental water (Daniels et al, 1980), it has long been thought that in methane produced from acetate, one H atom comes from water and the others come from the acetate methyl group (Pine and Barker, 1956). Recent evidence has suggested that there is an exchange of hydrogen between the acetate methyl group and water in the final stages of methanogenic acetate metabolism (de Graaf et al, 1996). Presumably this isotopic exchange is accompanied by an isotopic fractionation but this has not been measured.

The scientific community also needs to better describe the source of hydrogen and the isotopic fractionations involved during the production of methane from acetate. It is difficult to explain the very depleted values of i5D for acetate-derived methane. An excellent example is given by Whiticar (1999). If one assumes a <5D organic value of about — 120%o for the three methyl hydrogens donated, no isotopic fractionation associated with this donation, and a methane produced with a SD of —400%o, mass balance would require the strange value of — 1240%o for the final hydrogen added. Therefore it seems likely that some sort of isotopic fractionation is involved in the transfer of methyl H to methane.

Burke (1993) proposed that hydrogen concentration could affect the fractionation of H between water and methane, resulting in greater fractionation at higher H2 concentrations but not affecting the <513C of ch4. Burke (1993) observed that in incubation experiments and in the rumen of cows, where H2 concentrations are greater, the 5D of ch4 is consistently depleted relative to observations made in wetlands or sediments. If the H2 concentration varies in soils and sediments in a consistent manner along with the production mechanism, it may be that H2 concentration alone determines variations in the <5D of methane when environmental water 3D is constant.

In the next section we present a number of field observations that demonstrate antipathetic shifts in methane C and D isotopic composition across seasonal, depth, and vegetation gradients. These data are consistent with the hypothesis that shifts in dominant mechanisms in methane production, i.e., CO2 reduction and acetate fermentation, are associated with antipathetic changes in these isotopes. Further, in the same manner that sympathetic change in C and D isotopes driven by methane oxidation are described (Table 6.1), we want to develop consistent field estimates of the relative difference in the changes in C and D driven by changes in production mechanisms. The mechanisms controlling the observed variations certainly need considerable elucidation; future research should be focused in this direction.

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