Methane biogenic, reduction

The biogenic reduction of sulfate follows two biosynthetic pathways (1) assimilatory incorporation of sulfur into the amino acids e.g. cysteine) with very small sulfur isotope fractionation (2) dissimilatory production of sulfide coupled to oxidation of OM to CO2 (see Scheme 1). If we take, for example, methane as the electron donor, the reaction is... 

Whiticar MJ, Faber E, SchoeU M (1986) Biogenic methane formation in marine and freshwater environments CO2 reduction vs. acetate fermentation-isotopic evidence. Geochim Cosmochim Acta 50 693-709... 

Figure 7.4 shows the reduction in sulfates and the corresponding growth of both the parent carbonates and the offspring methane with subbottom depth. The methane production is parallel but lower in isotope production than the carbonates. In Figure 7.4 the sulfur isotope (< 34S) content is defined in an identical manner to Equation 7.2 with the replacement of the fraction 13C/12C by 34S/32S in both the numerator and the denominator, using Canon Diablo meteoritic troilite as a standard. The < 34S value increases from 20-60%c before substantial biogenic methane is produced. 

The decomposition of complex organic substances by microbial marine populations results in the in situ production of biogenic methane, which is found in trace amounts in all fresh and salt waters. The relative importance of acetate assimilation and CO reduction as primary methanogenic pathways is discussed in the chapter by R.A. Burke and W.M. Sacke. In the past, these pathways were studied by the use of C labelled substrates. 

Stable hydrogen and carbon Isotopic compositions of biogenic methane produced In the sediments of several freshwater and estuarine environments have been measured and Interpreted using a previously published model. The results Infer that acetate dissimilation Is the dominant methanogenlc pathway In these sediments, accounting for about 50 to 80% of the total methane production, with CO2 reduction responsible for the remainder. In general, 6D-CH and are Inversely correlated. Implying... 

Since methane formation is shifted at least as strongly as sulfate reduction towards sediments with high organic deposition, an equally small fraction of the global biogenic methane production presumably takes place in the deep sea below 1000 m... 

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