Methanogenesis, carbon cycle

The sulfur-isotope signal in sedimentary record of the early Earth provides evidence for the early evolution of life. Sulfate-reducing bacteria deplete the resultant sulfides in " S. The isotopic record of " S supports this, although some evidence now suggests that seawater sulfate concentrations were so low that they severely suppressed sulfate reduction rates allowed for a carbon cycle dominated by methanogenesis (Habicht et al, 2002). 

Discuss, in terms of energy yields of the appropriate reactions, the importance of the syn-trophic relationship between acetogens and methanogens, specifically the interspecies hydrogen transfer. If, in the absence of other electron acceptors, methanogenesis is inhibited, what impact would this have on the carbon cycle ... 

Martens, C.S., and Klump, J.V. (1984) Biogeochemical cycling in an organic-rich coastal marine basin. 4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis. Geochim. Cosmochim. Acta 48, 1987-2004. 

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