(Table S1) Calcium concentrations of ODP Site 201-1226

In situ Gibbs energies of reaction (DG) for acetate-oxidizing sulfate reduction, acetate-oxidizing iron reduction, and acetoclastic methanogenesis, and sulfate-reducing methanotrophy are consistently negative and relatively constant throughout most of the sediment column at the eastern equatorial Pacific Ocean Drilling Program (ODP) Site 1226. The energy yields (-DG) closely match the values (for acetate-oxidizing sulfate reduction and acetoclastic methanogenesis) in published culturing experiments with actively growing cells and, for sulfate-reducing methanotrophy, in other environments. Although microbes mediating these reactions compete for substrates, mutualistic interactions between them appear to sustain their co-existence in deep subseafloor sediments for millions of years (the interval over which the sediments have been deposited). These competing and mutualistic interactions collectively constitute a highly coupled reaction network where relative rates of reaction are regulated by the in situ Gibbs energies of reaction.