Numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of these activities often deviate from the standard model. Discrete zones of manganese reduction and iron reduction occur deep beneath sulfate-reducing zones at most sites. Sulfate reduction, iron reduction and methanogenesis co-occur throughout open-ocean sediment columns. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere. To assess the co-existence of several of these activities, we calculated in situ free energy yields of microbial activities at equatorial Site 1226. Our results indicate that this subseafloor ecosystem is a thermodynamic homeostat that sustains iron reduction, sulfate reduction, and methanogenesis. All three reactions are energetically favorable throughout most of the sediment column. Sulfate-reducing methanotrophy is also energetically favorable and may co-occur cryptically. Iron reduction and sulfate reduction yield approximately equal free energies. Sulfate reduction and methanogenesis have relatively constant energy yields downcore. Feedbacks between these multiple reactions may maintain the homeostat. We quantified rates of energy-yielding activities by using a diffusion/advection model, downhole profiles of dissolved chemicals, and sediment physical properties. Rates of activities and cell concentrations vary consistently from one subseafloor environment to another. Sulfate reduction and iron reduction are the principal forms of respiration in the subseafloor sedimentary ecosystem at most sites. From site to site, reliance on sulfate reduction declines with total activity; sulfate reduction is below detection at the lowest activity site (Peru Basin Site 1231), where metal reduction is the dominant activity. At all sites, the net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world.