Sulfate reducing microorganisms (SRM) may play a significant role altering upper oceanic crustal fluids when suitable electron donors, such as hydrogen or organic matter, are available. The habitability of such an environment with respect to sulfate reduction depends on the competition of microbial communities for substrates, which is largely dictated by the energetics of catabolic and anabolic processes. Although sulfate reduction has been observed in fluids taken from the upper ocean crust in Juan de Fuca Ridge flanks, the electron donors (EDs) used by SRM have not been identified, nor has the energy required for organic synthesis been determined. As a result, a collaboration is underway to characterize the EDs that are plausible candidates for the SRM in the Juan de Fuca system and to quantify the amount of energy these microorganisms require to synthesize biomolecules. This is accomplished by carrying out thermodynamic calculations that take into account the physicochemical properties of the resident fluids. Specifically, the Gibbs energy of reactions describing the reduction of sulfate by various EDs and the synthesis of amino acids from inorganic precursors is being calculated at the temperature, pressure and compositional conditions prevailing in particular Juan de Fuca sample site locations.