Blank reactions (where the mineral electrodes in the separate compartments were not connected) did not result in any observed succinate product. Likewise, reactions conducted with a weak vent electrolyte solution formed little to no product. FeS, FeS2 and Ni-FeS facilitated the reaction in moderate yield (5-17%), whereas a bare Fe electrode produced very little succinate (1%). The lack of reactivity without electrodes connected indicates that sacrificial oxidation of the mineral in the ocean side is not reducing enough to drive the conversion of fumarate to succinate in that ocean chamber; instead fumarate reduction must be coupled to H2 oxidation on the vent side (so without an electrical connection between these chambers, this reduction cannot occur). Additionally, the lack of reactivity with a dilute vent electrolyte suggests the role of sulfide as an important reducing agent in addition to H2 gas as well as the role of the electrolyte in charge mobility through the aqueous medium. The yields for fumarate reduction with the full strength electrolyte (5-17%) are on par with those demonstrated in a single-pot reaction for Fe/Ni minerals & metals, although this is the first such demonstrated case of fumarate reduction where the H2 provided is not oxidized in the same chamber, but instead it's oxidation is electrochemically coupled across a mineral electrocatalyst.