Simplified illustration of the dissimilatory sulfate reduction network and S isotope fractionation associated with each reaction step. Proposed modes of electron flow are indicated with dashed arrows. See text for a detailed description. 34εeq, equilibrium S isotope fractionation; 34εkin, kinetic S isotope fractionation; Apr, APS reductase; Dsr, dissimilatory sulfite reductase; ECred, reduced electron carrier; ECox, oxidized electron carrier; MK, menaquinone; MKH2, menaquinol; Sat, sulfate adenylyl transferase. The figure is adapted from Wing and Halevy (2014) and includes new insights into the DsrC cycle from Santos et al. (2015).