Marine archaea and bacteria that can carry out dissimilatory reduction of sulfate to sulfide (DSR) have a central role in sulfur cycles. A key step in this pathway is the reduction of a high-energy sulfite intermediate to sulfide, which is mediated by the dissimilatory sulfite reductase DsrAB. However, none of the enzymes involved in the DSR pathway is membrane-bound and thus cannot generate an electrochemical gradient, so how sulfite reduction is coupled to energy conservation is unknown. Now, Santos, Venceslau et al. report that sulfite reduction by an archaeal DsrAB involves another protein, DsrC, which interacts with DsrAB and uses a sulfite-derived sulfur to bridge two of its cysteines. This reaction forms a trisulfide intermediate that is then probably reduced to sulfide by the membrane-bound DsrMKJOP complex. This complex has previously been shown to interact with DsrC and may use the electrons released during the reduction of the trisulfide intermediate to generate a transmembrane proton gradient that enables energy conservation.