A system of two membrane biofilm reactors (MBfRs) was tested for the conversion of sulfate (1.5 g/L) in mining-process water into elemental sulfur (S0) particles. Initially, a H2-based MBfR reduced sulfate to sulfide, and an O2-based MBfR then oxidized sulfide to S0. Later, the two MBfRs were coupled by a recirculation flow. Surface loading, reactor-coupling configuration, and substrate-gas pressure exerted important controls over performance of each MBfR and the coupled system. Continuously recirculating the liquid between the H2-based MBfR and the O2-based MBfR, compared to series operation, avoided the buildup of sulfide and gave overall greater sulfate removal (99% vs 62%) and production of S0 (61% vs 54%). The trade-off was that recirculation coupling demanded greater delivery of H2 and O2 (in air) due to the establishment of a sulfur cycle catalyzed by Sulfurospirillum spp., which had an average abundance of 46% in the H2-based MBfR fibers and 62% in the O2-based MBfR fibers at the end of the experiments. Sulfate-reducing bacteria (Desulfovibrio and Desulfomicrobium) and sulfur-oxidizing bacteria (Thiofaba, Thiomonas, Acidithiobacillus and Sulfuricurvum) averaged only 22% and 11% in the H2-based MBfR and O2-based MBfR fibers, respectively. Evidence suggests that the undesired Sulfurospirillum species, which reduce S0 to sulfide, can be suppressed by increasing sulfate-surface loading and H2 pressure.