A bifunctional enzyme that catalyzes the reduction of elemental sulfur or polysulfide to hydrogen sulfide with NADPH as electron donor. Also functions as a reduced ferredoxin:NADP oxidoreductase with a very high affinity for reduced ferredoxin. Exhibits a broad specificity for various physiological and non-physiological substrates with varied reduction potentials such as methyl viologen, benzyl viologen, FAD, FMN, methylene blue, 2,6-dichlorophenolindophenol (DCIP), cytochrome C and ferricyanide with highest preference for benzyl viologen. Does not reduce fumarate, succinate, nitrate, nitrite, sulfate, sulfite or protons. Does not possess any hydrogenase activity or NADPH-dependent glutamate synthase activity. hydrogen sulfide + NADP(+) + n sulfur = NADPH + (n+1) sulfur H(+) + NADP(+) + 2 reduced [2Fe-2S]-[ferredoxin] = NADPH + 2 oxidized [2Fe-2S]-[ferredoxin] Binds 1 FAD per subunit. Binds 1 [3Fe-4S] cluster. Binds 1 [4Fe-4S] cluster. Measured for the whole complex. Optimum pH is 8.0 (for polysulfide as substrate) and 10.3 (for benzyl viologen as substrate). Optimum temperature is 80 degrees Celsius. Has a half-life of 12 h at 95 degrees Celsius. Activity increases by 50% after incubation for several hours at 82 degrees Celsius. Heterodimer of alpha and beta subunits.