Iron is an essential nutrient for almost all known organisms and is a cofactor in enzymes found in a number of important pathways, such as oxygen respiration, nitrogen fixation, methanogenesis, and hydrogen production and consumption. Under anaerobic conditions, iron reacts with sulfide, forming iron sulfide minerals and potentially limiting iron availability for growth, as well as affecting intracellular iron reactions such as iron-sulfur cluster biosynthesis. This is especially relevant to sulfide-producing organisms, such as those that reduce sulfate- and elemental sulfur (S0). This study investigated two effects of sulfide production on the iron metabolism of the S0-reducing hyperthermophilic archaeon, Pyrococcus furiosus. First, to determine if extracellular sulfide affects iron assimilation, the ability of P. furiosus to utilize iron sulfide minerals as an iron source was investigated. It was found that P. furiosus can utilize dissolved iron sulfide complexes of the form FexSx (x 150) that are larger than Fe40S40. These can either be formed from the reaction of iron with sulfide or dissolved from the mineral mackinawite. P. furiosus may also be actively involved in the dissolution of mackinawite. Second, to determine the effect of intracellular sulfide production on iron metabolism, the conserved hypothetical S0-induced protein A (SipA) was purified and characterized. The gene encoding SipA, along with several genes involved in iron metabolism, is expressed after S0 reduction and sulfide accumulation begin, with SipA production also dependent on sufficient iron concentrations. SipA self-assembles in the presence of iron and sulfide into homopolymeric complexes of primarily 90 MDa, which contain as many as 5,000 monomers of 19 kDa and 45,000 [4Fe-4S] clusters linked by polysulfide ions. Its N-terminal nitrogenase-like cofactor domain (residues 1-107) binds iron and sulfur, while the C-terminus (108-179) is required for polymerization. SipA is predicted to be an iron-sulfur cluster biosynthesis and storage protein, using sulfide produced during S0 reduction as the sulfur source.