A non-magnetic mutant of Magnetospirillum magneticum AMB-1, designated as NMA21, was generated by mini-Tn5 transposon mutagenesis to identify genes involved in bacterial magnetic particle (BMP) synthesis. Alignment of the DNA sequences flanking the transposon allowed the isolation of an open reading frame (ORF2) within an operon consisting of five genes. The amino acid sequence of ORF2 showed homology with tungsten-containing aldehyde ferredoxin oxidoreductase (AOR) from Pyrococcus furiosus (48% identity and 64% similarity), which functions for aldehyde oxidation. AOR was found to be expressed under microaerobic conditions and localized in the cytoplasm of AMB-1. Iron uptake and growth of NMA21 were lower than wild type. Transmission electron microscopy (TEM) of NMA21 revealed that no BMPs were completely synthesized, but polyhydroxybutyrate (PHB)-like granules were persistently produced. These results indicate that AOR may contribute to ferric iron reduction during BMP synthesis in M. magneticum AMB-1 under microaerobic respiration.