N-2-dependent growth and nitrogenase activity in the metal-metabolizing bacteria, Geobacter and Magnetospirillum species

Cells of Geobacter metallireducens, Magnetospirillum strain AMB-1, Magnetos pirillum magnetotacticum and Magnetospirillum gryphiswaldense showed N-2-de pendent growth, the first anaerobically with Fe(III) as the electron accept or, and the latter three species microaerobically in semi-solid oxygen grad ient cultures. Cells of the Magnetospirillum species grown with N-2 under m icroaerobic conditions were magnetotactic and therefore produced magnetosom es. Cells of Geobacter metallireducens reduced acetylene to ethylene (11.5 +/- 5.9 nmol C2H4 produced min(-1) mg(-1) cell protein) while growing with Fe(III) as the electron acceptor in anaerobic growth medium lacking a fixed nitrogen source. Cells of the Magnetospirillum species, grown in a semi-so lid oxygen gradient medium, also reduced acetylene at comparable rates. Unc ut chromosomal and fragments from endonuclease-digested chromosomal DNA fro m these species, as well as Geobacter sulphurreducens organisms, hybridized with a nifHDK probe from Rhodospirillum rubrum, indicating the presence of these nitrogenase structural genes in these organisms. The evidence presen ted here shows that members of the metal-metabolizing genera, Geobacter and Magnetospirillum, fix atmospheric dinitrogen.