MICROBIALLY-CATALYSED ANAEROBIC METAL REDOX CYCLING BY AN ACIDOPHILIC, GEOBACTER SP. FEAM09

Nitrate mediated metal-redox cycling has been recognized to occur in water and saturated soils and sediments through a series of biological and abiotic mechanisms. This process has been described using isolates obtained from saturated environments. An acidophilic (optimal growth pH 5.0), autotrophic Geobacter sp. strain FeAm09, was isolated from iron-rich unsaturated soils collected from a tropical forest (Luquillo Experimental Forest, Puerto Rico). Strain FeAm09 was isolated under Fe(III) reducing conditions and is capable of growth using soluble Fe(III), Fe(III)-NTA, as well as insoluble Fe(III) and Mn(IV) oxides as terminal electron acceptors with H₂ as the electron donor. Strain FeAm09 is also capable of coupling the reduction of nitrate to ammonium (DNRA) coupled to the oxidation of Fe(II) or Mn(II). Direct cell counts revealed growth of FeAm09 during metal reduction and oxidation. Dinitrogen (N₂) gas was also formed (denitrification) in equal molar amounts to ammonium with both Fe(II) and Mn(II) serving as the electron donor. The draft genome sequence of FeAm09 supports the reduction of nitrate to nitrite (napB, napD) and subsequent nitrite reduction to ammonium (nrfB). Gene responsible for nitrogen gas production have not been identified suggesting that nitrite produced as an intermediate may be abiotically reduced by Fe(II) and Mn(II). Nitrate was not reduced in killed controls. These results indicate that ammonium production coupled to metal oxidation is biological. Genes within the reverse TCA cycle were identified supporting an autotrophic lifestyle (citB, frdB, frdC, fumB, icdI, korA, korB). Carbon dioxide fixation was verified through assimilation of ¹⁴C-labelled CO₂ into biomass under metal-reducing and metal-oxidizing conditions. Together, these data describe the first acidophilic, autotrophic Geobacter species capable of nitrate-dependent metal-oxidation. The ability of FeAm09 to reduce nitrate to ammonium as the sole nitrate reduction pathway present a model organism that can be used to study the simultaneous biological and abiotic reduction of reactive intermediates such as nitrite.