Geobacter metallireducens —evaluation of its sustainability in nitrate-reducing Fe(II) oxidation under autotrophic conditions

Bacterial nitrate (NO3−) reducing Fe(II) oxidation is often regarded as an autotrophic process, although its long-term sustainability is rarely demonstrated. Here, we assessed whether Geobacter metallireducens (ATCC 53774) can sustain chemolithoautotrophic nitrate-reducing Fe(II) oxidation. Cultures were incubated with Fe(II) and NO3− as electron donor and acceptor, respectively, and CO2 as the sole carbon source. Negative controls contained either (1) electron acceptor, (2) donor or (3) no substrates. We found that first-generation cultures transferred from a washed NO3−/acetate-grown preculture oxidized 21.0±1.3% (2.09±0.14 mM) of 10 mM Fe(II) and reduced 5.2±3.9% (0.20±0.15 mM) of 4 mM NO3−. Nitrite remained below the detection limit and ammonium increase was not observed while total N2O accumulated up to 31.34±1.90 µM. However, following transfers under organic-free conditions did not sustain Fe(II) oxidation. Further, we did not observe a decrease in nitrate, although we found low N2O formation of total 9.10±0.42 µM and 14.10±3.90 µM in later generations. These results demonstrate that G. metallireducens does not sustain autotrophic nitrate-reducing Fe(II) oxidation, although it may contribute to coupled Fe(II) oxidation and nitrate reduction in organic environments.