초록
<P>Ammonia (NH<SUB>3</SUB>) is an important industrial chemical that is produced using the energy- and carbon-intensive Haber-Bosch process. Recovering NH<SUB>3</SUB> from microorganisms that fix nitrogen gas (N<SUB>2</SUB>) may provide a sustainable alternative because their specialized nitrogenase enzymes can reduce N<SUB>2</SUB> to ammonium (NH<SUB>4</SUB><SUP>+</SUP>) without the need for high temperature and pressure. This study explored the possibility of converting N<SUB>2</SUB> into NH<SUB>4</SUB><SUP>+</SUP> using anaerobic, single-chamber microbial electrolysis cells (MECs). N<SUB>2</SUB> fixation rates [based on an acetylene gas (C<SUB>2</SUB>H<SUB>2</SUB>) to ethylene gas (C<SUB>2</SUB>H<SUB>4</SUB>) conversion assay] of a microbial consortium increased significantly when the applied voltage between the anode and cathode increased from 0.7 to 1.0 V and reached a maximum of ∼40 nmol of C<SUB>2</SUB>H<SUB>4</SUB> min<SUP>-1</SUP> mg protein<SUP>-1</SUP>, which is comparable to model aerobic N<SUB>2</SUB>-fixing bacteria. The presence of NH<SUB>4</SUB><SUP>+</SUP>, which can inhibit the activity of the nitrogenase enzyme, did not significantly reduce N<SUB>2</SUB> fixation rates. Upon addition of methionine sulfoximine, an NH<SUB>4</SUB><SUP>+</SUP> uptake inhibitor, NH<SUB>4</SUB><SUP>+</SUP> was recovered at rates approaching 5.2 × 10<SUP>-12</SUP> mol of NH<SUB>4</SUB><SUP>+</SUP> s<SUP>-1</SUP> cm<SUP>-2</SUP> (normalized to the anode surface area). Relative to the electrical energy consumed, the normalized energy demand [MJ mol<SUP>-1</SUP> (NH<SUB>4</SUB><SUP>+</SUP>)] was negative because of the energy-rich methane gas recovered in the MEC. Including the substrate energy resulted in total energy demands as low as 24 MJ mol<SUP>-1</SUP>. Community analysis results of the anode biofilms revealed that <I>Geobacter</I> species predominated in both the presence and absence of NH<SUB>4</SUB><SUP>+</SUP>, suggesting that they played a key role in current generation and N<SUB>2</SUB> fixation. This study shows that MECs may provide a new route for generating NH<SUB>4</SUB><SUP>+</SUP>.</P><P>Electrically driving the microbial conversion of N<SUB>2</SUB> into NH<SUB>4</SUB><SUP>+</SUP> may provide a sustainable alternative to current energy- and carbon-intensive NH<SUB>4</SUB><SUP>+</SUP> production methods.</P><BR>[FIG OMISSION]</BR>