초록
<P><B>Significance</B></P><P>The nitrogen cycle and the fixation of atmospheric N<SUB>2</SUB> into ammonium are crucial to global food production. The industrial Haber–Bosch process facilitates half the global nitrogen fixation in the form of ammonia but it is energy- and resource-intensive, using natural gas as the source of energy and hydrogen at elevated temperature and pressure. Our alternative approach synthesizes ammonium from N<SUB>2</SUB> and H<SUB>2</SUB>O at ambient conditions powered by water splitting, which may be driven renewably. The inorganic–biological hybrid system fixes atmospheric nitrogen into NH<SUB>3</SUB> or soluble biomass with high fluxes and energy efficiency. Simultaneously, this system cultivates a living soil bacterium that acts as a potent biofertilizer amenable to boosting crop yields.</P><P>We demonstrate the synthesis of NH<SUB>3</SUB> from N<SUB>2</SUB> and H<SUB>2</SUB>O at ambient conditions in a single reactor by coupling hydrogen generation from catalytic water splitting to a H<SUB>2</SUB>-oxidizing bacterium <I>Xanthobacter autotrophicus</I>, which performs N<SUB>2</SUB> and CO<SUB>2</SUB> reduction to solid biomass. Living cells of <I>X. autotrophicus</I> may be directly applied as a biofertilizer to improve growth of radishes, a model crop plant, by up to ∼1,440% in terms of storage root mass. The NH<SUB>3</SUB> generated from nitrogenase (N<SUB>2</SUB>ase) in <I>X. autotrophicus</I> can be diverted from biomass formation to an extracellular ammonia production with the addition of a glutamate synthetase inhibitor. The N<SUB>2</SUB> reduction reaction proceeds at a low driving force with a turnover number of 9 × 10<SUP>9</SUP> cell<SUP>–1</SUP> and turnover frequency of 1.9 × 10<SUP>4</SUP> s<SUP>–1</SUP>⋅cell<SUP>–1</SUP> without the use of sacrificial chemical reagents or carbon feedstocks other than CO<SUB>2</SUB>. This approach can be powered by renewable electricity, enabling the sustainable and selective production of ammonia and biofertilizers in a distributed manner.</P>