Ambient nitrogen reduction cycle using a hybrid inorganic–biological system

Abstract

We demonstrate the synthesis of NH₃ from N₂ and H₂O at ambient conditions in a single reactor by coupling hydrogen generation from catalytic water splitting to a H₂-oxidizing bacterium <i>Xanthobacter autotrophicus</i>, which performs N₂ and CO₂ 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 NH₃ generated from nitrogenase (N₂ase) 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 N₂ reduction reaction proceeds at a low driving force with a turnover number of 9 × 10⁹ cell^-1 and turnover frequency of 1.9 × 10⁴ s^-1⋅cell^-1 without the use of sacrificial chemical reagents or carbon feedstocks other than CO₂ This approach can be powered by renewable electricity, enabling the sustainable and selective production of ammonia and biofertilizers in a distributed manner.