Ambient nitrogen reduction cycle using a hybrid inorganic–biological system
Monday, 2017/06/26 | 08:13:07
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Chong Liu, Kelsey K. Sakimoto, Brendan C. Colón, Pamela A. Silver, and Daniel G. Nocera SignificanceThe nitrogen cycle and the fixation of atmospheric N2 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 N2 and H2O at ambient conditions powered by water splitting, which may be driven renewably. The inorganic–biological hybrid system fixes atmospheric nitrogen into NH3 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. AbstractWe 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 Xanthobacter autotrophicus, which performs N2 and CO2 reduction to solid biomass. Living cells of X. autotrophicus 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 X. autotrophicus 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.
See: http://www.pnas.org/content/114/25/6450.full PNAS June 20 2017; vol.114; no,.25: 6450–6455
Fig. 1. Schematic of the electroaugmented nitrogen cycle. A constant voltage (Eappl) is applied between CoPi OER and Co–P HER electrodes for water splitting. The H2ases of X. autotrophicus oxidize the generated H2, fueling CO2 reduction in the Calvin cycle and N2 fixation by N2ases. The generated NH3 is typically incorporated into biomass (pathway 1), but can also diffuse extracellularly by inhibiting biomass formation (pathway 2). This process is powered by renewable, solar-derived electricity taking N2 and CO2 from the environment. Cells of X. autotrophicus form an electrogenerated biofertilizer which may be added to soils to improve plant growth. The pathway of natural N cycling/N2 fixation is indicated, with line width denoting relative flux of these pathways. Red pathways indicate carbon cycling; blue pathways indicate N cycling. |
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