Thermodynamic Constraints on Electromicrobial Protein Production

Global consumption of protein is projected to double by the middle of the 21st century. However, protein production is one of the most energy intensive and environmentally damaging parts of the food supply system today. Electromicrobial production technologies that combine renewable electricity and CO2-fixing microbial metabolism could dramatically increase the energy efficiency of commodity chemical production. Here we present a molecular-scale model that sets an upper limit on the performance of any organism performing electromicrobial protein production. We show that engineered microbes that fix CO2 and N2 using reducing equivalents produced by H2-oxidation or extracellular electron uptake could produce amino acids with energy inputs as low as 64 MJ kg-1. This work provides a roadmap for development of engineered microbes that could significantly expand access to proteins produced with a low environmental footprint.

A nitrite-oxidizing bacterium constitutively consumes atmospheric hydrogen

Chemolithoautotrophic nitrite-oxidizing bacteria (NOB) of the genus Nitrospira contribute to nitrification in diverse natural environments and engineered systems. Nitrospira are thought to be well-adapted to substrate limitation owing to their high affinity for nitrite and capacity to use alternative energy sources. Here, we demonstrate that the canonical nitrite oxidizer Nitrospira moscoviensis oxidizes hydrogen (H2) below atmospheric levels using a high-affinity group 2a nickel-iron hydrogenase [Km(app) = 32 nM]. Atmospheric H2 oxidation occurred under both nitrite-replete and nitrite-deplete conditions, suggesting low-potential electrons derived from H2 oxidation promote nitrite-dependent growth and enable survival during nitrite limitation. Proteomic analyses confirmed the hydrogenase was abundant under both conditions and indicated extensive metabolic changes occur to reduce energy expenditure and growth under nitrite-deplete conditions. Respirometry analysis indicates the hydrogenase and nitrite oxidoreductase are bona fide components of the aerobic respiratory chain of N. moscoviensis, though they transfer electrons to distinct electron carriers in accord with the contrasting redox potentials of their substrates. Collectively, this study suggests atmospheric H2 oxidation enhances the growth and survival of NOB in amid variability of nitrite supply. These findings also extend the phenomenon of atmospheric H2 oxidation to a seventh phylum (Nitrospirota) and reveal unexpected new links between the global hydrogen and nitrogen cycles.