Hydrogen (H2) gas is an obligatory byproduct of nitrogen (N2) reduction during biological nitrogen fixation by the metalloenzyme nitrogenase. Despite significant efforts, diazotrophic H2 production rates remain too low to compete with fossil fuel-derived H2. Here, we investigate the role of temperature (14, 19, 30 °C), carbon metabolism (acetate or succinate as carbon source), and nitrogenase isoform (molybdenum, vanadium, iron-only nitrogenase) in controlling N2 reduction and H2 production rates in the model anaerobic photoheterotroph Rhodopseudomonas palustris. Rates of H2 production are primarily controlled by growth rate and secondarily by nitrogenase enzymology. The iron-only nitrogenase exhibits the highest H2:N2 stoichiometries (6.3–12.7); H2:N2 stoichiometries for molybdenum and vanadiumnitrogenases are similarly lower (2.5–4.1 and 2.6–4.3, respectively) and uncorrelated with growth rate or temperature. Hydrogen inhibition of growth is lower than H2 inhibition of purified nitrogenase. These results help provide a framework for optimizing physical and metabolic conditions in diazotroph-based biohydrogen production efforts.
All Science Journal Classification (ASJC) codes
- Renewable Energy, Sustainability and the Environment
- Fuel Technology
- Condensed Matter Physics
- Energy Engineering and Power Technology
- H:N ratio
- Rhodopseudomonas palustris