A novel endo-hydrogenase activity recycles hydrogen produced by nitrogen fixation.

Background: Nitrogen (N(2)) fixation also yields hydrogen (H(2)) at 1:1 stoichiometric amounts. In aerobic diazotrophic (able to grow on N(2) as sole N-source) bacteria, orthodox respiratory hupSL-encoded hydrogenase activity, associated with the cell membrane but facing the periplasm (exo-hydrogenase), has nevertheless been presumed responsible for recycling such endogenous hydrogen.

Methods And Findings: As shown here, for Azorhizobium caulinodans diazotrophic cultures open to the atmosphere, exo-hydrogenase activity is of no consequence to hydrogen recycling.