Respiratory Membrane endo-Hydrogenase Activity in the Microaerophile Azorhizobium caulinodans Is Bidirectional

BackgroundThe microaerophilic bacterium Azorhizobium caulinodans, when fixing N2 both in pure cultures held at 20 µM dissolved O2 tension and as endosymbiont of Sesbania rostrata legume nodules, employs a novel, respiratory-membrane endo-hydrogenase to oxidize and recycle endogenous H2 produced by soluble Mo-dinitrogenase activity at the expense of O2. Methods and FindingsFrom a bioinformatic analysis, this endo-hydrogenase is a core (6 subunit) version of (14 subunit) NADH:ubiquinone oxidoreductase (respiratory complex I). In pure A. caulinodans liquid cultures, when O2 levels are lowered to M dissolved O2 tension (true microaerobic physiology), in vivo endo-hydrogenase activity reverses and continuously evolves H2 at high rates. In essence, H+ ions then supplement scarce O2 as respiratory-membrane electron acceptor. Paradoxically, from thermodynamic considerations, such hydrogenic respiratory-membrane electron transfer need largely uncouple oxidative phosphorylation, required for growth of non-phototrophic aerobic bacteria, A. caulinodans included. ConclusionsA. caulinodans in vivo endo-hydrogenase catalytic activity is bidirectional. To our knowledge, this study is the first demonstration of hydrogenic respiratory-membrane electron transfer among aerobic (non-fermentative) bacteria. When compared with O2 tolerant hydrogenases in other organisms, A. caulinodans in vivo endo-hydrogenase mediated H2 production rates (50,000 pmol 109·cells−1 min−1) are at least one-thousandfold higher. Conceivably, A. caulinodans respiratory-membrane hydrogenesis might initiate H2 crossfeeding among spatially organized bacterial populations whose individual cells adopt distinct metabolic states in response to variant O2 availability. Such organized, physiologically heterogeneous cell populations might benefit from augmented energy transduction and growth rates of the populations, considered as a whole.