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

Background The microaerophilic bacterium Azorhizobium caulinodans, when fixing N 2 both in pure cultures held at 20 µ M dissolved O 2 tension and as endosymbiont of Sesbania rostrata legume nodules, employs a novel, respiratory-membrane endo- hydrogenase to oxidize and recycle endogenous H 2 produced by soluble Mo-dinitrogenase activity at the expense of O 2 . Methods and Findings From 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 O 2 levels are lowered to <1 µ M dissolved O 2 tension (true microaerobic physiology), in vivo endo -hydrogenase activity reverses and continuously evolves H 2 at high rates. In essence, H + ions then supplement scarce O 2 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. Conclusions A. 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 O 2 tolerant hydrogenases in other organisms, A. caulinodans in vivo endo -hydrogenase mediated H 2 production rates (50,000 pmol 10 9 ·cells −1 min −1 ) are at least one-thousandfold higher. Conceivably, A. caulinodans respiratory-membrane hydrogenesis might initiate H 2 crossfeeding among spatially organized bacterial populations whose individual cells adopt distinct metabolic states in response to variant O 2 availability. Such organized, physiologically heterogeneous cell populations might benefit from augmented energy transduction and growth rates of the populations, considered as a whole.