Background
The microaerophilic bacterium Azorhizobium caulinodans, when fixing N(2) both in pure cultures held at 20 µM 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).
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.