Energy model and metabolic flux analysis for autotrophic nitrifiers

The behavior of pure cultures of nitrifying microorganisms under autotrophic growth operating conditions was investigated and the relations between their energy metabolism and their anabolism analyzed by means of metabolic network computation. The description of the metabolism of the nitrifiers is extended to their energy metabolism by introducing compartmentalization (cytoplasmic and periplasmic sides) and studying coupling between the electron transport chain and the proton gradient generation. The energy model of Nitrosomonas and Nitrobacter was developed based on the oxidoreduction reactions known to be involved. The electron transport chains and the associated proton translocation for these models are described. Several possible hypotheses are analyzed and discussed concerning the thermodynamic consistency of all the oxidoreduction reactions. For Nitrosomonas, the most delicate point is the second step of hydroxylamine oxidation. For Nitrobacter a new energy model is proposed in which NO plays an important role as node in the distribution of electrons from NO(2)(-) oxidation to the membrane electron transport chain. The compartmentalization enables us to consider a proton gradient dissipation flux as the expression of the overall energy loss in metabolic analysis (the so-called maintenance phenomena). The energy model (electron transport chain, proton gradient) is associated with an overall description of the metabolism of Nitrosomonas and Nitrobacter in terms of metabolic flux calculation. This representation demonstrates that a maintenance in nitrifiers expressed as a proton leak is no higher than for other aerobes. The yields calculated from the energy models integrated with the metabolic models of nitrifiers are consistent with the experimental yields in the literature.