Energy model and metabolic flux analysis for autotrophic nitrifiers

The behavior of pure cultures of nitrifying microorganisms under autotrophi c growth operating conditions was investigated and the relations between th eir energy metabolism and their anabolism analyzed by means of metabolic ne twork computation. The description of the metabolism of the nitrifiers is e xtended to their energy metabolism by introducing compartmentalization (cyt oplasmic and periplasmic sides) and studying coupling between the electron transport chain and the proton gradient generation. The energy model of Nit rosomonas and Nitrobacter was developed based on the oxidoreduction reactio ns known to be involved. The electron transport chains and the associated p roton translocation for these models are described. Several possible hypoth eses are analyzed and discussed concerning the thermodynamic consistency of all the oxidoreduction reactions. For Nitrosomonas, the most delicate poin t is the second step of hydroxylamine oxidation. For Nitrobacter a new ener gy model is proposed in which NO plays an important role as node in the dis tribution of electrons from NO; oxidation to the membrane electron transpor t 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 fl ux calculation. This representation demonstrates that a maintenance in nitr ifiers expressed as a proton leak is no higher than for other aerobes. The yields calculated from the energy models integrated with the metabolic mode ls of nitrifiers are consistent with the experimental yields in the literat ure. (C) 2001 John Wiley & Sons, Inc.