Modeling bacterial utilization of dissolved organic matter: Optimization replaces Monod growth kinetics

A bioenergetic model has been developed to examine growth kinetics associated with bacterial utilization of dissolved organic matter (DOM), NH4+, and NO3-. A set of 11 metabolic reactions are used to govern the incorporation, oxidation, and N remineralization of DOM and dissolved inorganic N associated with bacterial growth. For each reaction, free energies and electron transfer requirements are calculated based on the C, H, O, and N composition of the substrates and their concentration in the environment. From these reactions, an optimization problem is constructed in which bacterial growth rate is maximized subject to constraints on energetics, electron balances, substrate uptake kinetics, and bacterial C:N ratio. The optimization approach provides more information on bacterial growth kinetics than do the Monod-type models that are typically used to describe bacterial growth. Simulations are ran to examine bacterial C yield and growth rate, N remineralization or immobilization, and substrate preferences as resource concentrations and compositions are varied. Results from the model agree well with observations in the literature, which indicate that the premise of the model, that bacteria allocate resources to maximize growth rate, may be an accurate overall description of bacterial growth. Simulations indicate that bacterial growth rate and yield are strongly correlated to the oxidation state of the labile DOM, as determined from its bulk elemental composition. Furthermore, the model demonstrates that bacterial growth cannot always be explained by a single constraint (such as the C:N ratio of substrate), since several constraints are often active simultaneously and continuously change with environmental conditions.