Experimental evidence for the need of thermodynamic considerations in modelling of anaerobic environmental bioprocesses

Abstract

For modeling of biological processes that operate close to the dynamic equilibrium (eg. anaerobic processes), it is critical to prevent the prediction of positive reaction rates when the reaction has already reached dynamic equilibrium. Traditional Michaelis-Menten based models were found to violate the laws of thermodynamics as they predicted positive reaction rates for reactions that were endergonic due to high endproduct concentrations. The inclusion of empirical “product inhibition factors” as suggested by previous work could not prevent this problem. This paper compares the predictions of the Michaelis-Menten Model (with and without product inhibition factors) and the Equilibrium Based Model (which has a thermodynamic term introduced into its rate equation) with experimental results of reactions in anaerobic bacterial environments. In contrast to the Michaelis-Menten based models that used traditional inhibition factors, the Equilibrium Based Model correctly predicted the nature and the degree of inhibition due to endproduct accumulation. Moreover, this model also correctly predicted when reaction rates must be zero due to the free energy change of the conversion reaction being zero. With these added advantages, the Equilibrium Based Model thus seemed to provide a scientifically correct and more realistic basis for a variety of models that describe anaerobic biosystems.

Long chain fatty acids degradation in anaerobic digester: Thermodynamic equilibrium consideration

“…When interpreted in the context of our overall thermodynamic model it is clear that Mosche and Jordening [31] have simply demonstrated that propionate degradation was not the rate controlling step of their batch anaerobic digestions. A number of authors argue that thermodynamic considerations must be included in bio-kinetic studies of anaerobic digestion [20,30,32,33]. Kaspar and Wuhrman [30], and Hoh and Cord-Ruwisch [32,33] empirically demonstrated that propionate degradation in an operating digester proceeds close to equilibrium (DG = 0) despite the methanogenesis reaction being far from equilibrium.…”

“…A number of authors argue that thermodynamic considerations must be included in bio-kinetic studies of anaerobic digestion [20,30,32,33]. Kaspar and Wuhrman [30], and Hoh and Cord-Ruwisch [32,33] empirically demonstrated that propionate degradation in an operating digester proceeds close to equilibrium (DG = 0) despite the methanogenesis reaction being far from equilibrium. The observations of these groups show that the acclimatisation of anaerobic sludges selects the best microbial populations [47] which, under the proper oxidation/reduction conditions are able to effectively degrade the linear fatty acids produced in the acido-acetogenesis (fermentation) until the system approaches the equilibrium state.…”

“…A few researchers [20,[29][30][31][32][33][34][35][36] reported anaerobic short chain fatty acids degradation showing product inhibition when the system is close to the equilibrium point (DG = 0). The observations suggest that a thermodynamic study needs to demonstrate the equilibrium condition more generally.…”

Using the second law of thermodynamics for enrichment and isolation of microorganisms to produce fuel alcohols or hydrocarbons

“…2 as the difference between ΔG for acetate production and ethanol production from CO 2 and H 2 . Typically in the rumen or anaerobic digester, the ratio of H 2 to CO 2 is about 0.001 (Hoh and Cord-Ruwisch, 1997;Ungerfeld and Kohn, 2006). Under such conditions, it is thermodynamically feasible to convert these gases to methane with production of 1 ATP per mole of methane, and H 2 is readily used to make methane in the rumen, keeping the H 2 pressure low.…”

“…cow's rumen) is explained as near-equilibrium in accordance with the second law of thermodynamics. First, the values of ΔG are near zero for many reactions in the rumen (Kohn and Boston, 2000;Ungerfeld and Kohn, 2006), or a manure or a sewage digester (Hoh and Cord-Ruwisch, 1997;Jackson and McInerney, 2002) including methane or acetic acid production from CO 2 and H 2 and interconversion of acids. Furthermore, the rates of interconversion of one acid to another are similar in each direction as measured using isotopic labels (Ungerfeld and Kohn, 2006).…”