A Generalized Method for Thermodynamic State Analysis of Environmental Systems

Abstract

Chemotrophic life depends on the regular supply of new material to maintain a thermodynamic non-equilibrium situation. A thermodynamic non-equilibrium situation is characterized by significant driving forces to drive phase and/or electron transfer in redox reactions. Microorganisms play an important role as catalysts of thermodynamically feasible chemical redox reactions, and are capable of utilizing the energy generated for microbial growth. An improved insight in the functioning of ecosystems and the microbial life encountered can be established by analyzing the thermodynamic state of the ecosystem. Still, conducting an extensive thermodynamic state analysis based on measured information of an ecosystem is generally considered a rather complicated and laborious task. In this paper, we describe a generalized and step-wise method for conducting an extensive thermodynamic state analysis of an environmental ecosystem. The method is based on the identification of the reactants in the system and the derivation of the stoichiometry of the catabolic and anabolic reaction. In a subsequent step, the thermodynamic system properties are calculated, and different methods to establish a full description of microbial metabolism are obtained. Several examples are presented to clarify the method proposed. We hope this relatively straightforward method encourages researchers of microbial ecosystems to include thermodynamic state analysis as an integral part of their research.

[Supplemental materials are available for this article. Go to the publisher's online edition of Critical Reviews in Environmental Science and Technology for the following free supplemental resource: an elaboration in MS Excel of examples in the text regarding the fully generalized method described in this paper.]

KEY WORDS:

• thermodynamics
• stoichiometry
• Gibbs free energy
• microbial growth

ACKNOWLEDGMENTS

Robbert Kleerebezem wishes to thank Professor Ramón Méndez Pampín for giving the opportunity to elaborate on the concepts proposed here in the framework of an annual doctoral course on Environmental Thermodynamics at the University of Santiago de Compostela, Spain. He furthermore wishes to thank Dr. Jorge Rodriguez for extensive collaboration and numerous discussions on thermodynamics related issues during the past years. The authors acknowledge the financial support of the Dutch Technology Foundation (STW), project no. DPC.5904.

Notes

* The γ values for N and S compounds (N _x H _y O _z ^v and S _x H _y O _z ^v ) are calculated using γ = v − y + 2 · z. For organic compounds (C _x H _y O _z N ^{− III} _u ^{− II} _w ^v with γ _N = − 3 and γ _S = − 2), the degree of reduction is calculated using γ = v − y + 2 · z + 3 · u − 2 · w.

^†The COD-content is calculated using the equation derived in Eq. (A.3).

^‡The last two columns show the electron yield for oxidation to the most oxidized form in an electron donor reaction (Y _e ^D ) and the Gibbs energy change per electron of the reaction (Δ G _e ⁰¹).