Demystifying anaerobic respiration: a problem-solving exercise

Abstract

Anaerobic respiration reactions are of fundamental importance to global biogeochemical cycling of elements. Yet, the idea that cellular respiration can occur not only in the absence of oxygen but also involve the oxidation of inorganic substrates (e.g., AsO 3 3-, Fe 2+ , H 2 , H 2 S, Mn 2+ , NH 3 , and S 0) is often foreign to many undergraduate students. This article describes a problem-solving exercise where students are introduced to the thermodynamic fundamentals of respiration with a particular focus on the role of redox (reduction-oxidation) potentials (E 0 ´). In the exercise, the students investigate how the difference in redox potential (ΔE 0 ´) between different pairs of reductants and oxidants affects the range of permissible microbial metabolic reactions in natural environments when oxygen is absent. KEYWORDS active learning, problem-based learning, anaerobic respiration, redox potential, metabolism M icroorganisms, especially prokaryotes (1), display an astounding metabolic diversity that is rarely appreciated outside the field of microbiology. Undergrad­ uate courses in biochemistry tend to focus almost exclusively on aerobic energy metabolism (respiration) of carbohydrates and lipids. Many undergraduate students within biological subjects therefore tend to believe that respiration is synonymous with oxygen dependence and organic substrates. Without a firm grasp of the fundamental principles that govern respiratory processes, the idea that respiration not only can occur in the absence of oxygen but can also involve inorganic substrates can at first appear alien to undergraduate students. The author teaches introductory microbiology as part of transdisciplinary programs in food science, crop science, and environmental science and has observed that undergraduate students often struggle to fully appreciate both the possibilities and constraints of microbial respiration under anaerobic conditions. Improving student understanding is clearly important since anaerobic respiratory processes play critical roles in the biogeochemical cycling of essential elements such as carbon, nitrogen, and sulfur-processes that are inexorably linked to agriculture and the natural environment. Redox (reduction-oxidation) potentials lie at the heart of respiratory processes but only get a brief mention in most biochemistry textbooks (2). The redox potential reflects the relative affinity that a redox couple (e.g., O 2 /H 2 O) has for electrons, which determines whether a particular redox reaction is thermodynamically favorable or not. For example, the redox couple CO 2 (oxidized state)/CH 4 (reduced state) has a redox potential (E 0 ´) of-0.20 V under standard conditions (25°C and pH 7), while the redox couple O 2 (oxidized state)/H 2 O (reduced state) has a redox potential of +0.82 V. In practical terms, this means that the oxidation of methane (the reductant) by O 2 (the oxidant) to produce CO 2 and water is thermodynamically favorable, while the reverse scenario (oxidation of water by CO 2 to produce O 2 and methane) is not. The change in free energy (ΔG°´) can be approximated from the difference in redox potentials (ΔE 0 ´) between oxidant and reductant using equation 1. Month XXXX Volume 0 Issue 0