7. Electron Bifurcation in the Ecosystem and its Role in the Degradation of Cellulose

Abstract

Cellulose, one of the most common polymers on earth, is difficult to metabolize but can be broken down by select anaerobic bacteria though the process of fermentation. However, environmental circumstances may present challenges for anaerobic bacteria, especially the recycling of their electron carriers. Hydrogen production is a preferred method of electron carrier recycling but presents thermodynamic restrictions. One way of maximizing limited energy yields is through a process called electron bifurcation. Electron bifurcation is a recently recognized third mechanism of biological energy conservation which couples exergonic and endergonic reactions to limit free energy loss. Bifurcating enzymes are characterized by the presence of two electron donors and one receptor, or by one electron donor and two receptors. Anaerobic hydrogen production through bifurcation maximizes the free energy in the anaerobe’s metabolism. When negative feedback from hydrogen inhibits hydrogen production, alternative anaerobic pathways such as ethanol or butyrate fermentation take over. Our research is being conducted on samples taken from different environments and is designed to indicate which pathways dominate. In the presence of a bifurcating hydrogen producing pathway, ethanol will not be produced. In the case of an ethanol producing pathway, less hydrogen is likely to be produced. For our research, cultures obtained from four different ecological environments will be incubated in separate containers for several weeks. Chromatography paper will be added to determine the level of cellulose degradation. Hydrogen, acetate, ethanol, and other fermentation products will be measured to determine which pathway the anaerobic cellulose degraders have taken.