Effect of fermentation on the acetate-dominated rumen microbial fuel cells

Converting renewable biomass into electricity by the use of microbial fuel cells (MFCs) can produce clean and transportable energy. The performance of MFCs has been extensively evaluated on laboratory scales [1]. Thus far, electron suppliers for MFCs have been primarily limited to those soluble and rapidly metabolized organic compounds such as simple carbohydrates [2], [3], small organic acids [3], [4], starch [5], and amino acids [6]. Plant fiber carbohydrates, including waste from agricultural and industrial activity, are the most abundant and renewable biomass on Earth [7]. MFCs offer an opportunity to treat fibrous waste, such as straw, to concurrently generate electricity without the competition of food by humans and animals. In contrast to the non-fiber substrates mentioned above, plant fiber is relatively insoluble and is a large polymer with a diverse and complex structure [8]. Consequently, the biodegradation of fiber, coupled with the electrical output from MFCs, requires the collaborative actions of various microorganisms. In the rumen of ruminants there resides numerous symbiotic microorganisms, consisting of bacteria, protozoa, and fungi. By processing various enzymes, the microorganisms efficiently degrade plant fiber under anaerobic conditions [9], [10] with volatile fatty acids, such as acetate, propionate and butyrate, being produced in the process. During microbial fermentation of organic matter in the rumen a reducing equivalent is produced, which is accompanied by the release and translocation of protons and electrons [11]. These products could theoretically be transformed into