Generation of Electricity and Analysis of Microbial Communities in Wheat Straw Biomass-Powered Microbial Fuel Cells

Electricity generation from wheat straw hydrolysate and the microbial ecology of electricity-producing microbial communities developed in two-chamber microbial fuel cells (MFCs) were investigated. The power density reached 123 mW/m² with an initial hydrolysate concentration of 1,000 mg chemical oxygen demand (COD)/liter, while coulombic efficiencies ranged from 37.1 to 15.5%, corresponding to the initial hydrolysate concentrations of 250 to 2,000 mg COD/liter. The suspended bacteria found were different from the bacteria immobilized in the biofilm, and they played different roles in electricity generation from the hydrolysate. The bacteria in the biofilm were consortia with sequences similar to those of Bacteroidetes (40% of sequences), Alphaproteobacteria (20%), Bacillus (20%), Deltaproteobacteria (10%), and Gammaproteobacteria (10%), while the suspended consortia were predominately Bacillus (22.2%). The results of this study can contribute to improving understanding of and optimizing electricity generation in microbial fuel cells.Wheat straw is one of the most abundant renewable resources. According to the Food and Agriculture Organization of the United Nations, approximately 1.9 × 10⁹ tons of wheat straw annually are produced worldwide, accompanied by 6.2 × 10⁸ tons of wheat production. Wheat straw is composed of 35 to 45% cellulose and 20 to 30% hemicelluloses with a relatively low lignin content (<20%) (42). The hemicellulose fraction of the straw is easily hydrolyzed to its constituent sugars by a hydrothermal treatment process, forming a carbohydrate-enriched liquid hydrolysate (46). Chemical and biological approaches to sustainable energy production from the liquefied hydrolysates to energy carriers, such as methane, ethanol, and H₂, have been developed. However, many of these approaches encounter technical and economical hurdles (10, 12, 15, 16). An alternative strategy is direct conversion of wheat straw biomass to electrical energy in microbial fuel cells (MFCs).MFCs are bioelectrochemical reactors in which microorganisms mediate the direct conversion of chemical energy stored in organic matter or bulk biomass into electrical energy (12, 15, 16, 40). Various substrates, such as simple carbohydrates, low-molecular-weight organic acids, starch, amino acids, chitin, cellulose, domestic wastewater, food-processing wastewater, recycled paper wastewater, and marine sediment organic matter, have been successfully utilized for power generation in MFCs (16-18, 27, 30, 33). To understand the microbial constraints on various fuel-powered MFCs, microbial communities have been characterized by several groups. Microbial communities from various systems are very different and often diverse, ranging from well-known metal- and anode-reducing bacteria to unknown exoelectrogens (1, 20, 21). It has been found that parameters such as the substrates used as fuels and the inocula used for starting up the MFCs can influence the anode bacterial communities in an MFC, which subsequently influence the efficiency of the MFCs (3, 14, 22, 38, 44). Different pure substrates, such as acetate, glucose, and lactate, were used as fuel to compare the microbial communities that developed in the MFCs. Regardless of the different substrates, all anode communities contained sequences closely affiliated with Geobacter sulfurreducens (>99% similarity) and an uncultured bacterium clone belonging to the family Bacteroidaceae (99% similarity). Firmicutes were only found in glucose-fed MFCs (20). Microbial-community analyses of MFCs powered with complex substrates have also been performed by several researchers, and their results were very diverse. The microbial community in starch wastewater-powered MFC was dominated by unidentified bacteria (35.9%), followed by Betaproteobacteria (25.0%), Alphaproteobacteria (20.1%), and the Cytophaga/Flexibacter/Bacteroides group (19.0%) (21). The anode-attached consortia in a cellulose-powered MFC were related to Clostridium spp., while Comamonas spp. were abundant in the suspended consortia (13). Although many studies have reported the microbial compositions of MFCs, it is still unclear which microbial communities develop as a function of the external parameters.Wheat straw biomass constitutes a large source for bioenergy production and shows promising prospects for electricity generation in MFCs. Therefore, wheat straw biomass was used to study the microbial communities that develop during the operation of an MFC in order to better understand the microbial electrochemical roles and potentially improve MFC performance.The objectives of this study were to (i) test wheat straw hydrolysate as a potential fuel in an MFC for electricity generation and (ii) study the microbial composition and evolution of electricity-producing communities in a two-chamber MFC system. Phylogenetic-diversity analysis of the enriched consortia was conducted to verify the presence of hydrolytic and respiratory anaerobes that could couple hydrolysate oxidation with proton reduction in the anode chamber. This is the first report of exploiting microbial communities for direct conversion of wheat straw hydrolysate to electrical energy in an MFC.