The potential to optimize the activities of the microbial catalysts via culturing approaches and genetic engineering or adaptive evolution make this platform attractive for the processing of agricultural wastes, the researchers suggested in a paper published in the ACS journal Environmental Science & Technology.

Native lignocellulose degraders show promise as CBP catalysts because their hydrolysis and fermentation efficiencies are naturally evolved to maximize cell growth yields from biomass. However, these microorganisms are adapted to growing within specialized, synergistic consortia, where fermentation products are rapidly removed to prevent feedback inhibition of biomass decomposition and fermentation using various electron acceptors as final electron sinks.

The possibility of mimicking CBP consortia in bioelectrochemical cells is attractive because an electrode can be used to replace the natural electron acceptors and model exoelectrogens such as Geobacter sulfurreducens are available that conserve energy for growth by transferring electrons from waste fermentation products such as acetate, formate, lactate, and H₂ to electrodes.

Furthermore, with sufficient electrical input the current generated in the anode can be converted into H₂ in the cathode chamber in a microbial electrolysis cell (MEC), thus producing H₂ fuel as a coproduct.

—Speers and Reguera (2012)

MECs are attractive as CBP platforms for ethanol because the electrical input can be used to produce H₂ simultaneously in the cathode at much higher yields than those achieved fermentatively, the authors note. Furthermore, the applied potential removes cathodic limitations and promotes the growth of exoelectrogenic biofilms on the anode electrode, which in turn maximizes the conversion of fermentation byproducts to cathodic H₂ while preventing the accumulation of feedback inhibitors. However, they added, it is important to minimize electron losses by selecting CBP strains that produce fermentation byproducts that serve as electron donors for the exoelectrogen.

For their system, Gemma Reguera, MSU microbiologist, and Allison Spears, MSU graduate student, selected C. uda for its ability to produce ethanol from AFEX-CS and for producing electron donors for G. sulfurreducens fermentatively. G. sulfurreducens generates electricity used to generate hydrogen in the MEC to increase the energy recovery process even more.

The fermentation and electrical conversion efficiencies were high, but much of the AFEX-CS remained unhydrolyzed as nitrogen availability limited the growth of the CBP partner. Nitrogen supplementation stimulated the growth of C. uda, AFEX-CS hydrolysis and ethanologenesis.

Similar microbial fuel cells have been previously investigated. However, maximum energy recoveries from corn stover, a common feedstock for biofuels, hover around 3.5%. Reguera’s platform, despite the energy invested in chemical pretreatment of the corn stover, averaged 35 to 40% energy recovery just from the fermentation process.

This is because the fermentative bacterium was carefully selected to degrade and ferment agricultural wastes into ethanol efficiently and to produce byproducts that could be metabolized by the electricity-producing bacterium. By removing the waste products of fermentation, the growth and metabolism of the fermentative bacterium also was stimulated. Basically, each step we take is custom-designed to be optimal.

When the MEC generates hydrogen, it actually doubles the energy recoveries. We increased energy recovery to 73 percent. So the potential is definitely there to make this platform attractive for processing agricultural wastes.

—Gemma Reguera

Reguera’s fuel cells use corn stover treated by the ammonia fiber expansion process, an advanced pretreatment technology pioneered at MSU. AFEX is an already proven method that was developed by Dr. Bruce Dale, MSU professor of chemical engineering and materials science.

Dale is currently working to make AFEX viable on a commercial scale.

In a similar vein, Reguera is continuing to optimize her MECs so they also can be scaled up on a commercial basis. Her goal is to develop decentralized systems that can help process agricultural wastes. Decentralized systems could be customized at small- to medium-scales (scales such as compost bins and small silages, for example) to provide an attractive method to recycle the wastes while generating fuel for farms.

The MEC platform fed with AFEX-CS and described herein addresses the need to decouple bioenergy production from the food supply, to reduce processing costs through the use of lignocellulose substrates, and to carry out a single-step hydrolysis and fermentation while minimizing the accumulation of low-value fermentation byproducts that can also function as feedback inhibitors.

Relatively simple culturing approaches such as nitrogen supplementation were sufficient to improve the growth of the CBP partner and the electrical conversion of waste fermentation products by the exoelectrogen in the MEC. Further optimization of the culturing conditions shows promise to increase the activity of the microbial catalysts so as to improve the performance of the platform. This, and the possibility of genetically engineering and/or adaptively evolving the microbial catalysts for improved hydrolysis, saccharification, and electrical conversion, suggests that the processing of lignocellulose substrates in MECs can provide an economically and environmentally attractive CBP technology for ethanol and H₂.

—Speers and Reguera (2012)

Resources

• Allison M. Speers and Gemma Reguera (2012) Consolidated Bioprocessing of AFEX-Pretreated Corn Stover to Ethanol and Hydrogen in a Microbial Electrolysis Cell. Environmental Science & Technology doi: 10.1021/es3008497