Books and Arts

Nature 454, 943-944 (21 August 2008) | doi:10.1038/454943a; Published online 20 August 2008

Microbial fuels for the future

Bruce Logan1

BOOK REVIEWEDBioenergy

Edited by Judy D. Wall, Caroline S. Harwood and Arnold Demain
ASM Press: 2008. 454 pp. $149.95

An individual in the United States consumes the equivalent of 100 watts of continuous power from food, but it takes more than a hundred times this amount to sustain their lifestyle. Fossil fuels cannot provide this much power for every person on the planet, and we must reduce our dependence on these fuels to address global carbon dioxide emissions and climate change. To succeed, we need sustainable and carbon-neutral sources of energy. How can we find or make these fuels?

Microbes, according to the microbiologists and biochemists who contribute to Bioenergy, hold the answers. Editors Judy Wall, Caroline Harwood and Arnold Demain have assembled 31 impressive chapters that address the opportunities and challenges of using microbes to produce bioelectricity, to help in oil recovery or to make biofuels — including ethanol, methanol, methane and hydrogen.

Microbial fuels for the future

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Finding new biofuel sources, such as algae that make hydrogen gas, might help solve the energy crisis.

Bioenergy supplies a wealth of technical information. Each chapter has an accessible introduction and each author positions their favourite fuel within the larger context of energy production and utilization. Nancy Nichols and her colleagues note that in 2008, the United States will produce 30 billion litres of fuel ethanol, mainly from corn (maize). In 2006, around 20% of the US corn crop was used to make ethanol, which represented more than 2% of all liquid fuels used for transportation. Z. Lewis Liu and co-workers say that if all of the corn grown in the United States was used to make fuel, only 15% of current US fuel needs would be satisfied. These numbers show that we will need more than corn ethanol to fuel our cars.

Using any food source as a fuel is controversial. Perhaps reflecting this, only one of the twelve chapters on bioethanol directly addresses the use of food crops. Other chapters focus on the real challenge: how to turn cellulose, the main constituent of plant cell walls, into ethanol. Microbes can break down cellulose to produce sugar efficiently, but during the process they also consume the sugar. This loss can be avoided by using enzymes instead of microbes, but enzymes are expensive to make. After ethanol is formed by fermenting the sugar, another energy-intensive process is needed to remove the water by-product.

Gaseous biofuels, such as hydrogen and methane, are made easily from many source materials using mature technologies. Hydrogen gas and volatile fatty acids, such as acetic acid, can be made from cellulose by fermentation. According to Ann Wilkie, most biomass sources can produce biomethane after limited preparation, such as drying or shredding. Certain microbes can convert acetic acid into methane gas, and methane or hydrogen can be converted to methanol. Hydrogen and methane are highly insoluble, so they can be recovered from water more easily than ethanol. In one of the seven chapters on methane and methanol, Bakul Dave reminds us that single-carbon fuels such as methanol lack carbon–carbon bonds, and therefore do not leave residues during combustion.

Combustion of hydrogen gas is better than methane as it produces only water. Three chapters are devoted to the production of hydrogen by photosynthesis in algae or bacteria, but none describes the use of fermentation or microbial electrolysis cells to make hydrogen. According to Marc Rousset and Laurent Cournac, hydrogenase enzymes that catalyse both the production and the consumption of hydrogen offer excellent opportunities to capture energy directly from sunlight, rather than through biomass, by splitting water into hydrogen and oxygen. But the sensitivity of these enzymes to oxygen needs to be decreased. Caroline Harwood describes using whole cells of purple non-sulphur bacteria to form hydrogen without splitting water. She notes that these cells can be immobilized in thin latex sheets to form panels. If perfected, this wonderful method could make hydrogen through the use of biosolar collectors.

My favourite bioenergy approach involves using bacteria to make electricity directly in microbial fuel cells. Certain strains of Geobacter might power these, but Peter Aelterman and his colleagues explain that many different types of bacteria release electrons to electrodes and can yield useful current. Why such a variety of bacteria can transfer electrons, in both directions, across their outer cell membranes remains a mystery worthy of further investigation. In the near future, microbial fuel cells could harness energy from waste water by replacing the energy-consuming bioreactors used in conventional treatment systems with those that produce bioelectricity or biohydrogen.

What is missing from Bioenergy is a discussion of the social and political implications of a microbe-based, biofuel economy. For example, growing algae or certain crops for biofuel production requires enormous amounts of water. Nutrient releases from different crops into the environment also need to be critically evaluated. The possibility of extracting methane from gas hydrates in the ocean floor is addressed but, from a climate-change perspective, the release of stored carbon in this fuel could have disastrous consequences.

Solving the energy crisis using renewable biofuels will require microbiologists, electrochemists, engineers and politicians. This book is an excellent overview of the many possible methods for harnessing microbes to make energy, and I hope it will inspire researchers from fields outside microbiology to move into bioenergy production.

  1. Bruce Logan is the Kappe Professor of Environmental Engineering at Pennsylvania State University, and director of the Penn State Hydrogen Energy Center, University Park, Pennsylvania 16802, USA. He is author of Microbial Fuel Cells.
    Email: bel3@engr.psu.edu

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