As the effort to create renewable energy sources picks up, biofuels are attracting a lot of attention, not all of it good. Initial efforts have focused on extracting energy from sugars and starches in food crops, causing a competition for resources that has raised food prices globally. Researchers have already started looking for ways to move beyond this competitive state by avoiding the use of food plants, and three documents published this week explore the challenges and potential in these approaches.

Nature Reviews Genetics has published an extensive review of the potential of genetic engineering to improve the efficiency of cellulosic ethanol. The robust bodies of all plants, including everything from crops to trees, are actually composed of sugars. Unfortunately, those sugars are linked together and bundled into a complex structure called cellulose, which is insoluble and largely unreactive. Cellulosic ethanol relies on breaking down the cellulose to its component sugars, which are then converted to ethanol. It's flexible enough to work with non-crop species, such as switchgrass, or the non-edible portion of crop species, such as the body of a corn plant.

Right now, generating cellulosic ethanol is fairly energy intensive. The fuel source has to be extensively processed to be made accessible to enzymes that target cellulose, which need to be obtained separately from growing stocks of bacteria or fungi. The review focuses on ways that genetic engineering could make various stages of this process a bit easier. The simplest approach involves engineering the fuel plants to produce their own cellulose digesting enzymes and store them in a safe compartment inside a cell. Then, the initial processing of the plant could release the enzymes needed to start the fuel production process.

Other ideas discussed would improve the plants themselves. These include boosting plant growth by increasing a hormone that regulates plant height, or boosting the cellulose content by adding additional copies of the genes that catalyze its synthesis. Another approach would be to make the cellulose content easier to digest by reducing the dose of the enzymes that produce the crosslinks among cellulose fibers or adding modifications that make cellulose more soluble.

A note of caution about what plants to engineer, however, was sounded by the Global Invasive Species Programme, which published a set of recommendations (PDF) regarding a number of species that have been proposed as potential biofuels sources. Quite sensibly, most plants under consideration grow quickly and in a variety of climates. Unfortunately, these are precisely the properties that make a plant an invasive species. The long list of species that GISP examined indicates that nearly every plant being considered is an invasive species somewhere; the primary exceptions are existing crop plants, like wheat, peanuts, and soy.





Switchgrass, a potential biofuel.

The GISP recommendations are largely focused on keeping countries or industries from driving ahead with biofuels programs that haven't been carefully evaluated for the potential of unintended consequences. They call for basic evaluations like risk assessments and cost/benefit analyses to be done before any new program is started, and to have working programs include ongoing risk management work driven by a certification process. Obviously, where possible, native species should be used. It's difficult to find fault with any of these recommendations.

Meanwhile, Science ran a short editorial that pondered the other part of the equation, the bacteria that produce the enzymes that digest the cellulose and convert it to useful fuels. It also goes on to consider how symbiotic bacteria might help foster the growth of useful plants, or other strains might help us sequester carbon. It's short, but it wraps up by arguing that scientists need a better understanding of the diversity of bacterial ecosystems and the way they interact with the carbon cycle, and the public needs a better understanding of why that science is important.

Overall, the attention being paid to what might be termed second generation biofuels, ones that aren't based on food crops, appears to represent a significant shift, a shift from a "quick and dirty" approach to one involving some forethought and planning.

Nature Reviews Genetics, 2008. DOI: 10.1038/nrg2336

Science, 2008. DOI: 10.1126/science.1159999