Although the US and other nations currently produce ethanol from the sugars and starches of crops like sugar cane and corn, ethanol isn't a good match for our existing fuel infrastructure—and this form of production runs the risk of putting energy in competition with food production for resources like land and water. As a result, attention has shifted to figuring out how to produce a new generation of biofuels from different sources that more closely approximate the diesel and gasoline in use today.

Yesterday's edition of Science contained a perspective on the prospects for these next-generation biofuels. Although it's very short, only a page long, it contains an excellent list of references to very current publications. The diversity of approaches it covers highlights how many options there are to produce different fuels, each with its own advantages and drawbacks. In the following sections, we'll discuss different methods of producing biofuels; although the text presents them as alternatives, it's important to emphasize that there are significant overlaps among them, and more than one technology might emerge for widespread use.

Cellulose or lipids: The first question is what you want the initial fuel stock to be. Cellulose, a polymerized sugar, is found in combination with proteins and other compounds; together, they provide plants with their structure. It's available in abundance, and all sources of cellulose are more or less functionally equivalent, so a cellulose processing facility could work with a huge range of sources: agricultural waste, wood waste from lumber processing, leftover construction material, and dedicated biofuel crops like switchgrass. A recent USDA report suggests that the US has an annual supply of a billion tons of cellulose to work with.

The problem is that cellulose doesn't break down easily or quickly, and its component parts—sugars and proteins—lend themselves most readily to the production of ethanol. The alternative is to go with something like lipids, which are long-chained hydrocarbons that are chemically more similar to fuels.

As with cellulose, a lipid is a lipid, regardless of the source. Unfortunately, there are far fewer sources, since plants don't generally contain an excess of them. In a few cases, lipid-processing facilities have been built to operate using the waste from meat packing plants, but a larger source of lipids would require the development of an algae that grows well in waste or salt water.

Industrial or biological: Once the material itself is isolated, it generally needs processing before it's ready for use as fuel. Here, working with lipids has huge advantages, as they can generally be fed into the same sort of reactions that are used during oil refining.

Cellulose is another matter. Several industrial processes can quickly break it down into more chemically-flexible components, but all of them require elevated temperatures and pressures, and thus a substantial energy input.

Once broken down, catalysts can rearrange the simpler components into usable fuels. But there are significant barriers to using the processes which we've already developed, designed to handle relatively pure mixtures. It's difficult to fully eliminate water from cellulose, the plant material contains an unwanted mix of nitrogen and sulfur compounds, and the pH of the reaction material can be difficult to control. Any one of these issues can interfere with the action of an essential catalyst, so finding a combination that provides high energy payback and works well with cellulose isn't a simple thing.

The alternative is to go biological, as there are organisms that make their living by digesting cellulose. Unfortunately, as anyone knows who has ever seen downed trees rot, the process is extremely slow. There are ways to speed it up, of course, but there's a very clear trade-off between speed and energy input. Once it's digested, researchers still face the challenge of finding a way to convert its component sugars to something more useful than ethanol.

Existing or engineered organisms: The ideal situation, of course, would be to have an organism that does everything for us, from digesting ethanol to spitting out some sort of lipid derivative that can be fed directly into fuel tanks. Life, however, is rarely that simple; evolution tends to frown upon organisms that release energetically valuable chemicals into their environments, and we'd be growing any biofuel organisms in the sorts of numbers that allow evolution plenty of opportunity.

That said, there are some organisms that happen to do some very convenient things on their own, like a bacterial strain that stores energy in lipids to such a degree that nearly 80 percent of its dry mass is in lipids. Unfortunately, we know almost nothing else about the strain, including the details of metabolism and whether they can be engineered in the same way we handle more common bacteria, like E. coli. And some form of engineering would almost certainly be needed if we're going to have them growing on cellulose.

The alternative approach would be to take an organism we do know well, like E. coli, and engineer it so that it could digest cellulose and directly produce a chemical that could be easily converted to fuel. (A review of microbial fuels lists a variety of intermediate-chain hydrocarbons and alcohols that might work.) A sufficiently hydrophobic liquid, once outside the cell, should separate from the liquid the bacteria are growing in, allowing easy harvesting. But, at the moment, we only have limited experience with re-engineering an organism's metabolism to do something that might not do said organism much good.

Because of all the complex trade-offs, it's likely that there won't be a one-size-fits-all biofuel solution, but the energy available to us (based on the USDA report) is substantial and could dramatically cut our imports of fossil fuels—up to 30 percent, based on a report that predates the recently tightened fuel economy standards.

Fortunately, it appears that the market is taking the advice of the National Academies and developing a portfolio of approaches. The Science perspective notes that a handful of companies already claim they're on track to begin producing non-ethanol biofuels on the scale of millions of gallons within the next five years, and more will join them within the next decade. Although none of them are giving away the precise details of what they're up to, it's clear that they are taking a variety of different approaches to producing that fuel. Gas has never looked greener.