foreseeable future (Pacheco, 2006). Many of the impediments are engineering challenges associated with how and where to grow the algae to achieve needed productivity. Because of the metabolic burden associated with the biosynthesis of high-energy lipids, production strains that accumulate high levels of oil tend to grow and reproduce more slowly than strains that do not. As a consequence, open cultures are subject to contamination by undesirable species unless the production strain is able to grow in specialized conditions that restrict the growth of those other species (for example, high-alkalinity environments).
Alternatively, production strains selected for high growth rates and high biomass yields without regard for oil content can often compete satisfactorily with contaminating strains, but the chemical composition of the algae would be better suited for anaerobic digestion than liquid fuel production. The use of closed photobioreactors can significantly lower the risk of culture contamination, although the capital costs of such systems are high.
Algal biodiesel has properties similar to those of biodiesel made from vegetable oil, except that algal biodiesel has better cold-weather properties. The energy content per gallon of biodiesel is about 93 percent that of petroleum-based diesel fuel, and it has a cetane number between 50 and 60, with 55 being typical. Also, it is somewhat more viscous. Biodiesel can be distributed by existing infrastructure and used in unmodified diesel-engine vehicles.
Butanol is a four-carbon-atom alcohol—as opposed to ethanol, which is a two-carbon-atom alcohol. Biobutanol, the name given to butanol that has been made from biomass, is another potential entrant into the automotive biofuel market, and several technologies for producing it are in the R&D stage. The one receiving the most attention is the acetone-butanol-ethanol process. As currently envisioned, it involves the biochemical conversion of sugars or starches (from sugar beets, sugar cane, corn, wheat, or cassava) into biobutanol using a genetically engineered microorganism, Clostridium beijernickii BA101. The midterm goal is to start with cellulose, but that goal awaits the demonstration of economic success in converting cellulose and hemicelluloses into sugars.
Biobutanol has many attractive features as a fuel. Its energy content is close to that of gasoline, it has a low vapor pressure, it is not sensitive to water, it is less hazardous to handle and less flammable than gasoline is, and it has a slightly higher octane than gasoline has. Thus it is likely to be compatible with the exist-