The nation is abuzz with talk of replacing imported oil with "biofuels" produced from homegrown materials. This past April, for example, the U.S. Environmental Protection Agency honored singer Willie ("On the Road Again") Nelson for his efforts to promote the use of biodiesel through his "BioWillie" brand, which is now being distributed at filling stations nationally. Clearly, many hurdles stand in the way of making such biofuels commercially competitive with traditional sources. Indeed, it remains very difficult to forecast whether powering our vehicles with crop derivatives will ever be a truly economic proposition. Still, it's not too early to ponder what the widespread adoption of biofuels would mean for the environment and to take a hard look at the best strategy available, which may require tapping a very unconventional crop.

Some back-of-the-envelope calculations are helpful in this regard. Michael S. Briggs, a biodiesel advocate at the University of New Hampshire, has estimated that the United States would need about 140 billion gallons of biodiesel each year to replace all the petroleum-based transportation fuels currently being used. This calculation is premised on the idea that Americans could over time switch to using diesel vehicles, as European drivers are clearly doing. (Half the cars sold in Europe now run on diesel.) Although one could make a similar appraisal for the amount of biomass-derived ethanol needed to fill all of our transportation-fuel needs, it would be unlikely that drivers would ever want to tank up entirely on ethanol, which contains only two-thirds the energy of gasoline gallon for gallon, whereas biodiesel (according to Briggs) ends up being only 2 percent less fuel-efficient than petroleum-based diesel. Hence a switchover would demand no new technology and would not significantly reduce the driving range of a car or truck.

The chief feedstock for biodiesel is plant oil derived from one crop or another. Many candidates have been considered—including hemp. Perhaps a more reasonable choice is rapeseed oil. An acre of rapeseed could provide about 100 gallons of biodiesel per year.  To fuel the country this way would thus require 1.4 billion acres of rapeseed fields. This number is a sizable fraction of the total U.S. land area (2.4 billion acres) and considerably more than the 400 million or so acres under cultivation in this country. So the burden on freshwater supplies and the general disruption that would accompany such a switch in fuel sources would be immense.

This simple exercise is sobering. It suggests that weaning ourselves from petroleum fuels and growing rapeseed instead to power the nation's vehicles would be an environmental catastrophe. Are more productive oil crops the answer? Oil palms currently top the list because they can provide enough oil to produce about 500 gallons of biodiesel per acre per year, which reduces the land requirement fivefold. Yet its cultivation demands a tropical climate, and its large-scale production, which currently comes from such countries as Malaysia and Indonesia, is a significant factor in the ongoing destruction of what rainforest remains there. Conservationists have been warning that palm oil production poses a dire threat to the dwindling population of orangutans, for example, which exist in the wild only in Borneo and Sumatra. Even the World Bank attributes the accelerating rate of forest clearing in Indonesia largely to the establishment of oil-palm plantations. So here again, the prospect of dedicating sufficient land to growing feedstock for the world's transportation needs promises to be an environmental nightmare.

There is, however, a "crop" that is widely recognized as having the potential to meet the demands of a biodiesel-based transportation fleet without devastating the natural landscape: algae. Some varieties of these single-celled plants can contain 50 percent or more oil. And they grow much more rapidly than ordinary cultivars—with doubling times that can be as short as several hours.

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The U.S. Department of Energy funded considerable research on biofuel production using algae after the oil shocks of the 1970s, an effort known as the Aquatic Species Program. Although this DOE program was terminated in the mid-1990s, much experience was gained through research and various demonstration projects. The results suggested that algae can be grown in sufficient density to provide for the production of several thousand gallons of biodiesel per acre per year—a full order of magnitude better than can be expected using palm oil and two orders of magnitude better than soybeans.

So it is no wonder that many scientists and entrepreneurs are once again looking hard at the prospects for using algae to produce transportation fuels. And sizable amounts of money are being invested in various schemes for doing so. "When Katrina hit and the price of diesel went to $3.20, that's when the flood gates opened," says David J. Bayless, a professor of mechanical engineering at Ohio University in Athens, Ohio.

Bayless and his colleagues have been working with scientists at Oak Ridge National Laboratory to engineer a device that can grow cyanobacteria ("blue-green algae"). It uses carbon dioxide from power-plant flue gases and sunlight that is captured by a parabolic dish and distributed to the growing surfaces through optical fibers. With his enclosed bioreactor, Bayless claims to be able to produce as much as 60 grams of biomass per square meter of growing surface per day, which is about twice the highest long-term productivity achieved by the Aquatic Species Program in a large-scale demonstration in Hawaii using open ponds. Whether Bayless's system can be scaled up and operated economically remains to be seen, but some people appear to think there's a possibility: Manhattan-based Veridium Corporation has licensed this invention in hopes of commercializing it.

Isaac Berzin, who left MIT to found a Cambridge startup named GreenFuel Technologies Corporation, is developing algal bioreactors that would similarly exploit the carbon dioxide coming out of power plants to fertilize the growth of algae that can then be used to produce biodiesel. John Lewnard, who is vice president of process development for GreenFuel, is keenly aware of the challenges involved in devising a bioreactor that costs little and supports sufficient productivity that excessive land use is not a factor.

GreenFuel Technologies operated a prototype system at the MIT cogeneration facility and is currently setting up a more advanced bioreactor at a natural-gas fired power station at an undisclosed location in the American Southwest, where having abundant sunlight for growing algae is presumably less of a problem than in Cambridge. Lewnard says that productivities of about 100 grams of algae per meter squared per day (about three times what was demonstrated during the Aquatic Species Program) is needed to achieve commercial viability, adding that "we're working very hard to meet that target."

Another recent effort of this type is being carried out by Kent SeaTech Corporation, which has its headquarters in San Diego. This company has gained experience growing algae in conjunction with its striped bass aquaculture operations near California's Salton Sea.  So it was poised to respond when the California state government started looking for ways to treat the water flowing into this closed basin, which receives huge quantities of nutrient-laden runoff from adjacent agricultural lands.

"It's no real difficult feat to turn nutrients into algae," says Kent SeaTech's director of research, Jon C. Van Olst, "but how do you get it out of the water? They are almost impossible to harvest." Van Olst and his coworkers have been devising ways to enhance the settling of the algae, and they are currently doing research on turning algal biomass into useful fuel. Van Olst believes that several separate benefits have to come together to make growing algae an attractive proposition—the removal of nutrients from wastewater, the capture of carbon dioxide that would other otherwise go into the atmosphere, the production of biodiesel from algal oils and the use of the remaining biomass for animal feeds. "All those things together might [make] this cost effective," he says.

The people now working on these and several similar commercial ventures are clearly eager to make growing algae a going business in this country. Yet it's not hard to find experts who view such prospects as dim indeed. John R. Benemann, a private consultant in Walnut Creek, California, manages the International Network on Biofixation of CO₂ and Greenhouse Gas Abatement with Microalgae for the International Energy Agency. He helped author the final report of the Aquatic Species Program and has decades of experience in this field. "Growing algae is cheap," he says, but "certainly not as cheap as growing palm oil." And he is particularly skeptical about attempts to make algal production more economical by using enclosed bioreactors (rather than open ponds, as were used for the Aquatic Species Program). He points out that Japan spent hundreds of millions of dollars on such research, which never went anywhere. Asked to comment about why there is so much effort in that direction now, he responds, "It's bizarre; it's totally absurd."

Even more telling is the reaction of Gerald R. Cysewski, president and chief executive officer of Cyanotech Corporation, a Hawaii company that grows algae for sale as a food supplement. Cysewski, who holds a doctorate in chemical engineering from UC Berkeley, is no stranger to the biofuels concept: For his Ph.D. (which he earned more than two decades ago), he studied the enzymatic production of ethanol from cellulose. Yet he has turned down recent offers to collaborate on projects to use algae for producing biofuels, preferring to keep his business focused on products that sell anywhere from $18 to $380 per kilogram (fuel oil, he points out, goes for something like 45 cents per kilogram). "In the laboratory, you can create some very efficient bioreactors, but it just isn't scalable," he says. Asked whether biodiesel will ever be made this way, he responds: "Not from microalgae—I just can't see it."

Even Kent SeaTech's Van Olst has serious reservations. "I'd put myself in with the skeptics," he says. "It may work, but it's going to take a while and a lot of research before we get anywhere."