Power plants: Algae churn out hydrogen.

Could the green scum that grows on the walls of a fish tank produce the fuel of the future? Some scientists think so. They've found a way to coax green algae into producing significant amounts of hydrogen gas. In these researchers' view, large pools of algae could generate clean-burning hydrogen fuel for cars and other applications.

As microscopic plants, algae use photosynthesis to create sugars from water, carbon dioxide, and sunlight. Algae also have the biochemical machinery to produce hydrogen, notes Tasios Melis of the University of California, Berkeley. Under some conditions--in the absence of oxygen, for example--algae strip hydrogen from some of their proteins. This process allows the cell to maintain its production of adenosine triphosphate, or ATP, an energy molecule that powers many cell functions.

Scientists trying to tap the hydrogen-making potential of algae faced a difficult problem. Plants produce oxygen during photosynthesis, and oxygen deactivates the enzyme that makes hydrogen.

"The algae know that hydrogen and oxygen don't mix," Melis says. The two gases react easily to give off a burst of heat. The enzyme probably evolved its sensitivity to oxygen to protect against that danger, he says.

Melis and his coworkers discovered a way around this dilemma. By depriving the algae of sulfur, which the cells need to make several important proteins, the researchers can turn off normal photosynthesis. This shuts down the algae's oxygen production and forces the cells to make hydrogen instead. Melis presented his group's findings this week in Washington, D.C., at the annual meeting of the American Association for the Advancement of Science.

To prevent the algae from dying during the hydrogen production, the researchers must permit them every few days to photosynthesize for a few days before beginning another round of hydrogen generation.

"We enrich them, then bleed them off," says Melis. "They can cycle between [these two stages] for a very long time."

Currently, a liter of an algal culture can produce about 3 milliliters of hydrogen per hour. Melis predicts that his group can increase that yield 10-fold. At that higher rate, a 500-square-foot pond about a foot deep could generate enough fuel to power a car under average use, estimates Margaret K. Mann, a chemical process engineer at the National Renewable Energy Laboratory in Golden, Colo.

It's too soon to say how much it would cost to use algae for hydrogen production. "It's a very new process," Mann says. "We don't have the data for an economic analysis yet."

Other technologies--for example, splitting water with solar cells (SN: 4/18/98, p. 246)--are being developed for energy companies to generate hydrogen. "Hydrogen has the potential to solve many of the environmental problems associated with energy use," says Mann. Algal production could eventually fill part of the need, she adds.