Power From Pond Scum

Jocelyn is a staff writer for Science magazine.

WASHINGTON, D.C.--Suppose you could fuel up your car by dipping a hose into your garden pond. That's roughly the idea behind a decades-old dream of using algae to split water into oxygen and hydrogen, the cleanest fuel there is. Now, two new discoveries have brought the idea a little closer to reality, researchers reported here on 21 February at the annual meeting of the American Association for the Advancement of Science, ScienceNOW's publisher.

Since the only product of burning hydrogen is water, hydrogen-powered vehicles or machines would produce no pollution at all--not even the greenhouse gas carbon dioxide, if the hydrogen were made from renewable sources. Green algae could potentially churn out lots of hydrogen because they can split water into hydrogen and oxygen. But there's a major catch: These one-celled plants also produce oxygen through photosynthesis, which shuts off the water-splitting enzyme, hydrogenase. As a result, the algae produce only trace amounts of hydrogen.

Plant biologist Tasio Melis of the University of California, Berkeley, reported one trick to get around this problem in Chlamydomonas reinhardtii, a common type of green algae. When Melis and his co-workers at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, starved the algae of sulfate salts, their photosynthesis machinery shut down; instead, the microbes turned to the hydrogen-producing pathway. As the team reported in the January issue of Plant Physiology, they could get a modest 3 milliliters of hydrogen per hour to bubble up from a 1-liter bottle of algae. After 4 days, though, they had to let the algae switch back to photosynthesis to rebuild burned-up proteins.

Another approach, described by biophysicist Elias Greenbaum of Oak Ridge National Laboratory in Tennessee, is to leave a lot of nitrogen gas in the space above the bottled algae solution, so oxygen wafts out of the water and the reaction keeps going. Greenbaum flushes out the oxygen that eventually builds up with a stream of nitrogen. That way, he made one algae culture produce about as much hydrogen as Melis did off and on for 1400 hours, "by far the world's record for sustainable production" from microbes, Greenbaum says.

Right now, both methods produce only 10% of their theoretical yield. Even if production increased 10-fold, you'd need a 500-square-foot shallow pond in a sun-drenched location to power one car, says NREL engineer Margaret Mann. Still, experts say the technology is well worth pursuing as an alternative to using photovoltaic cells or wind power to split water. "We don't know which technology will come out ahead," says Mann.

Posted in Plants & Animals, Technology