Enzyme Lets Algae Produce Hydrogen To Use As Clean Fuel
Chronicle, Jan 31, 2000
Berkeley scientist says discovery is like 'striking oil'
Researchers have found a metabolic switch in algae that allows the primitive plants to produce hydrogen gas - a discovery that could ultimately result in a vast source of cheap, pollution-free fuel. Hydrogen, which can be used as a clean-burning fuel in cars and power plants, is virtually limitless in availability, because it is part of the water molecule. It is a candidate to become the world's primary fuel in coming decades. But until now, it was obtainable in quantity only through relatively expensive extraction procedures involving the electrolysis of water or processing natural gas.
The breakthrough, by scientists at the University of California at Berkeley and the U.S. Department of Energy, would make possible the commercial production of hydrogen gas by photosynthesis in tanks, ponds or the open ocean. I guess it's the equivalent of striking oil, said Tasios Melis, a microbial biology professor at UC Berkeley. It was enormously exciting. It was unbelievable. Melis made the discovery with UC Berkeley researcher Liping Zhang and with Michael Seibert, Maria Ghiardi and Marc Forestier of the National Renewable Energy Laboratory, a Department of Energy project in Golden, Colo. The team's findings appear in this month's issue of Plant Physiology, a science journal.
Seibert said it has been known for decades that algae give off small amounts of hydrogen. The problem from a commercial perspective, he said, was hydrogenase - an enzyme that produces hydrogen, but does so only in the absence of oxygen. All green photosynthetic plants - including algae - consume carbon dioxide in the presence of light to build tissue, respiring oxygen as a waste product. But because hydrogenase shuts down in the presence of oxygen, it doesn't function during photosynthesis, he said. It basically only works during darkness, when photosynthesis isn't occurring.
Because plant functions are at low ebb during darkness, Seibert said,the amount of hydrogen produced is minimal. But the team was able to solve the problem, said Seibert, by imposing a nutrient stress to the algae. First we grow out the algae, `fatten' it under normal photosynthetic conditions, he said. Then we withhold sulfur. Sulfur is critical for the completion of normal photosynthesis, Seibert said. In the absence of the element, the algae ceased emitting oxygen and stopped storing energy as carbohydrates, protein and fats. Instead, the algal cells began using an alternative metabolic pathway to exploit stored energy reserves anaerobically - in the absence of oxygen. The hydrogenase was activated, splitting large amounts of hydrogen gas from water and releasing it as a byproduct.
The significant thing is that the plant is using the energy of the sunlight to produce hydrogen, not oxygen, said Melis. Without sulfur, it produces a great deal more hydrogen in the presence of light than it does under normal circumstances in the dark. The algae ultimately would die if the nutrient stress were maintained for more than a few days, but they can be fattened again with sulfur and sunlight, allowing for repetitions of the process and continued harvesting of hydrogen gas. Eventually, the process could be used for the production of huge quantities of hydrogen. Hydrogen burns clean and hot, and it constitutes one-third of the water found in the Earth's oceans, rivers, lakes and atmosphere.
Cars already have been developed that run on hydrogen-powered devices known as fuel cells. These vehicles are virtually pollution-free; the only substance emitted from the tailpipe is water vapor. They do not release carbon dioxide or other heat-trapping gases, which are widely considered the primary culprits in global warming. Fuel cells big enough to power electrical generating plants could also be built. Our long-term goal is to develop strains of algae that we would grow in mass cultures to produce enormous quantities of hydrogen gas, said Melis. But at this point, we have to improve the production performance.