WASHINGTON (AP) -- Hydrogen may be an ideal fuel when the supply of oil and natural gas runs out, but the problem has been finding a way to produce it cheaply. Scientists now say the answer may be an ordinary pond scum. Green algae, a simple plant that grows all over the world, has the unique ability to convert water and sunlight into hydrogen gas, researchers said Monday at the national meeting of the American Association for the Advancement of Science.

Now scientists have found a new way to force the algae to make hydrogen gas on demand, a process that could lead to an almost limitless supply of fuel that burns without pollution and produces only water as a waste product. Tasios Melis of the University of California, Berkeley, said that the algae, one of the most ancient plants known, evolved the ability to live in two radically different environments.

When living in ordinary air and sunlight, it uses photosynthesis like other plants. This process converts sunlight, water and carbon dioxide into waste oxygen and the life-sustaining chemicals the plant needs. But when the algae is deprived of a key nutrient, sulfur, and forced to live in an anaerobic, or oxygen-free, environment, the plant reverts to an alternate life style in order to survive. Under these conditions, the algae makes hydrogen, said Melis.

``It is sort of a metabolic switch,'' said Melis. ``It is an alternative way of breathing'' that the plant developed over millions of years to survive where there is no oxygen or sulfur. Those same conditions would kill other plants.

In experiments, Melis said his laboratory first grows algae cells in the ordinary way, giving the plant sunlight, nutrients and water. The plant happily reproduces, growing millions upon millions of new cells. Then, the researchers cut off the supply of sulfur and oxygen to the algae, forcing it to click the metabolic switch to its hydrogen-producing life style.

``Within 20 hours, the algae turns on its switch, converting from oxygen production to hydrogen production,'' said Melis. ``We have to seal the culture to prevent exposure to oxygen. Then we collect the hydrogen as it bubbles out of the culture.''

The gas comes out as pure, pollution-free hydrogen, he said. Melis said the hydrogen-making process has operated experimentally for up to four days, the time it takes for the algae to exhaust its internal resources. The researchers then converted back to normal photosynthesis and the plant revives itself, ready to start another cycle.

``This has the promise of generating fuel from some of nature's most plentiful resources -- sunlight and water,'' said Melis. One liter of algae culture, a little over a quarter of a gallon, produces three milliliters of hydrogen, about a tenth of a fluid ounce, per hour, said Melis. Researchers believe this efficiency can be increased at least 100 fold, but that has yet to be demonstrated.

Algae growing in a small pond, he said, may eventually be enough to power 10 cars, although Melis admitted, ``I'm not saying how big the pond would have to be.''

The algae's alternate life style uses an enzyme, along with sunlight, to extract hydrogen from water. The enzyme, hydrogenase, is not found in higher plants. Some researchers are looking at ways to transfer the gene for this enzyme into other plants and, possibly, to force the enzyme to work in the presence of oxygen.

Hydrogen has long been promoted as a fuel to replace fossil energy sources. In the space program, hydrogen and oxygen are combined to make a rocket propellant, such as in the main engines of the space shuttle. Oxygen and hydrogen are an explosive mixture, but they can be combined in a fuel cell to produce electricity and water. Fuel cells electrify the space shuttle and were used successfully in the Apollo program.

Margaret K. Mann of the National Renewable Energy Laboratory, a federal facility in Golden, Colo., said that auto makers are already developing fuel cells to drive automobiles. Other researchers, she said, are studying ways of changing the nation's energy infrastructure -- pipelines, fuel transports and service stations -- to make use of hydrogen.

She said it will be at least 20 years before hydrogen becomes a major part of the energy picture, but the gas could eventually power the nation, providing a renewable fuel source for both transportation and electrical generation.

Right now, hydrogen is most commonly separated from natural gas. This makes hydrogen a negative in the energy equation, since natural gas is a cheaper, more efficient fuel.

Hydrogen may come into its own for environmental reasons, said Mann. Carbon dioxide from the burning of fossil fuels is the primary cause of global warming, many believe. The only waste generated from hydrogen fuel is pure water, a resource that theoretically can be recycled to produce more hydrogen.