Petrol stations could be replaced by stagnant ponds if a breakthrough in hydrogen fuel technology fulfils its potential.

The new approach harnesses an emergency survival strategy that green algae use to survive during hard times. The microscopic plants switch from normal photosynthesis, producing oxygen, to an alternative way of "breathing" which produces hydrogen gas.

The theoretical yields are high enough for the process to be exciting experts in the energy field as a future source of fuel, perhaps in 20 years time.

The fuel could be used to power fuel cells in cars. The big advantage of hydrogen as a fuel is that it does not produce carbon dioxide or other pollutants when it is burnt in pure oxygen. And if produced by using solar energy to split water, it is entirely sustainable.

The new research was led by Tasios Melis, from the University of California-Berkeley. "I guess it's the equivalent of striking oil," he told the annual meeting of the American Association for the Advancement of Science (AAAS).

He estimates that a small pond in which the growth of the algae is controlled could provide enough fuel for 12 cars for a week.

Airtight pond

The key to the process is the element sulphur. The algae need this nutrient to grow and when it is not available, the algae begin to consume the oxygen in the water. In a short time, the system becomes stagnant and, said Professor Melis, "every other plant on Earth would suffocate and die".

"This algae do not - they have a trick." They switch to a different metabolic pathway which produces hydrogen.

Cars are already in development that use hydrogen as a fuel

So to use the algae is a two step process. First they are grown, then the sulphur is removed and a day later the hydrogen starts to flow. This continues for a few days before the algae are so starved they must be returned to normal photosynthesis.

However, there appears to be no limit to the number of times this cycle can be repeated, according to Professor Melis. This approach could be applied to a pond by using an airtight cover.

Currently, the process produces three millilitres of hydrogen per litre of algae solution, but improvements to the system should deliver 10 times this amount. The conversion efficiency of the sunlight energy would be about 10%.

Maggie Mann, from the National Renewable Energy Laboratory, Colorado, said biophotosynthesis was likely to be one important way of producing hydrogen fuel in the future.

Fuel from sewage

It also has advantages over using solar energy to electrically produce hydrogen as there is no need to manufacture solar panels.

Future work in the area involves developing mutants of the algae which can tolerate oxygen whilst still producing hydrogen - oxygen destroys the key enzyme in the process, called hydrogenase.

Elias Greenbaum, from Oak Ridge National Laboratory, is using laboratory techniques to select out algae that have a natural tolerance for oxygen.

"We do this by telling them that they have to produce hydrogen or they die," he said.

If oxygen-tolerance could be achieved, then the two-stage production cycle could be simplified to one, continuous process.

It may even be possible to convert sewage into hydrogen fuel. Tadashi Matsunaga, at the Tokyo University of Agriculture and Technology, has found a photosynthesising bacterium which produces hydrogen from waste water.