The green goo which thrives in stagnant ponds might one day keep your car running in an energy-starved future. Certain types of algae and bacteria can produce recoverable amounts of a powerful, ecologically-safe and renewable fuel - namely, hydrogen.
Efforts to tap into this rich natural resource were described yesterday at the American Association for the Advancement of Science meeting in Washington DC.
The world could be on the verge of the hydrogen era, said Prof Tasios Melis, of the University of California-Berkeley.
He said green plants used photosynthesis to produce food, in the process converting carbon dioxide into oxygen. Certain species of algae, however, also had the ability to split water into hydrogen and oxygen. "The hydrogen production system is an alternative method for breathing."
There is a trick to the process which Dr Melis and colleagues discovered and which makes the bioproduction of hydrogen a possibility.
He said the two algal breathing systems were mutually exclusive. "The algae apparently know that oxygen and hydrogen don't mix."
The team discovered a "metabolic switch" associated with the availability of sulphur which causes the algae to convert from one form of breathing to the other.
During normal photosynthesis, the algae store up supplies of energy in the form of nutrients. However, when sulphur and any residual carbon dioxide are removed, the switch is thrown and the algae begin using up energy supplies to produce hydrogen.
He said it was a self-limiting process, with the algae only able to survive for about four days in these conditions. They quickly convert back to normal photosynthesis when sulphur and carbon dioxide are restored and can be repeatedly cycled back and forth in this way.
An 850ml container of the variety under study, chlamydomonas reinhardtii, can produce 3ml of hydrogen an hour. However, the theoretical output limit was 10 times this amount.
This would mean that a "small" algae-laced pond equipped with a collection system could produce enough hydrogen each week to power a dozen family cars.
It was a process "which we feel may produce unlimited amounts of a clean, renewable fuel," Prof Melis said.
The group said that if hydrogen production could be increased they would have "a commercially-viable production system" on their hands.
The ability to convert between the two forms of respiration was probably an "evolutionary relic", said Dr Elias Greenbaum, of Oak Ridge National Laboratory.
It would have allowed early algae to survive in the harsh environment billions of years ago.
He said the hydrogen process had been known about for 50 years, but more recent research had shown how the process worked. There were many varieties of algae which had this characteristic, along with varieties of bacteria.
