Scientists in Israel have discovered a new way to test for water pollution by "listening" to what the plants growing in water have to say.

By shining a laser beam on the tiny pieces of algae floating in the water, the researchers said they hear sound waves that tell them the type and amount of contamination in the water.

"It is a red light, telling us that something is beginning to go wrong with the quality of water," said Zvy Dubinsky, an aquatic biologist at Israel's Bar Ilan University. "Algae is the first thing to be affected by a change in water quality."

Although most of the earth is covered in water, 44 per cent of the world's population live in areas with high water stress, and the number is likely to increase because of factors such as global warming and rising population.

As water sources deteriorate worldwide, the testing of algae could be used to monitor water quality faster, more cheaply and more accurately than techniques now in use, Prof. Dubinsky said.

The secret, he said, is to measure the rate of photosynthesis in the algae, meaning the plant's ability to transform light into energy. During photosynthesis, plants also release oxygen into the air. Prof. Dubinsky's technique is easy to perform because of the over-abundance of algae in the planet's water. Most of the oxygen in the atmosphere comes from algae.