BLOOMS of algae in lakes and seas, sometimes called red tides, can release neurotoxins into the food chain or suffocate the local ecology by sucking up too much oxygen. When one occurs, the safest option is usually just to wait for the bloom to clear of its own accord, but now scientists at the University of Hull, UK, think they have found a way to put a stop to these deadly algal explosions- by exposing them to blasts of ultrasound.
The use of ultrasound has been explored before, but with mixed results. That may be because the mechanism was not well understood, say Michiel Postema and his colleagues, who successfully used ultrasound to kill off algae. Postema believes it affects buoyancy cells, known as heterocysts, which keep the algae afloat by enclosing a bubble of nitrogen gas. He reckons the ultrasound pressure wave causes the gas in the cells to resonate. At high intensity it bursts the cell, and the algae sink. 鈥淲ithout sunlight they will then die,鈥� he says.
Postema and his team tested three different frequencies on a particularly harmful species of blue-green algae, Anabaena sphaerica, which can cause respiratory disease and liver cancer in humans who come into contact with it. Although all three frequencies worked to some extent, the most effective was close to 1 megahertz. That value matches the expected resonant frequency of this alga鈥檚 buoyancy cell, which is about 6 micrometres across (Applied Acoustics, ).
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If they are right about the resonance mechanism, it would be good news, says Postema. Any method for clearing toxic algal blooms should do as little damage as possible to the rest of the ecosystem. 鈥淵ou need to be sure you avoid other harmless species,鈥� says Deborah Long, conservation officer for the charity in the UK. The ultrasound could be targeted to specific species of algae, because the resonant frequency of heterocysts varies from species to species in accordance with their size. What鈥檚 more, such a measure should not damage ordinary water-filled plant cells, which are relatively impervious to pressure waves.
These high frequencies are absorbed rapidly as they travel through water, and at 1 megahertz the effective radius is less than 20 metres, says Postema. So the technique may be more practical for clearing algal blooms in lakes and ponds than for large-scale red tides that can cover hundreds of square kilometres of sea.
