Research work, undertaken
at the Centre for Aquatic Plant Management, showed that exposure of
Spirogyra and Scenedesmus to ultrasound for less than a week
induced changes in the algal cells. The interior of the cell shrunk
away from cell wall and the cells died. Measurements of chlorophyll
content of cells also declined, and exposed cells did not regrow after 3
weeks exposure. In outdoor pond tests, algal growth was much lower
than in tanks without any treatment, and there were no effects on
Daphnia, an invertebrate used as a test organism. Fish are not
effected because the sound level produced is half the required decibels
to effect fish. They do not affect submerged plants, water lilies or
other desirable aquatic plants, and are highly specific to algae.
The pictures below show
examples of how algae respond to exposure to ultrasound. The filamentous
alga is Spirogyra exposed to treatment with an Aquanet device in
glasshouse conditions for a period of 21 days. The disintegration
of the cell is clear, even after 7 days. This is due to functional
membrane changes in the cell induced by ultrasound. The
unicellular algae is Scenedesmus. This species show
characteristic banding of the cytoplasm when exposed to ultrasound for 3
days.
Spirogyra - 7 days exposure to ultrasound
14 Days exposure to ultrasound
21 Days exposure to ultrasound - all dead
Scenedesmus no ultrasound treatment
Scenedesmus treated with ultrasound 3 days - note banding
The graph shows the growth
rate of exposed (pink) and untreated (blue) algae after exposure to
ultrasound for three weeks. This shows that permanent damage has been
caused by only a brief exposure. Continued exposure will eradicate all
algae except algae that swim and move, so called motile algae, but the
invertebrates and fish must have something left to feed on, and this is
the key to the environmentally friendly nature of ultrasound treatment -
it does not kill all algae, it reduces the numbers of nuisance algae and
the biomass of filamentous algae to point where balance with other
algae and organisms is restored.
