Ultrasound Effects on Algae, Cyanobacteria and Biofilm

South Santee Aquaculture
Green algae and diatoms have been found to be damaged by sound waves in studies with frequencies ranging from 20 khz to 60 khz putting it in the ultrasonic region.  The damage is done by resonating the inner cell wall that generates a sympathetic rebound and ultimately causes a tear between it and the contractile vacuole that regulates the internal pressure.  The inner cell wall is called the plasmalemma and its structure and design can vary widely between different types of algae.  Once it is broken from the contractile vacuole, it will collapse inside the outer algae sheath an d c ause the cell to die.  It is impossible to know what is the exact frequency that damages each type of algae.  There are estimated to be upwards to 100000 different species.  Our control strategy takes the range of 24 to 64 khz and covers it with 79 frequencies within that range.  The idea is to get close enough to a resonating frequency to create the damaging effect.  As algae grow and reproduce, they likely go through a number of frequencies that may be damaging to them, so you don’t have to be exact, just close enough.
 
Cyanobacteria are different than green algae.  They have gas vesicles that are only one protein layer thick and easy to break.  They only learned what these things looked like in the early 1970’s.  The ultrasonic device does not kill them directly, but breaking the gas vesicles causes them to lose buoyancy so they sink out of light and die.  Since the device is not strong enough to break the outer cell wall, the trapped gas from the vesicles migrates to the outer wall and slowly leaks through in about 3 days.  To see this effect you need an electron microscope.  Gas vesicles are extremely tiny and there are hundreds in each cyanobacteria cell.
 
Some scientists believe that it is cavitation that causes the damage, but if you do the math you will see that it would be impossible to create a cavitation zone large enough to be effective and even if you did, the ultrasonic device would be prohibitively expensive.   This belief stems from early work in this field by Paul Langevin, the French scientist that developed sonar in the early 1920’s.  They noted that algae died around the sonar test sight and since the device created a cavitational field, they assumed that cavitation, a very strong force related to turbulence, was responsible.  In the 1890’s they noted similar effects when algae blooms disappeared around mining sites where dynamite was exploded underwater.  In this case the shockwave of the explosions caused the damage.
 
Some algae are so complex internally that it is difficult to cause them to resonate and be damaged.  Types that are similar to plants with true branching filaments are generally not affected by the technology.  In the last two years we learned that a difficult blue-green algae (Lyngbya) can be damaged by a slightly higher energy output level.  We were the first to note this and are working to improve upon this finding.
 
One last piece of information that you may find interesting is that facultative anaerobic bacteria are influenced by the sound waves.  They behave as if they are in turbulent water conditions where they do not seek to reproduce in such turbulent water.  We have used this finding to keep tanks and water processing equipment clean from biofilm growth.
 
George Hutchinson, President, AlgaeControl.US

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