Effects of a High-Amplitude 1-MHz Standing Ultrasonic Field on the Algae Hydrodictyon

When portions of a Hydrodictyon reticulatum colony were exposed to a high-amplitude l-MHz ultrasonic standing wave, the cells of this green algae were typically destroyed by cavitation in the sur- rounding water. If cavitation did not occur, however, another ultrasonic hioeffect could be discerned: regularly spaced markings gradually ap- peared in the cytoplasm of the cells. Peak pressure amplitudes in the field were 1-2 MPa, with exposure durations of up to 30 min. The markings appear to be the result of acoustic streaming generated in the cytoplasm when a transverse standing wave is induced in the cell walls. A theoretical prediction of the spacing, based on theory for a membrane under tension, is in rough agreement with the observed spacing of the markings. This phenomenon is a rare example of an ul- trasonic bioeffect, which is not attributable either to heating or to cav- itation.