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Certain cyanobacteria, such as open ocean strains of Synechococcus, are ableto swim at speeds up to 25 diameters per second, without flagella or visiblechanges in shape. The means by which Synechococcus generates thrust forself-propulsion is unknown. The only mechanism that has not been ruled outemploys tangential waves of surface deformations. In Ehlers et al, the averageswimming velocity for this mechanism was estimated using the methodsinaugurated by Taylor and Lighthill in the 1950's and revisited in differentialgeometric language by Shapere and Wilczek in 1989. In this article we proposean entirely different physical principle self propulsion based on acousticstreaming (AS). Micro-pumps in silicon chips, based on AS, have beenconstructed by engineers since the 1990's, but to the best of our knowledgeacoustic streaming as a means of microorganisms locomotion has not beenproposed before. Our hypothesis is supported by two recent discoveries: (1) InSamuel, et al, deep-freeze electron microscopy of the motile strain WH8113revealed a crystalline outer layer (CS) covered with a forest of "spicules"(Sp) extending from the inner membrane through the CS, projecting 150 nm intothe surrounding fluid. (2) In Pelling et al, atomic force microscopy (AFM) wasused to find that the cell wall of yeast cells periodically oscillates onnano-scale amplitudes at frequencies of 0.8 to 1.6 kHz, and that theoscillations are generated metabolically. We propose that the spicules, incontact with the cell's power systems, could perform high frequency motionsgenerating acoustic streaming (AS) in the surrounding fluid. We compare twomodels for self-propulsion employing acoustic streaming: the quartz wind effect(QW) and boundary induced streaming generated by surface acoustic waves (SAW).

Synechococcus as a "singing" bacterium: biology inspired by micro-engineered acoustic streaming devices