2 3 M ar 2 00 9 Synechococcus as a “ singing ” bacterium : biology inspired by micro - engineered acoustic streaming devices

Certain cyanobacteria, such as open ocean strains of Synechococcus, are able to swim at speeds up to 25 diameters per second, without flagella or visible changes in shape. The means by which Synechococcus generates thrust for self-propulsion is unknown. The only mechanism that has not been ruled out employs tangential waves of surface deformations. In [1] the average swimming velocity for this mechanism was estimated using the methods inaugurated by Taylor and Lighthill in the 1950’s and revisited in differential geometric language by Shapere and Wilczek in 1989. The procedure consists of solving quasi-statically the Stokes equations with no slip boundary conditions. (These are given by the instantaneous velocity field defined by the current deformation of a localized shape. The physical condition of no net force and torque yields a rigid body counterflow, whose average on a stroke cycle gives the average swimming velocity.) In this article we propose making a break with the no slip boundary condition paradigm. In fact, we are proposing here an entirely different physical principle self propulsion based on acoustic streaming. Micro-pumps in silicon chips, based on AS, have been constructed by engineers since the 1990’s, but to the best of our knowledge acoustic streaming as a means of microorganisms locomotion has not been proposed before. Our hypothesis is supported by two recent remarkable discoveries: (1) In [3], deep-freeze electron microscopy of the motile strain WH8113 revealed a crystalline outer layer (CS) covered with a forest of ”spicules” (Sp) extending from the inner membrane through the CS, projecting 150 nm into the surrounding fluid. (2) In [2], atomic force microscopy (AFM) was used to find that the cell wall of yeast cells periodically oscillates on nano-scale amplitudes at frequencies of 0.8 to 1.6 kHz, and that the oscillations are generated metabolically. We propose that the spicules, in contact with the cell’s power systems, could perform high frequency motions generating acoustic streaming (AS) in the surrounding fluid. We compare two models for self-propulsion employing acoustic streaming: the quartz wind effect (QW) and boundary induced streaming generated by surface acoustic waves (SAW). Based on an estimate of the power required, the former would require an enhancement mechanism similar to a laser to be viable. In striking contrast, we find that the efficiency of the SAW mechanism compares favorably with known strategies for bacterial self-propulsion. The required amplitude is below the resolution limit of light microscopy and the required frequency is biologically attainable. Moreover SAW produce an ”atmosphere” (the Stokes layer) surrounding the cell, within which the fluid motion is essentially chaotic and thus acoustic streaming may turn out to be biologically advantageous, enhancing nutrient uptake and chemical reactions. Some possible experiments are outlined.