  We demonstrate that the permeability of the cell wall of microalga
$\textit{C. reinhardtii}$ can be enhanced in controlled microstreaming flow
conditions using single bubbles adherent to a wall and driven by ultrasound
near their resonance frequency. We find that microstreaming flow is effective
at acoustic pressures as low as tens of kPa, at least one order of magnitude
lower than those used in bulk ultrasonication. We quantify the increase in
number of fluorescent cells for different acoustic pressure amplitudes and
ultrasound exposure times. We interpret the increase in dye uptake after
microstreaming flow as an indication of enhanced permeability of the cell wall.
We perform microscopic visualizations to verify the occurrence of non-spherical
shape oscillations of the bubbles, and to identify the optimal conditions for
dye uptake. In our experimental conditions, with acoustic pressures in the
range 20-40 kPa and frequencies in the range 25-70 kHz, we obtained spherical
oscillations of bare microbubbles and non-spherical shape oscillations in
microbubbles with algae attached to their surface. Only the latter were able to
generate a non-linear microstreaming flow with sufficiently large viscous
stresses to induce algae permeabilization. The results demonstrate that
controlled microbubble streaming flow can enhance cell wall permeability in
energy-efficient conditions.
