  Unassisted ion transport through lipid membranes plays a crucial role in many
cell functions without which life would not be possible, yet the precise
mechanism behind the process remains unknown due to its molecular complexity.
Here, we demonstrate a direct link between membrane potential fluctuations and
divalent ion transport. High-throughput wide-field second harmonic (SH)
microscopy shows that membrane potential fluctuations are universally found in
lipid bilayer systems. Molecular dynamics simulations reveal that such
variations in membrane potential reduce the free energy cost of transient pore
formation and increase the ion flux across an open pore. These transient pores
can act as conduits for ion transport, which we SH image for a series of
divalent cations (Cu$^{2+}$, Ca$^{2+}$, Ba$^{2+}$, Mg$^{2+}$) passing through
GUV membranes. Combining the experimental and computational results, we show
that permeation through pores formed via an ion-induced electrostatic field is
a viable mechanism for unassisted ion transport.
