Biophysical model of EPS-based mobility enhancement captures the observed fingering behavior during phototaxis.

A) As shown schematically, each cyanobacterium (green) is assumed to undergo a biased random walk toward the light source (LED). Each cell secretes EPS (blue), whose local concentration increases the cell's mobility M (red). B) The cell deposition, crescent formation, and finger formation observed in experiments (top) are recapitulated by simulations using our model (bottom) with and ; cells are shown in green, EPS in red. Immediately after deposition (left), cells are distributed randomly and the boundary with the substrate is smooth (inset). Cells collect at the edge of the initial deposition area as they migrate toward the light source (middle, 20 hr), with small variations at the interface (inset). The front matures into discrete groups of cells that separately migrate toward the light source (right, 45 hr). Scale bars are 1 mm.