Cells secrete an extracellular substance that enhances their motility.

<p>A) Typical fingering of a <i>Synechocystis</i> community toward a light source during a phototaxis assay. The contrasted inset shows EPS deposited by motile groups. B) Phase-contrast microscopy of motile groups immediately before, and then at specified intervals after a change in the light direction (orange arrows). The white dashed lines indicate the bounds of a region through which a finger recently passed. The motile group changes course after a change in the light direction, and upon intersection with the EPS trail of a neighboring finger, group speed toward the light increases and the cells become more dispersed. C) Median velocity of cells in the motile group in both the <i>x</i> and <i>y</i> directions with respect to the coordinate system shown in (B); dashed horizontal lines indicate 95% confidence intervals. Left: prior to the change in light direction, cells have positive velocity toward the light source (top), and approximately zero net velocity perpendicular to the light (bottom). Middle: following the change in light direction, the cells reorient and velocity in the <i>x</i> direction rises to a value comparable with the <i>y</i> velocity prior to the light change (bottom), while the net y velocity approaches zero (top). Right: when the group of cells merges with the trail of the neighboring finger (dashed vertical lines), the spread in <i>y</i> velocities increases (top) and the median <i>x</i> velocity increases by approximately three-fold (bottom). D) Histograms of speeds for the same cells (<i>n</i> = 95) before and soon after merging with the trail of secreted extracellular substance, with mean speed and standard deviation indicated in the legend. E) Individual cells experience an increase in speed after the group merges with the trail of another finger, indicating that the change in group dynamics upon merging is coupled to a change in the motility of individual cells.