<i>Synechocystis</i> cells on a surface accumulate in finger-like projections when moving toward a directional light source.

<p>A) Time-lapse imaging of a <i>Synechocystis</i> community in the presence of a directional light source; images were taken at 0, 11, 24, 36, and 49 hr. White rectangles indicate regions highlighted in (B–F). Scale bar is 1 mm. B) Cells are initially randomly distributed across the surface. (phase contrast image taken at 10× magnification, scale bar is 10 µm.) C) Regions of heterogeneity are evident where the local cell concentrations are slightly higher than in the rest of the initial deposition area. D) Regions of higher cell concentrations start to form small finger-like projections within the initial deposition. E) Cells collect at the edge of the initial deposition and create a ruffled interface, from which finger-like projections will emerge. F) Finger-like projections form as distinct, motile groups of cells that move toward the light source, leaving behind a trail of lower density cells.