  Motility coupled to responsive behavior is essential for many microorganisms
to seek and establish appropriate habitats. One of the simplest possible
responses, reversing the direction of motion, is believed to enable filamentous
cyanobacteria to form stable aggregates or accumulate in suitable light
conditions. Here, we demonstrate that filamentous morphology in combination
with responding to light gradients by reversals has consequences far beyond
simple accumulation: Entangled aggregates form at the boundaries of illuminated
regions, harnessing the boundary to establish local order. We explore how the
light pattern, in particular its boundary curvature, impacts aggregation. A
minimal mechanistic model of active flexible filaments resembles the
experimental findings, thereby revealing the emergent and generic character of
these structures. This phenomenon may enable elongated microorganisms to
generate adaptive colony architectures in limited habitats, or guide the
assembly of biomimetic fibrous materials.
