  Euglena, a swimming micro-organism, exhibited a characteristic bioconvection
that was localized at the center of a sealed chamber under bright illumination
to induce negative phototaxis. This localized pattern consisted of high-density
spots, in which convection was found. These observations were reproduced by a
mathematical model that was based on the phototaxis of individual cells in both
the vertical and lateral directions. Our results indicate that this convection
is maintained by upward swimming, as with general bioconvection, and the
localization originates from lateral phototaxis.
