Localized Bioconvection Patterns and Their Initial State Dependency in <i>Euglena gracilis</i> Suspensions in an Annular Container

Abstract

Localized patterns of bioconvection in Euglena gracilis suspensions were experimentally analyzed in an annular container. Near the critical mean density of convection, we succeeded in isolating two basic types of localized convection patterns. One was an almost stationary pattern consisting of two convection cells centered by an isolated high-density region of the microorganism where a downflow was generated, which we call a “bioconvection unit”. The other was a traveling wave pattern consisting of an array of moving high-density waves bounded in a certain area. The effect of the mean density of E. gracilis on the emergence of the localized convection pattern was also examined. Near the critical mean density, we found that the emergence probability of the localized convection pattern depends on the initial state, i.e., whether E. gracilis has a uniform or localized distribution, which suggests that the system is bistable. Such bistability is often accompanied by localized structures in spatially extended dissipative systems.