Bioconvective dynamics: Dependence on organism behaviour

Bioconvection occurs when a macroscopic nonuniformity of the concentration of microbial populations is generated and maintained by the directional swi mming of the organisms. This study investigated the properties of the patte rns near the onset of the instability and later during its evolution into a fully nonlinear convection regime. In suspensions of the bacteria Bacillus subtilis, which tend to swim upwards in a gradient of oxygen concentration that they create by consumption, we discovered that the dominant wavelengt h at the onset of the instability is determined primarily by the cell densi ty and is influenced only weakly by the fluid depth. This observation contr asts strongly with previous observations on the gravitactic alga Chlamydomo nas nivalis, in which the opposite dependence was found. Considerable diffe rences were also found in the long-term evolution of the convection pattern s. These results demonstrate the existence of readily distinguishable types of bioconvection systems, even at early stages of the instability. The obs erved differences are clearly and causally correlated with disparate reason s for upward swimming by these microorganisms, leading to different geometr ic distributions of the density of the suspension.