An Experimental Investigation of Bioconvection in Three Species of Microorganisms*

Abstract

SYNOPSIS. A series of experiments was carried out on bioconvection using cultures of Polytomella agilis, Tetrahymena pyriformis and Chlamydomonas moewusii. During convection these microorganisms disperse into more and less dense regions causing visible patterns. These patterns, called the cell plan forms, were found to change with increasing Rayleigh numbers above Rc, the lowest concentration gradient at which bioconvection occurs. Hexagonal convection cells were observed near the critical Rayleigh number, Rc. As the Rayleigh number was increased the cell plan form changed from hexagons to 2‐dimensional rolls and then to square convection cells. The size of the observed convection cells was less than expected for such a physical phenomenon. This appears to result from an increasing tendency of the microorganisms to circulate near the upper surface at higher Rayleigh numbers. The square convection cells appear to be unique to bioconvection. The inhibition of bioconvection is directly linked to an increased mortality rate. Observations on the nature of negative geotaxis were made which tend to support the statocyst hypothesis.