This paper examines the combined effects of density stratifications in porous media due to both gravitactic microorganism motion and thermal or solutal solicitation. These effects are coined in the literature by the term "double diffusion." In this case, an equation of conservation of energy or mass is added to the gravitactic bioconvection system. The problem is then governed by two Rayleigh numbers, a Peclet number (normalized swimming speed), and a Lewis number (ratio of thermal diffu-sivity/microorganism diffusivity). It is found that gravitactic bioconvection with double diffusion may contain a subcritical instability regime due in part to the gravitactic motion of microorganisms and in part to the weak diffusivity of the thermal/solutal solicitation. It is also found that there is an oscillatory regime that is established below the critical threshold of steady bioconvection. This oscillatory convection flow may arise when the diffusivity of the stabilizing component is weaker than that of the destabilizing component.
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