  Microorganisms are ubiquitous in nature and technology. They inhabit diverse
environments ranging from small river tributaries and lakes to oceans, as well
as wastewater treatment plants and food manufacturing. In many of these
environments, microorganisms coexist with settling particles. Here, we
investigate the effects of microbial activity (swimming \textit{E. coli}) on
the settling dynamics of passive colloidal particles using particle tracking
methods. Our results reveal the existence of two distinct regimes in the
correlation length scale ($L_u$) and the effective diffusivity of the colloidal
particles ($D_{e\!f\!f}$), with increasing bacterial concentration ($\phi_b$).
At low $\phi_b$, the parameters $L_u$ and $D_{e\!f\!f}$ monotonically increases
with increasing $\phi_b$. Beyond a critical $\phi_b$, second regime is found in
which both $D_{e\!f\!f}$ and $L_u$ are independent of $\phi_b$. We demonstrate
that the transition between these regimes is characterized by the emergence of
bioconvection. We use experimentally-measured particle-scale quantities ($L_u$,
$D_{e\!f\!f}$) to predict the critical bacterial concentration for the
diffusion-bioconvection transition.
