  In addition to enabling movement towards environments with favourable living
conditions, swimming by microorganisms has also been linked to enhanced mixing
and improved nutrient uptake by their populations. Experimental studies have
shown that Brownian tracer particles exhibit enhanced diffusion due to the
swimmers, while theoretical models have linked this increase in diffusion to
the flows generated by the swimming microorganisms, as well as collisions with
the swimmers. In this study, we perform detailed simulations based on the
force-coupling method and its recent extensions to the swimming and Brownian
particles to examine tracer displacements and effective tracer diffusivity in
squirmer suspensions. By isolating effects such as hydrodynamic or steric
interactions, we provide physical insight into experimental measurements of the
tracer displacement distribution. In addition, we extend results to the
semi-dilute regime where the swimmer-swimmer interactions affect tracer
transport and the effective tracer diffusivity no longer scales linearly with
the swimmer volume fraction.
