The way single colloidal objects behave in presence of active forces arising from within the bulk of the system is crucial to many situations, notably biological and ecological (e.g. intra-cellular transport, predation), and potential medical or environmental applications (e.g. targeted delivery of cargoes, depollution of waters and soils). In this talk I will present experimental findings that my collaborators and I have obtained over the past years on the dynamics of single Brownian colloids in suspensions of biological micro-swimmers, especially the green alga Chlamydomonas reinhardtii. I'll show notably that spatial heterogeneities and anisotropies in the active particles statistics can control the preferential localisation of their passive counterparts. The results will be rationalized using theoretical approaches from hydrodynamics and stochastic processes.