Few studies have been conducted on the impact of air pollutants in relation with environmental variables on microbial communities (including bacteria, auto-and heterotrophic protists and micro-metazoa). We studied the effect of atmospheric particles on microorganisms living in Bryophytes in situ and in controlled conditions. In situ, we observed a seasonal variation of the different groups. The total biomass of microorganisms and the fungal biomass were significantly higher in the coolest sites in winter. At the end of the experimentation, the biomass of cyanobacteria increased in the polluted sites while the biomass of predators (testate amoeba and ciliates) decreased. In the experimental study, the total microbial biomass and the biomass of cyanobacteria, testate amoebae and fungi decreased significantly in the polluted situations. These results suggest that the atmospheric pollution could have direct and indirect (i.e. trophic) effects on microorganisms and that environmental factors such as temperature could modify the impact of pollutants on microbial groups.