Modelling the effect of size on the aerial dispersal of microorganisms

Aim: We investigate the long-standing question of whether the small size of microbes allows most microbial species to colonize all suitable sites around the globe or whether their ranges are limited by opportunities for dispersal. In this study we use a modelling approach to investigate the effect of size on the probability of between-continent dispersal using virtual microorganisms in a global model of the Earth's atmosphere. Location: Global. Methods: We use a computer model of global atmospheric circulation to investigate the effect of microbe size (effective diameters of 9, 20, 40 and 60 μm) on the probability of aerial dispersal. Results: We found that for smaller microbes, once airborne, dispersal is remarkably successful over a 1-year period. The most striking results are the extensive within-hemisphere distribution of virtual microbes of 9 and 20 μm diameter and the lack of dispersal between the Northern and Southern Hemispheres during the year-long time-scale of our simulations. Main conclusions: Above a diameter of 20 μm wind dispersal of virtual microbes between continents becomes increasingly unlikely, and it does not occur at all (within our simulated 1-year period) for those of 60 μm diameter. Within our simulation, the success of small microbes in long-distance dispersal is due both to their greater abundance and to their longer time in the atmosphere - once airborne - compared with larger microbes.