Staying local: Rate and composition of microbial dispersal across ecosystems

Kendra E. Walters, Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA and Jennifer B.H. Martiny, Department of Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA

Background/Question/Methods
Dispersal, as with larger organisms, is a key process influencing microbial communities. The taxonomic composition of dispersal, in particular, influences its effect on communities. However, where dispersing taxa originate from or how far they disperse remains unknown because microbial dispersal has yet to be studied in a spatially explicit way. Here we examined microbial dispersal into the soil surface along three 30 m transects that span the boundary of two ecosystems: a grassland and a shrubland. We quantified the rate (by flow cytometry) and composition (by 16S and ITS metabarcoding) of immigrating bacteria and fungi on sterile microscope slides. Simultaneously, we characterized potential sources of dispersal. We mapped the vegetation in the field site with aerial imagery and characterized microbial communities in the surrounding air, leaf litter, and soil, likely sources of microbial dispersal. We then tested the influence of geographic distance and surrounding vegetation on the microbial communities dispersing in the surface leaf litter.
Results/Conclusions
Bacterial and fungal dispersal was highly localized. The communities dispersing to the glass slides were highly correlated with the surrounding vegetation within a 1 m radius. Further, when variation explained by surrounding vegetation was removed, the communities showed no relationship with geographic distance. These results suggest that the majority of dispersing taxa originate from their immediate surroundings with dispersal kernels likely much smaller than prior assumptions. While previous studies on the air microbiome show that some microbial taxa can disperse long-distance, even between continents, our study suggests that most taxa stay local. As climate and vegetation continue to shift, the microorganisms responsible for key ecosystem functioning such as decomposition might not be able to disperse far enough to establish in newly favorable conditions.