Effects of dispersal on diversity and composition of bacterioplankton following a salinity change

Although dispersal can influence the response of bacterial communities to environmental changes, the importance of dispersal under different environmental conditions remains unclear. To disentangle the effect of dispersal from that of the environment, we conducted a transplant experiment using dialysis bags where bacterioplankton originating from brackish and marine regions of Saint Lawrence Estuary were reciprocally incubated into both environments. Additionally, half of the microcosms in each environment were subjected to dispersal of cells between the two inocula during 5 days of incubation (daily exchange of 6 % v/v). We used bacterial 16S rRNA sequencing to examine the response of the active communities. Dispersal interacted significantly with incubation environment to influence the transplanted marine communities, resulting in greater species richness and evenness. These results suggest that more competitive dispersers were able to dominate the marine communities in the brackish environment, even if such exposure per se led to a decrease in diversity. However, the overall diversity of brackish communities was not affected by dispersal in any of the two environments. Dispersal also decreased the mean difference in variability among communities, which was likely driven by changes in the relative abundances, rather than replacement of abundant taxa. These patterns were, however, less pronounced under marine than brackish conditions. Moreover, independent of the origin of the communities, bacterial family-specific changes in relative abundances occurred in the brackish environment following dispersal events. Together, this study shows that a relatively low dispersal of microorganisms among local communities can alter the importance of environmental effects on bacterial communities, and potentially facilitate local adaption of some bacteria under brackish environmental condition.