Experimental insights into the importance of ecologically dissimilar bacteria to community assembly along a salinity gradient

The response of local communities to marine–freshwater transitions and the processes that underlie community assembly are unclear, particularly with respect to bacteria that differ in their life strategies. Here, we implemented a transplant experiment where bacterioplankton from three regions of the Baltic Sea with differing salinities (～3, 7 and 28 psu) were exposed to each other's environmental conditions. We found that habitat specialists were more abundant than generalists after exposure to salinity changes, irrespective of their origins. Most specialists that were selected following a salinity change were rare in the starting communities. Selection for generalists, however, was not specifically driven by the recruitment of either rare or abundant members, suggesting that taxon's initial abundance is minor relevant to the growth of generalists. Patterns in phylogenetic relatedness indicated that environmental filtering was the most influential assembly mechanism for specialists, whereas competitive interaction was more important for the assembly of generalists. Altogether, this study shows that large salinity changes promote the establishment of habitat specialists and that deterministic processes vary during community assembly for ecologically dissimilar taxa. We, therefore, propose that distinguishing assembly mechanisms of different community members helps understand and predict community dynamics in response to environmental change.