A matter of salt: Global assessment of the effect of salt ionic composition as a driver of aquatic bacterial diversity

While the influence of salinity on microbial diversity is well documented in marine and brackish ecosystems, the impact of different dissolved inorganic ion types remains largely unexplored. In this study, we assessed how ionic composition shapes planktonic bacterial community structure in inland saline aquatic habitats, compared to the effects of salinity alone, spatial factors, and other environmental variables. We collected and analyzed 16S rRNA gene amplicon datasets from freshwater to hypersaline aquatic environments worldwide (375 samples from 130 lakes). The composition of major ions explained more variability in bacterioplankton structure than bulk salinity. Taxa contributing the most to the observed dissimilarity between communities included lineages characteristic of specific habitat types, such as Actinobacteria acI in freshwater, Halomonadaceae in saline waters, or Nitriliruptorales in soda- and soda-saline systems. Many of these indicator lineages for specific habitat types were monophyletic, further underpinning ionic composition as a crucial eco-evolutionary driver of aquatic microbial diversity.