Patterns of bacterial generalists and specialists in lakes and reservoirs along a latitudinal gradient

Aim: The geographical distribution of bacteria is an important, but poorly understood, topic in microbial ecology. A major question is how broadly distributed generalist taxa, and limitedly distributed specialist taxa, vary across a latitudinal gradient in freshwater ecosystems. We predict that: (a) generalists and specialists exhibit latitudinal diversity gradient with different patterns; (b) their community assemblies are mainly driven by stochastic processes; and (c) generalists coexist through niche differences, while specialists coexist through fitness differences. Location: Sixty lakes and reservoirs in China, ranging from 18 degrees N to 50 degrees N. ime Period: July-August in 2012 and 2018. Taxa Studied: Bacterioplankton. Methods: We clustered zero-radius operational taxonomic unit (zOTU) (subspecies-like level) into operational taxonomic unit (OTU) (species-like level) and calculated the intra-specific variation and spatial asynchrony of OTU. We examined the relative effects of environment versus space (latitude) on bacterial community and quantified the community assembly processes of generalists and specialists, respectively. The binary-state speciation and extinction (BiSSE) model has been used to estimate the evolutionary characteristics. The abundances of the top zOTUs within each OTU were identified to reveal their niche preferences (distribution patterns at different sites). Results: We found that bacterial generalists had a higher intra-specific variation, greater asynchrony and wider niche width than the specialists. With increasing latitude, bacterial diversity decreased with abundance variations in generalists and occurrence variations in specialists, respectively. Diversification in stochastic processes was the main driving factor in both groups, though deterministic processes showed greater effects on specialists than generalists in local communities. Bacterial generalists coexisted through niche differences, whereas specialists coexisted through average fitness differences. Main Conclusions: We demonstrated distinct latitudinal distribution patterns of bacterial generalists and specialists. These results highlight differences in biogeography of microbial generalist and specialist taxa mediated by intra-specific variation and emphasize their distinctly vital roles mediated by differing mechanisms in freshwater ecosystems.