Abundant and rare bacteria in anthropogenic estuary: Community co-occurrence and assembly patterns

In estuary, bacteria are critically vital to ecosystem function and sustainability. However, the effect of geographic distance and its related factors (i.e., salinity and nutrients) on bacterial diversity, co-occurrence, as well as community assembly on the abundant and rare species scale remained to be discussed. In this study, we deciphered the bacterial dynamics and ecological processes in the anthropogenic estuary of the Haihe River, in China. The results revealed that salinity was the major abiotic factor to shape bacterial community composition, diversity, and network complexity, and the role of biotic cytometric characteristics was considerable as well. Furthermore, the salinity gradient tended to select bacteria with low nucleic acid (LNA) such as Candidatus Pelagibacter and Longivirga, and abundant bacteria were more sensitive to the salinity gradient in comparison with rare ones. The similar keystone number and network topology values indicated that both abundant and rare bacteria played important ecological functions in degrading pollutants, developing co-occurrence networks, and maintaining compositional stability and complexity. In addition, drift and dispersion limitation dominated the ecological processes in the community assembly, suggesting that although salinity caused the migration of species, its ecological role was still weaker than that of stochastic processes. Further, niche differentiation contributed to different stochastic components of abundant and rare species. In high-salinity areas, rare bacteria with small population sizes but high diversity were enriched, which made the bacterial community more affected by ecological drift. Our study illustrated the different responses of abundant and rare bacteria to salinity and the assembly process of bacterial communities in anthropogenic estuary, deepening the understanding of their ecological processes in the estuarine ecosystem