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• Deterministic processes prevail in the subcommunity assembly of river sediments. • Rare subcommunity assembly was governed more by interspecific interactions. • Introduction and colonization processes disentangle the abundant assembly. • Metals dominate the two processes, but nutrient availability prevail in the latter. • A balance between metal filtering and species coexistence within subcommunities. It is unknown how anthropogenic pollutants released into freshwater ecosystems affect the assembly processes of microbial communities in river sediment. We used high-throughput sequencing to examine the assembly of rare and abundant subcommunities in a heavily polluted urban river: the Beiyun River in Beijing, China. Although deterministic processes overrode stochastic processes in shaping local rare and abundant subcommunities, there were distinctly different assembly mechanisms of rare and abundant subcommunities. Rare subcommunity assembly was governed more by interspecific interactions, and environmental selection and dispersal limitation explained only a small fraction of the variation. However, both factors seemed to govern the assembly of abundant subcommunities. Our results implied that microbial co-occurrence associations tended to be higher when rare subcommunities were less driven by community assembly, and that these associations tended to be lower when abundant subcommunities were more driven by community assembly. A balance between the community assembly and species coexistence was exhibited at the subcommunity level. Importantly, we tried to disentangle the assembly process of abundant subcommunities into introduction and colonization processes characterized by the presence/absence and relative abundance datasets. Interestingly, metals explained the highest percentage of spatial variation in the species introduction process. By affecting nutrient availability, metals also shaped the abundant subcommunity in the species colonization process, but this did not surpass nutrient availability. Therefore, disentangling the introduction and colonization processes enhances our understanding of the assembly mechanisms of microbial communities in heavily polluted running water ecosystems at fine geographical scales. [ABSTRACT FROM AUTHOR]

Interviews Min Weifang, Secretary of the Party Committee of Beijing University in China, concerning the university's personnel reform. Kind of position that the reform occupies in Weifang's work; Implementation of a hiring and appointment system; Reasons the university wants reform.