Depth induced assembly discrepancy of multitrophic microbial communities affect microbial nitrogen transformation processes in river cross-sections.

Understanding how the structures and functions of bacterial and microeukaryotic communities vary within cross-sections will improve managements aimed at restoring river ecological functions. However, no comprehensive investigation has examined how microbial community characteristics vary within cross-sections, which makes the accurate calculation and prediction of microbial metabolic processing of substances in rivers difficult. Here, the distributions, co-occurrence networks, and assemblies of bacterial and microeukaryotic communities and their feedback to nitrogen transformation in cross-sections of the Yangtze River were studied by coupling ecological theory, biogeochemistry, and DNA meta-barcoding methods. The study found that depth in cross-sections was the primary driving factor regulating the composition of sediment bacterial and microeukaryotic communities. Co-occurrence network analysis indicated that the effect of bacteria on the co-occurrence network decreased and the network become more simplified and instability with depth in river cross-sections. Quantified using the β-nearest taxon index, the H2 layer sediment (depth 10-20 m) displayed the largest variation in selection processes for microbial assemblies, while homogeneous selection and homogenizing dispersal contributed most to the bacterial and microeukaryotic assemblies in the H3 layer (depth >20 m). Cross-sectional depth and denitrification genes had a significant quadratic correlation, with the highest microbial nitrogen-removal potential occurring in the H2 layer sediment. Structural equation models showed that the sediment nitrogen distributions were regulated by distinct environmental pathways at different depths, and that the H2 layer sediment was primary driven by bacterial community. In this layer, river cross-sectional depth influenced nitrogen transformation by regulating the distribution of sediment particle sizes, which then influenced the assembly of the multitrophic microbial communities. This study will improve river management by clarifying the importance of cross-sectional depth to the ecological function of rivers.