Longitudinal patterns of dissolved organic matter quality and bacterial community degradation function along the mainstream of Three Gorges Reservoir during impoundment period

Longitudinal patterns of dissolved organic matter (DOM) influence the biogeochemical cycle in river ecosystems. Yet, linkages between DOM quality and bacterial degradation of emerging contaminants over a river continuum are unclear. In combination with DOM characterization, bioassay, and microbial analysis, this study unraveled the spatial variations of DOM composition and biodegradability as well as bacterial community structures, which determined the estrone biodegradation potential (EBP) along the mainstream in Three Gorges Reservoir during impoundment period. While urban and agricultural utilization increased the protein-like and biodegradable DOM components (BDOC%) in upstream, the in-situ biotransformation preferentially utilized tryptophan-like substances followed by tyrosine-like substances, resulting in the accumulated recalcitrant DOM with higher aromaticity, molecular weight, and humic-like components in downstream. Due to the promotion of biodegradable DOM on bacterial abundances, a significant and positive correlation was constructed between BDOC% and EBP (P < 0.001, R2 = 0.56), both of which were controlled by the ratio of fresh-to humic-like fluorescence. Nevertheless, a higher biomass-normalized EBP was found under recalcitrant conditions in downstream, demonstrating that the degradation function was also associated with the biogeographical distribution of bacterial communities. Results of microbial ecology revealed that the in-situ transformation of aromatic and humiclike DOM exhibited more correlations with bacterial taxa than protein-like DOM, leading to increased diversity and decreased niche breadths along the mainstream. The triggered deterministic selection of community assemblages in downstream favored potential estrone-degraders and up-regulated their specific metabolic capacity. These findings will provide critical insights toward understanding the crucial role of DOM in biogeochemical cycles and the self-purification of emerging contaminants along the river continuum.