Eutrophication Reshapes Microbial Communities and Life-History Strategies in the Riverine Ecosystems

Rivers are increasingly affected by human activities, leading to widespread eutrophication. However, the responses of riverine microbiomes to eutrophication remain poorly understood. In this study, we compared microbiomes between eutrophic urban rivers (UR) and relatively undisturbed natural rivers (NR) to elucidate how eutrophication influences community structures, assembly processes, functions and life-history strategies. Amplicon and metagenomic sequencing revealed that eutrophication substantially enhanced microbial abundance and diversity in riverine ecosystems, with UR harbouring a higher proportion of fast-growing, nitrogen-transforming and antibiotic-resistant taxa. Neutral and null model analyses further revealed that, while stochastic processes predominantly shaped communities in NR, deterministic environmental selection exerted stronger control under eutrophic conditions in UR. Correspondingly, microbial communities in UR exhibited higher 16S rRNA gene copy numbers (median 4.69 vs. 4.28), stronger codon usage bias (0.0209 vs. 0.0204), greater predicted growth rates (0.2664 vs. 0.1567 h^-1), larger genomes (5.91 vs. 5.19 Mb), higher guanine-cytosine content (57.68% vs. 56.41%) and enriched transposase genes (4.37% vs. 2.98%), collectively indicating a community-wide shift from K-selected to r-selected life-history strategies under eutrophication. Overall, this work elucidates how human activities reshape riverine microbial communities and life-history strategies, providing a basis for predicting the ecological outcomes of nutrient over-enrichment in fluvial environments.