Mora, J. E.; Olson, M.; Rocks, S.; Zahn, G. L.

Urban expansion, projected to triple globally from 2000 to 2030, significantly impacts biodiversity and ecosystem processes, including those of microbial communities. While extensive research has examined urbanization effects on macro-organisms, the impacts on microbial communities, particularly those associated with aquatic plants, remain underexplored. This study investigates the fungal endophyte communities in the pollution-tolerant aquatic plant Ranunculus aquatilis along an urbanization gradient in the Provo River, Utah, USA, a rapidly urbanizing region. We collected plant and adjacent water samples from ten locations along the river, spanning from rural to urbanized areas, and conducted DNA amplicon sequencing to characterize fungal community composition. Our results show a significant decline in fungal alpha diversity downstream, correlated with increased urbanization metrics such as impervious surface area and developed land cover. Specifically, fungal ASV richness and Shannon diversity decreased as urbanization intensified, driven primarily by a reduction in rare taxa. Despite a stable core microbiome dominated by a few taxa, the overall community structure varied significantly along the urban gradient, with notable shifts in dominant fungal taxa. Contrary to expectations, no detectable levels of heavy metals were found in water samples, suggesting that other urbanization-related factors, potentially including organic pollutants or plant stress responses, influence fungal endophyte communities. Our findings underscore the need for further investigation into the mechanisms driving these patterns, particularly the roles of organic pollution, nutrient loads, and plant stress. Understanding these interactions is crucial for predicting the impacts of continued urbanization on freshwater ecosystems and their associated microbial communities.