Characterization of microbial communities in the rhizosphere and water of a field-scale artificial floating island system for nutrient removal.

Eutrophication, caused by excessive nutrient inputs, threatens water quality and aquatic ecosystems worldwide. Artificial Floating Islands (AFIs) offer a promising phytoremediation solution, leveraging aquatic plants and their associated microbes for nutrient removal. However, the microbial communities within AFI systems, particularly under natural conditions, remain poorly understood. In this study, microbial communities were characterized in a field-scale AFI system treating residential raw sewage, with a focus on the rhizosphere of Carex comosa (bristly sedge), Eleocharis obtusa (blunt spike-rush), and their polyculture. Microbial communities in both rhizosphere and wastewater were analyzed using 16S rRNA gene sequencing, and water physicochemical parameters and plant nutrient assimilation were monitored throughout the study. Results showed significantly higher microbial diversity in the rhizosphere compared to the water. Proteobacteria dominated the rhizosphere (47 %-52 %) and Cyanobacteria dominated the water (30 %). Polyculture system was associated with greater abundance of beneficial microbial taxa and metabolic pathways, which likely supported plant growth and enhanced nutrient assimilation. These effects were attributed in part to species-specific microbial contributions, particularly by E. obtusa, which introduced functionally relevant taxa despite limited plant nutrient uptake. Plant nutrient assimilation, rhizosphere microbial communities, and environmental factors such as pH, dissolved oxygen, and temperature were found to be closely interconnected. These findings suggest that polyculture is an effective strategy for enhancing nutrient removal in field-scale AFI systems by promoting microbial diversity and functional potential.