Effect of substrates on functional microbial communities and functional metabolic pathways of constructed wetlands...

Constructed wetlands represent an eco-friendly approach for mariculture wastewater treatment, yet the mechanistic role of substrates, particularly their "biological influence" under high-salinity conditions (i.e., shaping and modulating functional microbial communities) remains poorly understood. To address this gap, horizontal flow simulated wetlands were established using Sesuvium portulacastrum L. combined with bay mud (BM), fine sand (FS), oyster shells (OS), volcanic rock (VR), and composite substrate (CS) to treat Epinephelus spp. breeding tailwater. Results demonstrated that the metabolic pathways of functional species rarely varied across habitats in the CWs, however, the functional bacterial community structure of the rhizosphere which,mainly contribute to the cycling of nitrogen, phosphorus and organic matters was significantly shaped by the characteristics of substrates. For example, substrates FS and VR demonstrated the highest COD removal efficiencies but poorly supported the growth of Sesuvium portulacastrum L. root systems, as well as the rhizosphere functional bacterial community structure. In contrast, the large voidage of OS resulted in lower removal efficiency (e.g., 45.6 % of COD removal), however, OS was suggested to favor the growth of plant roots, promoting the enrichment of keys functional species-Flavobacteriaceae, and "amino acid metabolism" and "energy metabolism" genes in both rhizosphere and water column, realizing the synergistic bacterial cooperation between aquatic and root niches for nutrient removal. Under high-salinity conditions, substrates with higher porosity were supposed to mainly regulate the accumulation of aerobic functional genera in the substrate and water column, while, substrates that could provide enough growth space for the root system prefer to exhibit optimal "biological role" in shaping functional flora in the CWs. This study provides critical insights into substrate-driven microbial selection in high-salinity wetlands, facilitating informed substrate design for enhanced pollutant remediation.