Macrophytes stimulate microbial interaction and carbon, nitrogen, sulfur, and iron cycling in iron-carbon micro-electrolysis constructed wetlands

Abstract

Little attention was paid on vital roles of macrophytes in iron-carbon micro-electrolysis constructed wetlands (ICME-CWs), especially focused on rhizosphere microbial interaction and metabolisms. In this study, ICME-CWs with and without macrophytes were constructed. Results showed efficient removal of total nitrogen (TN) in unplanted (73.28–76.57 %) and planted group (84.80–88.25 %). Superior TN removal was due to 24.26–38.87 % (p < 0.05) higher ammonium transformation with macrophytes present. Besides, both total phosphorus and organic matter removal were 10.41–16.83 % (p < 0.05) and 11.65–17.40 % (p < 0.05) higher in planted group, respectively in whole duration and 0–90 d. Microbial interaction was stimulated by macrophytes, with 20.48–74.65 % higher relative activity of key enzymes in rhizosphere. Moreover, macrophytes improved biofilm growth, increased microbial alpha diversity, and changed microbial community components at level phylum to genus. Functional bacteria were also enriched in planted ICME-CW, including nitrifying bacteria (Nitrospira, Nitrosomonas, Ellin6067, and Candidatus_Alysiosphaera) and hydrogenotrophic denitrifying bacteria (Thauera, Pseudomona, Hydrogenophaga, Dechloromonas, and Paracoccus). Meanwhile, enrichment of electroautotrophic and sulfur-oxidizing denitrifying bacteria, nitrate-dependent ferrous oxidation bacteria, and iron reducing/electroactive bacteria revealed plant enhancement on iron-carbon mircoelectrolysis and combined nitrogen-iron-sulfur cycling. Most functional classifications at the second level were up-regulated by macrophytes, with specific up-regulation of Glycolysis/Gluconeogenesis, Nitrogen metabolism, Sulfur metabolism, ABC transporters, and Phosphotransferase system at the third level. Simultaneously, relative abundance of genes involved in carbon, nitrogen, and sulfur metabolism, and iron cycling were also increased in planted group, thus promoting electron transferring, energy support, and nutrients removal.