Biofilms attached to Myriophyllum spicatum play a dominant role in nitrogen removal in constructed wetland mesocosms with submersed macrophytes: Evidence from 15N tracking, nitrogen budgets and metagenomics analyses.

The mechanisms behind nitrogen removal by the submersed macrophyte-biofilm complex in wetlands remain to be fully elucidated. This study investigated the role of Myriophyllum spicatum and the biofilm on their leaves in nitrogen removal in mesocosm experiments. ¹⁵N tracking showed that 61.9% and 30% of the ¹⁵N, respectively, was removed from the system and assimilated by the macrophyte-biofilm complex after loading with 5.4 mg L⁻¹¹⁵N labelled NH 4 ⁺ for 17 days. Nitrogen budget results showed that about 0.2%, 0.2% and 3.6% of the nitrogen were emitted as water-, HCl- and NaOH-soluble nitrogen-gas species, respectively. Bacteria (76.7–91.8%) were the predominant domain in all samples, followed by eukaryotes (8.0–23.0%), archaea and viruses. Network analyses showed that there were positive- and negative-correlative relationships among nitrogen-cycling genes and nitrifiers and denitrifiers. Our data highlight the important role of biofilm on submersed macrophytes for nitrogen removal. Image 1 • M. spicatum restoration promoted nitrogen removal in constructed wetlands (mesocosm scale). • The biofilms had a higher competitiveness for waterborne nitrogen than M. spicatum. • Bacteria were the most dominant domain, followed by eukaryotes, archaea and viruses. • There were complex interactions among nitrogen-cycling genes and genera. The biofilms had a higher competitiveness for nitrogen than M. spicatum and there were complex interactions among nitrogen-cycling genes and genera in biofilms. [ABSTRACT FROM AUTHOR]