Microbial Iron Utilization Pathways in Constructed Wetlands: Analysis of Substrates Affecting Iron Transformation, Absorption, and Utilization

Iron materials are known to enhance the nitrogen removal efficiency in constructed wetlands (CWs) by coupling iron transformation with nitrogen removal. However, current research lacks detailed explanations of the microbial processes involved in utilizing iron substrates, such as iron transformation, cellular iron uptake, and metabolism, leaving a gap in the understanding of these connections. This study addresses this gap by constructing four microcosm CW systems using Fe–C, various ratios of pyrite, and zerovalent iron (ZVI) as substrates. Experimental results indicated that the iron transformation was the most thermodynamically favorable with pyrite. Microbial communities on pyrite: gravel in a 2:1 volume ratio (2P1G) exhibited a greater propensity for Feammox, with a 0.76% increase in the functional microbial network of Feammox and a 31.20% increase in the abundance of the nirA gene associated with Feammox process compared to the Fe–C group. Conversely, the iron transformation in the Fe–C group was thermodynamically less favorable. To maintain intracellular iron homeostasis, microorganisms in the Fe–C group increased the siderophore activity. The gene abundances related to the release and absorption of siderophore were 22.12% and 17.26% increased, respectively, compared to 2P1G. This research employs the siderophore indicators to elucidate the link between iron transport and nitrogen metabolism, providing insights for improving nitrogen removal in CWs.