Distinct microbial assembly and structures of iron-dependent autotrophic denitrifying communities enriched by different ferrous salts

Iron-dependent autotrophic denitrification （IDAD） is a promising biological technology for nitrogen removal from wastewater with a low C/N ratio. While significant differences in community structure among IDAD consortia from different studies have been reported， the underlying reasons remain unclear. Hypothesizing that the anion type of ferrous salts could be a key contributing factor， this study enriched two IDAD consortia （R1 and R2） using FeCl₂ and FeSO₄ as respective electron donors under identical inoculum sludge conditions. Their nitrogen removal performance， extracellular polymeric substance （EPS） composition， iron oxidation product properties， microbial community structure， and functional gene distribution characteristics during long-term operation were systematically compared. The results showed that R1 exhibited higher denitrification efficiency and Fe（Ⅱ） oxidation activity. Key iron-oxidizing bacterial genera such as Gallionella showed a significantly higher relative abundance in R1 than in R2， and R1 also maintained higher community diversity. Metagenomic analysis further revealed a higher abundance of functional genes related to iron oxidation and denitrification in R1. In contrast， R2 was enriched with more genera associated with sulfate metabolism and complex organic matter degradation. Distinct differences were also observed in EPS composition and iron mineral surface properties between the two consortia. This study confirms that the type of ferrous salt significantly regulates the structural assembly and metabolic functions of IDAD consortia， providing a theoretical basis for optimizing their application in practical wastewater treatment.