Analysis of the β-cyclodextrin enhancing Bio-denitrification from the Perspective of Substrate Metabolism, Electron Transfer, and Iron Acquisition

• β-CD promoted denitrification and mitigated intermediate products generation. • β-CD enhanced glucose metabolism to generate energy and electron for denitrification. • The electron generation, transfer, and utilization in bacteria were promoted by β-CD. • The bacterial peripheral iron was enriched due to β-CD stimulating the EPS secretion. • β-CD enhanced bacterial iron acquisition via regulated iron transmembrane transport. Denitrification is widely applied in wastewater treatment systems for nitrogen removal, but it is still disturbed by unsatisfactory efficiency, nitrite accumulation, and nitrous oxide emission. In this study, the presence of 0.5 mM β-cyclodextrin (β-CD), an oligosaccharide chemical compound, effectively promoted the nitrate reduction by Paracoccus denitrificans from 68.02% to 82.78% after 20 h of cultivation. Meanwhile, the exogenous addition of β-CD significantly up-regulated the expression of denitrification-related genes ( narG , nirS , norB , and nosZ ), and mitigated the accumulation of nitrite and nitrous oxide emission. Notably, different from traditional organic substances for enhancing denitrification, this study not only identified β-CD increased electron generation and ATP synthesis from glucose metabolism of P. denitrificans instead of being utilized as an external carbon source, but also observed the relative promotion of β-CD on electron transfer and denitrification were positively correlated with ambient iron concentration. The addition of β-CD stimulated bacterial EPS secretion and indirectly enriched peripheral iron, and simultaneously benefited the iron transmembrane transportation and intracellular iron concentration. Given the whole process of iron enrichment and transportation, β-CD was demonstrated to improve denitrification and electron transfer via promoting iron acquisition. This study first revealed the enhancement of β-CD on bio-denitrification via iron acquisition, which provided a novel basis for its application in nitrogen removal.