Evaluating the role of high NO affinity complete denitrifiers and non-denitrifying NO reducing bacteria in reducing NO emissions in river.

Freshwater rivers are hotspots of NO greenhouse gas emissions. Dissolved organic carbon (DOC) is the dominant electron donor for microbial NO reduction, which can reduce NO emission through enriching high NO affinity denitrifiers or enriching non-denitrifying NO-reducing bacteria (NORB), but the primary regulatory pathway remains unclear. Here, field study indicated that high DOC concentration in rivers enhanced denitrification rate but reduced NO flux by improving nosZ gene abundance. Then, four NO-fed membrane aeration biofilm reactors inoculated with river sediments from river channel, estuary, adjacent lake, and a mixture were continuously performed for 360 days, including low, high, and mixed DOC stages. During enrichment stages, the (nirS+nirK)/nosZ ratio showed no significant difference, but the community structure of denitrifiers and NORB changed significantly (p < 0.05). In addition, NORB strains isolated from different enrichment stages positioned in different branches of the phylogenetic tree. NORB strains isolated during high DOC stage showed significant higher maximum NO-reducing capability (V: 0.6 ± 0.4 ×10 pmol h cell) and NO affinity (a: 7.8 ± 7.7 ×1 L cell h) than strains isolated during low (V: 0.1 ± 0.1 ×10 pmol h cell, a: 0.7 ± 0.4 ×1 L cell h) and mixed DOC stages (V: 0.1 ± 0.1 ×10 pmol h cell, a: 0.9 ± 0.9 ×1 L cell h) (p < 0.05). Hence, under high DOC concentration conditions, the primary factor in reducing NO emissions in rivers is the enrichment of complete denitrifiers with high NO affinity, rather than non-denitrifying NORB.