Effect of fulvic acids with different characteristics on biological denitrification

The influences of three fulvic acids, referred to as SAFA (purchased from Shanghai Aladdin Reagent Company), SRFA (Suwannee River Fulvic Acid from International Humic Substances Society (IHSS)) and PPFA (Pahokee Peat Fulvic Acid from IHSS), with different chemical composition, structure, hydrophobicity and aromaticity degree on biological wastewater denitrification were investigated. It was found that SAFA remarkably enhanced the denitrification performance, while SRFA and PPFA had no obvious effect. Mechanistic study revealed that SAFA remarkably improved the metabolism of a carbon source (glucose) by stimulating the activities of key enzymes (hexokinase, 6-phosphofructose kinase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and pyruvate kinase) involved in glycolysis. The increase of glucose utilization resulted in the increase of intracellular NADH/NAD+ ratio that favored microbial denitrification. Meanwhile the bacterial growth was significantly improved in the presence of SAFA. Further studies revealed that SAFA also increased the activities of key denitrifying enzymes, including nitrate reductase (NAR), nitrite reductase (NIR), nitric oxide reductase (NOR), and nitrous oxide reductase (N2OR), thus enhancing the reduction of nitrate and transformation of its intermediates, especially nitrite and nitrous oxide. All the above positive effects posed by SAFA might be attributed to its lower molecular weight, less complex aromatic structures with predominantly aliphatic carbons and higher hydrophilicity than SRFA and PPFA, which lead to a more active interaction with denitrifying microorganisms via metabolism process regulation. A better knowledge of the relationship between molecular structure of fulvic acids and microbial denitrification activity may be of practical interest in the nitrogen cycle in nature's niches.