Metagenomic insights into responses of microbial population and key functional genes to fulvic acid during partial nitritation

• 50mg/L is the threshold for PN process. • Sludge granulation is enhanced during proper FA exposure. • High FA is beneficial to maintain the perfect relative abundance of AOB • The down-regulation of amoABC and hao is responsible for the inhibition of PN • The addition of FA is expected to be a novel strategy to suppress NOB The long-term impact of fulvic acid (FA) on partial nitritation (PN) system was initially examined in this study. The obtained results revealed that the FA lower than 50mg/L had negligible effect on the nitrite accumulation rate (NAR nearly 100%) and ammonium removal rate (ARR 56.85%), while FA over 50 mg/L decreased ARR from 56.85% to 0.7%. Sludge characteristics analysis found that appropriate FA (<50mg/L) exposure promoted the settling performance and granulation of PN sludge by removing Bacteroidetes and accumulating Chloroflexi . The analysis of metagenomics suggested that the presence of limited FA (0-50mg/L) stimulated the generation of NADH, which favors the denitrification and nitrite reduction. The negative impact of FA on the PN system could be divided into two stages. Initially, limited FA (50-120 mg/L) was decomposed by Anaerolineae to stimulate the growth and propagation of heterotrophic bacteria ( Thauera ). Increasing heterotrophs competed with AOB ( Nitrosomonas ) for dissolved oxygen, causing AOB to be eliminated and ARR to declined. Subsequently, when FA dosage was over 120 mg/L, Anaerolineae were inhibited and heterotrophic bacteria reduced, resulting in the abundance of AOB recovered. Nevertheless, the ammonium transformation pathway was suppressed because genes amoABC and hao were obviously reduced, leading to the deterioration of reactor performance. Overall, these results provide theoretical guidance for the practical application of PN for the treatment of FA-containing sewage. Graphical abstract