Ammonia-oxidizing activity and microbial structure of AOB, AOA and comammox in biofilm systems with different salinities

Ammonia-oxidizing activity of different ammonia-oxidizing microorganisms (AOMs), such as ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and complete ammonia oxidizers (comammox), were investigated by adding the inhibitors 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), octyne (OCT) and KCLO3 in biofilm systems with different salinities. It was found that the ammonia-oxidizing activity of all AOMs gradually decreased with increasing salinity. The ammonia-oxidizing activity of AOB was consistently higher than that of AOA and comammox at different salinities. In addition, nitrite-oxidizing bacteria (NOB) were more sensitive to changes in salinity than AOMs. Metagenomic analysis revealed that nitrifiers were detected at high level, with the AOB Nitrosomonas sp. comprising 24.9% and the NOB Nitrospira sp. comprising 47.2% of all nitrifiers. The main functional genes of the nitrification reaction were amoABC, hao and nxrAB. This study demonstrates that higher abundances of functional microorganisms and genes are related to the ammonia-oxidizing activity and ammonia removal contribution rate.