Differential responses of encoding-amoA nitrifiers and nir denitrifiers in activated sludge to anatase and rutile TiO2 nanoparticles: What is active functional guild in rate limiting step of nitrogen cycle?

Abstract

The long-terms effects of different crystal-composition TiO2 nanoparticles (NPs) on nitrogen-cycle-related functional guilds in activated sludge remain unclear, especially under natural light irradiation. Accordingly, activated sludge was exposed to anatase TiO2-NPs (TiO2-A) and rutile TiO2-NPs (TiO2-R) for up to 45 days. With markedly (p < 0.05) reducing nitrification-/denitrification-enzymatic-activities and abundances of ammonia-oxidizing-microorganisms (AOMs) and nitrite-reducing-bacteria (NRB), TiO2-NPs triggered bacteria and archaea UPGMA clustering and a deep modification of N-cycling functional diversity guided by crystal structure. in situ13C-DNA-SIP confirmed ammonia-oxidizing-bacteria (AOB) (Nitrosomonas and Nitrosospira) in original sludge as main active AOMs with 75.4 times more abundance than ammonia-oxidizing-archaea (AOA), while AOA within Nitrosopumilus and Nitrososphaera genera were the main active AOMs and tended to aggregate inside sludge after 10-mg/L TiO2-NPs exposure. Encoding-nirK NRB were more sensitive, while encoding-nirS Zoogloea with a total share of 4.97% to 14.93%, etc. were the main active NRB. AOB was more sensitive to TiO2-A, while TiO2-R showed the stronger toxicity to AOA and NRB resulting from differences in water environmental behaviors and crystal characteristics of two TiO2-NPs. This work expands understanding of the ecological risks of titanium-dioxide-crystal-NPs in aquatic environment and may help devise better methods to alleviate environmental stress caused by NPs at wastewater treatment plants.