Differential responses of encoding-amoA nitrifiers and nir denitrifiers in activated sludge to anatase and rutile TiO 2 nanoparticles: What is active functional guild in rate limiting step of nitrogen cycle?

The long-terms effects of different crystal-composition TiO ₂ 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 TiO ₂ -NPs (TiO ₂ -A) and rutile TiO ₂ -NPs (TiO ₂ -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), TiO ₂ -NPs triggered bacteria and archaea UPGMA clustering and a deep modification of N-cycling functional diversity guided by crystal structure. in situ ¹³ C-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 TiO ₂ -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 TiO ₂ -A, while TiO ₂ -R showed the stronger toxicity to AOA and NRB resulting from differences in water environmental behaviors and crystal characteristics of two TiO ₂ -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.
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