Mechanistic insights into stress response and metabolic activity resilience of Nitrosomonas europaea cultures to long-term CeO2 nanoparticle exposure

Abstract

Although the negative effects of CeO₂ nanoparticles (nano-CeO₂) on wastewater biological nitrogen removal (BNR) systems have been documented, the potential and mechanisms of the stress tolerance and resilience of nitrifiers to nano-CeO₂ exposure remain unclear. Herein, we explored the long-term impacts of nano-CeO₂ on a continuously cultured model ammonia oxidizer, Nitrosomonas europaea, and assessed the recovery capacities of nano-CeO₂ stressed cells at the physiological and transcriptional levels. Nano-CeO₂ at 50 mg L⁻¹ remarkably inhibited the bacterial growth, the membrane integrity, the ammonium removal performance and the specific ammonia monooxygenase activity. Meanwhile, N. europaea exhibited a tolerance capacity to the nano-CeO₂ stress and their metabolic activities were maintained even after 40 d nano-CeO₂ exposure. The batch recovery incubation study revealed the inhibition level-dependent recovery potential of nano-CeO₂ impaired N. europaea cultures. Transcriptomics-based investigation indicated that nano-CeO₂ induced the disruptions of the membrane structure and the associated transport systems. Nevertheless, the peptidoglycan biosynthesis and ATP-binding cassette transport relevant metabolism regulation probably played a pivotal role in cellular stress tolerance/recovery. In particular, the transcriptional regulation of energy metabolism pathways, such as CO₂ fixation, respiratory chain and ATP production, might favor the bacterial protection/recovery from the nano-CeO₂ stress. Moreover, diverse metabolic regulation at the transcriptional levels, including DNA repair, oxidative stress quenching, and signal transduction would be actively involved in cellular protection against nano-CeO₂ and impaired cells' metabolic activity recovery. These findings improved our understanding of stress responses and resilience of nitrifiers to NPs and provided novel insights into adjustment strategy determination for the potential NP impacts on BNR systems.