Long-term cumulative toxicity of low-dose lanthanum on nitrifying bacteria in partial nitrification process: Mechanism of disruption of intracellular calcium homeostasis

Due to differences in ion uptake, aerobic bacteria are more prone to metal accumulation toxicity than anaerobic bacteria, while the potential long-term environmental risks of lanthanum (La(III)) on aerobic ammonia oxidizing bacteria are unknown. In this study, the effects and mechanisms of long-term (Within 99 days) exposure to low-dose (0.5–5.0 mg/L) La(III) on partial nitrification (PN) process were illuminated. Firstly, after long-term exposure to La(III), the performance of PN process was decreased by 8–48 % as both Nitrospira and Nitrosomonas are enriched simultaneously when La(III) of 0.5–2.5 mg/L, and collapsed with a sharp decrease in Nitrospira and Nitrosomonas when La(III) of 5.0 mg/L. Secondly, when La(III) reaches 5 mg/L, electrostatic interactions and hydrogen bonds are enhanced through the formation of La(III)- alginate network structure, which promotes the synthesis of lanthanum nanoparticles. Finally, the formed lanthanum nanoparticles bind to calcium ion (Ca(II)) channels on the cell membrane, which disrupted intracellular calcium homeostasis and resulted in oxidative damage to functional bacteria. This study fills the knowledge gap in the long-term effects and mechanisms of low-dose lanthanum on autotrophic nitrifying bacteria.