The alleviation of Cr(Ⅵ) stress on simultaneous nitrification and denitrification process of Acinetobacter...

Bioremediation of Cr(Ⅵ) and ammonia is considered as a promising and cost-effective alternative to chemical and physical methods. However, Cr(Ⅵ) could inhibit nitrogen removal by inhibiting intra-/extracellular electron (IET/EET) transfer or nitrifying and denitrifying enzymes activity due to its higher solubility. In this study, we isolated a simultaneous nitrification and denitrification (SND) microorganism Acinetobacter haemolyticus RH19, capable of outcompeting oxygen to take nitrogen oxides/ammonia as electron acceptors, and studied a combined accelerant (cysteine, biotin and cytokinin) to relive the Cr(Ⅵ) stress. Respiratory chain inhibited experiments and intermediates showed that strain RH19 had the intact intracellular respiratory chain. Despite the inhibited complex Ⅳ favoring the electrons transfer to NO_x^--N, the SND process was still greatly inhibited with Cr(Ⅵ), likely attributed to lower electron flow to the electron acceptors (nitration/nitrition/denitrification enzyme). Instead, the accelerant detoxified Cr(Ⅵ) mainly at CoQ site responsible for electron transfer to AMO and NAP, as well as complex Ⅳ (related with aerobic denitrification), favoring the shortcut SND (SSND, NH₄⁺-N→NH₂ON→NO₂^--N→N₂) process by directly converting nitrite to nitrogen gases. Additionally, accelerant could stimulate the secretion of c-Cyts and flavin mononucleotide (FMN) to improve the electron transfer. Overall, this study highlighted the accelerant-alleviated mechanism in the SND process under Cr(Ⅵ) stress, and deepened the theoretical SND basis for the treatment of co-existing pollutants.