Salinity acclimation of nitrifying microorganisms: Nitrification performance, microbial community, osmotic adaptation strategies

Wastewater with high salinity (> 1%) presents a significant challenge to conventional wastewater treatment, particularly for the nitrification process. However, the osmotic adaptation strategies of nitrifying microorganisms remain poorly understood. In this study, we examined the impacts of salinity on the ammonia and nitrite oxidation processes in wastewater. The biofilm samples without salinity acclimation (0 g NaCl/L), after 1% salinity acclimation (10 g NaCl/L), and after 3% salinity acclimation (30 g NaCl/L) were inoculated to conical flasks containing synthetic high-salt wastewater (30 g NaCl/L), respectively. The research findings indicate that, following the salinity acclimation of biofilm, the activity of ammonia oxidation surpassed that of nitrite oxidation. 16S rRNA gene amplicon analysis revealed a noteworthy increase in the abundance of Nitrosomonas (ammonia-oxidizing bacteria) and an unclassified ammonia-oxidizing archaeon within the Nitrososphaeraceae family. In contrast, Nitrospira (nitrite-oxidizing bacteria) exhibited a significant decrease (p < 0.01). Metagenomic analysis indicates certain strains, such as Nitrosomonas sp. PL2, Nitrosomonas mobilis PL3, and Nitrososphaeraceae gen. sp. PL5, possessed various genes related to Na⁺ efflux, K⁺ uptake, glutamate synthesis or transport. However, Nitrospira sp. PL6 and Nitrospira sp. PL7 lacked K⁺ uptake genes. This study elucidates the microbial mechanisms underlying the variations in nitrification observed before and after salinity acclimation of biofilm, which helps to develop microbial evolution strategies to remove nitrogen pollutants under high salinity conditions.