Deciphering autotrophic and heterotrophic partial nitrification under salinity stress

Abstract

Autotrophic (R1) and heterotrophic (R2) partial nitrification exhibited differential nitrifying performance under salinity free conditions, however, their responses to salinity stress haven't been clearly established. Comparing their salinity response mechanisms provides intriguing insights for the treatment of saline wastewater. In the current study, assisted by organic matter, R2 excelled at ammonia removal efficiency (ARE) at 0 g/L NaCl. Gradual elevation of salinity load (>10 g/L) highlighted the higher salinity sensitivity of R2 as evidenced by precipitous drop in ARE. Furthermore, ARE of R2 reached to 73.25% at 50 g/L NaCl, while, ARE of R1 remained as high as 85.22%. The elevation in the ratio of protein to polysaccharides (PN/PS) in the tightly attached extracellular polymer substances of R1 suggested its enhancement of nutrients transferring efficiency by secreting more hydrophilic proteins, while the drop in the PN/PS of R2 implied its actions in stabilizing the aggregate structure under salinity stress. Subsequently, microbial analysis linked the higher salinity sensitivity of R2 with its greater loss of AOB density. Important AOB such as uncultured Nitrosomonas sp. exhibited differing salinity tolerances in two reactors. In a nut shell, the cliff-like drop of total bacterial density contrasted strongly with greater levels of total EPS in R2, indicating EPS may offer limited microbial protection under salinity stress.