Salinity effect on freshwater Anammox bacteria: Ionic stress and ion composition

•Both ionic stress and ion composition contribute to salinity-induced inhibition on FAB.•For FAB, inhibition increases in the order: NaCl < KCl < Na2SO4.•NaCl and Na2SO4 inhibit both substrate transport and enzyme activity; KCl mainly inhibits enzyme activity.•The combined inhibition of different ions is either additive or synergistic. The biggest challenge to apply Anammox to treat wastewater with elevated salt content is the inhibitory effect of salinity on freshwater Anammox bacteria (FAB). Most of the research into salinity inhibition has focused on the osmotic pressure effect, while the inhibitory effect and its mechanisms induced by ion composition are poorly understood. In this study, the individual and combined effect of NaCl, KCl and Na2SO4 on FAB (>99% belonging to Ca. Brocadia genera) were systematically investigated by batch tests. The corresponding responses of mRNA abundance of three functional genes (including nitrite reductase gene (nirS), hydrazine synthase gene (hzsB) and hydrazine dehydrogenase gene (hdh)) under different salt conditions were analyzed. The results indicated that NaCl, KCl and Na2SO4 have different inhibition effects, with the 50% inhibition at 0.106, 0.096 and 0.063 M, respectively. The combined inhibition of NaCl+KCl and NaCl+Na2SO4 on FAB were both synergistic; while the combined inhibition of NaCl+KCl+Na2SO4 was additive. The responses of mRNA (of genes: nirS, hzsB and hdh) suggested NaCl inhibited the transport of ammonium; Na2SO4 inhibited both nitrite and ammonium transport; high salinity inhibited functional enzyme activity. These results suggest both ionic stress and ion composition contributed to the observed inhibition. [Display omitted]