From freshwater anammox bacteria (FAB) to marine anammox bacteria (MAB): A stepwise salinity acclimation process

Highlights: • Saline induced stress correlates uniquely to the saline adaptation of anammox. • Improvement in N-removal efficiency is observed at higher saline concentrations. • Granules formation and bacteria aggregation tailored the saline input profile. • MAB dominated the granules in the reactor by the latter end of 10 g·L⁻¹ salinity. An investigation into the effect of stepwise saline introduction (3–20 g·L⁻¹ NaCl) on the anaerobic ammonium oxidation (anammox) process in a lab-scale sequencing batch reactor was carried out for 252 days by evaluating the changes in influent and effluent nitrogen concentrations, conductivity, microbial extracellular polymeric substances' (EPS) ionic content, as well as stresses due to salinity, via microbial ATP analysis. It was observed that, effluent nitrogen concentrations remained stable at low saline levels of 3 g·L⁻¹ to 10 g·L⁻¹. Nonetheless, midway through 10 g·L⁻¹ and the preliminary phase of 15 g·L⁻¹ salinity presented a very unstable, highly fluctuating as well as deteriorating effluent nitrogen concentrations. A more satisfactory nitrogen removal efficiency of 83.7 ± 5.9% was obtained at higher saline concentrations implying that, the adaptation mechanism to tolerate increasing salinity was taking place. Saline induced stress, which measures the variation in viable anammox bacteria, was correlative to the formation of EPS and changes in its cationic contents along the increasing salinity. Although the specific anammox activity (SAA) dropped by approximately 15% from the beginning of the process to the midpoint, the drop in SAA after the midpoint was not as drastic as the initial phase. A change in microbial aggregation and dominance proved the existence of new saline-dependent species that can withstand high saline stresses. Recovery from abrupt high saline shocks in batch experiment was seen to be almost impossible.