Physiological response and tolerance of Myriophyllum aquaticum to a wide range of ammonium concentrations.

Myriophyllum aquaticum (M. aquaticum) can be used in constructed wetlands (CWs) to effectively purify swine wastewater with high-ammonia nitrogen (NH-N and NH-N) concentrations. However, the understanding of its tolerance mechanism to ammonia nitrogen is limited. The physiological response and tolerance mechanism of M. aquaticum to a wide range of NH concentrations (0-35 mM) were investigated in the present study. The results indicated that M. aquaticum can tolerate NH concentrations of up to 30 mM for 21 days and grow well with high nutrient (N, P) uptake. A suitable concentration of NH for a better growth of M. aquaticum was 0.5-20 mM. The free NH content was no obviously increase at NH concentration below 15 mM, indicated there was no obviously ammonium accumulation. Exogenous NH inhibited K absorption and improved Ca absorption, indicating mineral cation could mediate NH homeostasis under NH stress. Moreover, comparison with those in the control group, the activities of glutamine synthetase (GS), glutamate synthetase (GOGAT) in M. aquaticum increased by 52.7%-115% at 1-20 mM NH, and superoxide dismutase (SOD) increased by 29.2-143% at 1-35 mM NH. This indicated that the high NH tolerance of M. aquaticum was mainly due to the balance of free NH content in tissues, as well as improved nitrogen metabolism and antioxidant system. This could be attributed to the role of the GS-GOGAT cycle and SOD. In conclusion, M. aquaticum, which tolerates high NH concentration and has a high N uptake ability, can be used as a good candidate specie to help develop more efficient management strategies for treating high-NH wastewater in CW systems.