Effects Of Microorganism On Ammonia Tolerance Of Vallisneria Natans And Their Mechanism

With the acceleration of water eutrophication,a sharp declined diversity of the aquatic plants,especially the submerged plant,was commonly observed.Ammonia is generally considered as an important factor for the retirement of submerged plants in aquatic environment.However,results of ammonia toxicity generated from different laboratories under open condition varied a lot,even for the same tested aquatic plant.In present study,we treated macrophyte Vallisneria natans with different concentrations of ammonia(1-75.9mg/L)under aseptic or open conditions.Physicochemical and physiological responses were measured and analyzed to evaluate the impacts of microorganisms on ammonia toxicity test.The results showed that ammonia has minor direct effects on the growth of Vallisneria natans after 10d exposure under aseptic condition.Briefly,no significant differences of physicochemical responses(such as total chlorophyll,root activity,malonaldehyde(MDA),soluble carbohydrate(SC),the soluble protein and T-SOD activity)from control were detected.Comparatively,remarkable decreased ammonia tolerance of Vallisneria natans was observed under open condition.Plants treated with 7.5 mg/L ammonia had significant differences(p<0.05)of free amino acids(FAA)and soluble carbohydrate(SC)from the rest treatments.Besides,total chlorophyll decreased dramatically when ammonia was over 22.5 mg/L.The addition of ammonia accelerated the growth of microorganism,which resulted a sharp retirement of the plant,especially under favorable condition(sugar addition).Additionally,dissolved oxygen(DO)decreased to a very low level(i.e.,0 mg/L)and nitrate concentration raised to 13.38 mg/L.Exposure to hypoxia condition did not lead the sharp retirement of plant,but aeration could alleviate the ammonia toxicity under open condition.Nitrate(i.e.,15 mg/L)also had low toxicity,but the toxicity increased sharply once mixed with other metabolites.Besides,ammonia would reduce the microorganism diversity and increase the growth of mono-microorganism(Gammaproteobacteria).In conclusion,the metabolites(i.e.,nitrate and rest)probably derived from Gammaproteobacteria would be the major reason responsible for the low ammonia tolerance of Vallisneria natans.Besides,the hypoxia condition also contributed to the low ammonia tolerance.Therefore,the impacts of microorganism should be considered when using aquatic plants for ecotoxicology risk assessment.