The Effects Of AgNP And Ag~+ On Nitrosomonas Europaea Toxicity And Soil Ammonia Oxidation

With the extensive use of AgNP products,the AgNP content in the environment continues to increase,and the broad-spectrum antibacterial properties of AgNP pose a potential threat to environmental microbes.Ammonia oxidation is the rate-limiting step of nitrification,which affects the nitrogen cycle of the soil.AgNP may have an adverse effect on the soil ammonia oxidizing microorganisms and thus affect the progress of ammonia oxidation.Therefore,it is necessary to determine the environmental toxicity of AgNP,to clarify the effect of AgNP on the soil ammonia oxidizing microorganisms and ammonia oxidation process,and to lay a foundation for the study of soil ecotoxicity of AgNP.The effects of different AgNP with different particle sizes on the toxicity of ammonia-oxidation model strains?Nitrosomonas europaea?were studied to investigate the effects of nAg₁₀ and nAg₅₀ on Nitrosomonas europaea activity,apoptosis,membrane structure,oxidative stress and gene expression.The results showed that exposure of AgNP inhibited the activity of N.europaea and the nitrogen conversion.The bacterial mortality rates after exposure to 1 mg/L Ag⁺,10 mg/L nAg₁₀,and 20 mg/L nAg₅₀0 for 12 h were18.4%,42.2%,and 42.1%,respectively.At this time,the cell membrane was destroyed,and the expression of amoA and merA was down-regulated.The interaction between AgNP and cell membrane,release of silver ions,and oxidative stress are the main causes of AgNP toxicity,while nAg₁₀ is more toxic than nAg₅₀.The number of nitrifying bacteria,enzyme activity,and amoA gene abundance in yellow brown soil and paddy soill exposed to AgNP and silver ions were taken as research objects to investigate the effects of AgNP and silver ions on soil ammonia-oxidizing microorganisms.The results showed that after exposure to AgNP and silver ions for 7 days,the number of soil nitrite bacteria and nitrate bacteria decreased,urease activity was inhibited,and the abundance of soil AOB and AOA decreased,but the effect of AgNP and silver ions on soil catalase smaller,the effect of amoA gene abundance on ammonia-oxidizing bacteria is greater than that of ammonia-oxidizing archaea.The effects of AgNP and silver ions are positively correlated with their doses.The higher the dose,the greater the inhibition effect.AgNP and silver ions have stronger effects on the yellow brown soil than paddy soil.Under the exposure of AgNP and silver ions,the contents of NH₄⁺-N?NO₃^--N and potential ammoxidation rate in yellow-brown soil,paddy soil were studied.The effects of AgNP and silver ions on soil N conversion and ammonia oxidation were investigated.The results showed that the effect of AgNP and silver ions on soil NH₄⁺-N output was significant under short-term exposure?1 d,7 d?,and the effect of long-term?21 d,28 d?exposure was small.The effect of AgNP on soil NO₃^--N increased gradually with time,while the effect of silver ions was greatest on the 7th day of exposure,and then gradually decreased.The effect of AgNP and silver ions on the potential ammonia oxidation rate of the soil was evident at the 1st day of culture.The medium and high doses of AgNP and silver ions significantly inhibited the potential ammonia oxidation rate of the soil.The effect of AgNP changes little with time,and the effect of silver ions is inversely related to time.