Characteristics And Mechanisms Of Cd-Induced Hormesis On Typical Hydrolase And Oxidoreductase In Soil

Hormesis refers to the phenomenon where low doses of toxic chemicals or stress conditions can stimulate an organism’s response,while high doses result in inhibition.In recent years,the potential application of heavy metal-induced Hormesis studies in ecological risk assessment has received much attention.Previous studies have primarily focused on evaluating the compensation ability of soil itself（excluding plants and soil animals）to heavy metal stress using single test endpoints such as soil enzymes,soil respiration,and microbial community abundance.However,there is a lack of proper evaluation regarding whether the information obtained from these individual indicators can objectively reflect the overall situation.On the other hand,maximum stimulation,response dose width,and effect duration are currently common parameters for quantifying hormetic characteristics.Although the"30%-60%"rule of maximum stimulation is considered to exist,in fact,with the successive reports of multiple Hormesis phenomena and the time dependence of Hormesis,the results reflected by different parameters are often inconsistent.In addition,the quorum sensing of soil microorganisms is considered to be an important mechanism for the Hormesis of soil enzyme activity.However,it remains unclear that the response mechanism between microbial quorum sensing and soil enzyme activity.In this paper,soil hydrolases and oxidoreductases involved in carbon（C）,nitrogen（N）,and phosphorus（P）metabolism,such as urease（URE）,alkaline phosphatase（ALP）,dehydrogenase（DHA）,and denitrifying enzyme activity（DEA）,were selected as endpoints.Based on the maximum stimulation(M_max)and its corresponding dose（d）,as well as response dose width（Q_i）,the parameter of hormetic compensation capacity(Hor_zone)was constructed to study the dose-response of soil enzyme activity under Cd stress at different exposure times.The kinetic mechanism of the Hormesis of soil enzyme activity under Cd stress was studied by means of changes in the maximum reaction rate(V_max),Michaelis constant（K_m）,etc.,with the help of enzymatic reaction kinetic methods.Additionally,the population changes of abundant and rare bacteria,Gram-positive（G+）and Gram-negative（G-）bacteria,and community diversity under Cd stress were investigated at different exposure times and doses using second-generation high-throughput sequencing technology.This paper aims to provide a theoretical basis and new insights into the ecological risk assessment of heavy metal-contaminated soils and the regulation of soil Hormesis.The main results and conclusions of this thesis are as follows:（1）The changes of soil hydrolase and oxidoreductase activities under Cd stress（0-48 h）showed a typical inverted"U"-shaped dose-response characteristic.The period of typical Hormesis/Hormetic-like of all four soil enzymes under Cd stress was 24 h after exposure,and the duration of compensation of stress injury by redox enzymes was longer than that of hydrolytic enzymes.The compensation intensity varied as DHA>ALP>URE>DEA,and the corresponding dose interval for the appearance of maximum compensation intensity was 0.03-6.0 mg·kg^-1.（2）URE,ALP and DEA,showed significant Hormesis/Hormetic-like response characteristics for both V_max and K_m,but DHA only had V_max to respond to.This suggests that the appearance of Hormesis/Hormetic-like in the first three enzyme activities is closely related to the changes in enzyme-substrate affinity under Cd stress and the changes in enzyme amount in response to microorganisms,while DHA may be mainly dominated by the changes in microorganisms.From the response time,it seems that the two hydrolases,URE and ALP,may be more sensitive to Cd stress,responding not only earlier but also in terms of both enzyme structure and amount.In particular,ALP activity was significantly and positively correlated with both its V_maxand K_m at 24 h and 48 h,respectively.This not only confirms that the change in ALP activity is the result of the combined effect of enzyme structure and enzyme quantity,but also that the two modes of action may be in a"shift"relationship.In contrast,the responses of DEA and DHA oxidoreductases were more sluggish and homogeneous.（3）Hormetic response of the bacterial population under Cd stress was mainly in the form ofα-diversity,rare taxa,and G-of bacteria,with the magnitude of compensation intensity in the order of rare taxa>G->α-diversity.Of interest,the V_max of DHA showed significant or highly significant correlations with the shannon index,the relative abundance of G-and six phyla and five genera at the onset of Cd stress（0 h）,in contrast to the other three enzymes significantly.This confirms the inference that the Hormesis/Hormetic-like of DHA activity is mainly caused by microorganisms by altering the amount of enzymes.（4）The tolerance compensation force parameter(Hor_zone)was defined and constructed,and the relative magnitude of the compensation force of the four enzyme activities to Cd stress was found to be DHA>ALP>URE>DEA,in which the contribution originating from the structural response of the enzyme was 74.52%and the contribution from the enzyme quantity change was25.48%.The contributions of bacterial community parameters that had a significant effect on the variation of enzyme amount were in the order ofα-diversity index,rare taxa,and G-.The contribution of the three was 39.07%,35.90%and 25.04%,respectively.Therefore,it is hypothesized that the mechanism of Cd-induced Hormesis of soil enzyme activity is that Cd enters the soil and acts on both enzyme molecules and microorganisms,and the response of enzyme molecules to Cd causes changes in enzyme-substrate affinity（contributing74.52%）,while microorganisms dominate enzyme amount changes through abundance and metabolic processes（contributing 25.48%）.The initial response to Cd stress is observed in hydrolases,followed by oxidoreductases.Hydrolases accelerate hydrolysis under Cd stress,releasing nutrients and facilitating microbial proliferation.Oxidoreductases respond by reducing the oxidative toxicity of Cd to microorganisms.Notably,the enzymes with the highest compensatory power are DHA and ALP.The involvement of C and P biogenic elements associated with the catalytic process of these enzymes may be critical for maintaining homeostasis in the soil.This finding offers a novel perspective for regulating stress resistance in soil systems.It is evident that differences in enzyme sensitivity reflect their ability to transmit stress signals to the system,while compensatory power indicates their capacity to mitigate stress-induced damage.The interplay between enzyme sensitivity and compensatory force is crucial for maintaining system homeostasis.