Bt Research 2025, Vol.16, No.5, 214-223 http://microbescipublisher.com/index.php/bt 216 3 Degradation of Bt Toxins in Soil Matrix 3.1 Effects of soil physical and chemical properties on the degradation of Bt toxins The soil environment is one of the main destination media of Bt toxin, and the physical and chemical properties of the soil itself significantly affect the degradation dynamics of Bt toxin. Soils with high clay content often have a stronger adsorption and fixation effect on Bt protein, causing its concentration in the soil solution to drop rapidly and are not easily degraded by microorganisms, thereby prolonging the half-life of Bt toxin. On the one hand, soil organic matter provides a rich adsorption site, which can combine Bt toxin to form macromolecular complexes to slow down its decomposition; on the other hand, high organic matter content usually means that soil microorganisms are active and enzyme content is high, which is conducive to the degradation of Bt protein in the long run. Soil pH affects the solubility and enzymatic degradation activity of Bt protein. In neutral and alkaline soils, Bt protein is more easily soluble and exposed and can be acted by microbial enzymes; while in acidic soils, Bt protein may precipitate or bind strongly to organic matter, and the degradation rate is reduced. It is reported that in acidic red soils around pH 6.0, Bt toxin degrades slowly than black soils at pH 7.5 (Huang, 2009). Soil moisture content and temperature jointly determine microbial activity and enzyme activity, thereby affecting the degradation of Bt protein. Generally speaking, moderate humidity is conducive to the decomposition of Bt protein, and excessive dryness or excessive waterlogging will inhibit the effect of microorganisms. 3.2 The role of soil microbial communities in the degradation process Soil microorganisms are the main body of Bt toxin biodegradation. Whether it is indigenous microorganisms or exogenous microbial communities, their composition and function will significantly affect the degradation efficiency of Bt protein. Li Yujie et al.'s research analyzed the changes in soil fungal communities after adding Bt toxin through high-throughput sequencing. It was found that the application of Bt toxin increased the abundance of groups in soil fungal communities that are involved in protein degradation and carbon-nitrogen circulation functions, suggesting that these microorganisms play a role in Bt protein degradation. Some bacteria and fungi with protease secretion are considered key functional bacteria for the degradation of Bt toxins. For example, actinomycetes in soil can secrete a broad-spectrum protease Pronase, which can proteolyze Bt toxin protease into small-molecular peptides. Zhang et al. (2020) extracted Bt Cry1Ac protein from cotton field soil and added common actinomycetes culture. They detected that Bt protein was degraded by Pronase enzyme to produce 6 peptide products, and some of the short peptides can be further utilized by soil bacteria or converted into inorganic nitrogen. 3.3 Soil enzymatic reaction and decomposition mechanism of Bt toxin The degradation process of Bt toxin in soil is essentially the result of the combined action of multiple enzymatic reactions. Extracellular enzymes (such as proteases) secreted by soil microorganisms are directly responsible for decomposing polymer Bt protein into small molecules, thereby reducing their biological activity and accelerating subsequent degradation. Studies have shown that changes in some enzyme activities in soil are closely related to the degradation of Bt protein. For example, increased protease and peptidase activity in soil is often accompanied by a decrease in Bt toxin content (Ge et al., 2023). When the Bt toxin enters the soil, it must be recognized by the protease and hydrolyzed. Neutral proteases, metalloproteases, and microbial secreted extracellular proteases may be involved (Padmaja et al., 2008). Regarding the decomposition mechanism of Bt toxin in soil, a large amount of evidence shows that it is not mainly through abiotic chemical degradation (such as hydrolysis and photolysis), but mainly depends on biological enzymatic effects. In pure water or inorganic media without microorganisms, the Bt protein remains basically stable and does not degrade on its own. If protease or soil extract is added, the concentration of Bt protein will be significantly reduced and the polypeptide fragment product will appear. 4 Bt Toxin Degradation in Water Environment 4.1 Effects of light and physical and chemical parameters of water on the stability of Bt toxins In natural water bodies, Bt toxins usually degrade faster than in soils, due to the effects of light and chemical
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