Bt Research 2025, Vol.16, No.5, 214-223 http://microbescipublisher.com/index.php/bt 217 factors in the open environment of water bodies. One of the most important factors is solar radiation. Bt toxin is very sensitive to ultraviolet rays, and UV-B in sunlight will destroy the molecular structure of Bt protein, causing its insecticidal activity to rapidly decrease. Field experiments show that the half-life of Bt preparation suspension exposed to summer sun often less than 48 hours; while the activity of Bt toxin can be maintained for longer under light-shielding conditions. Photodegradation is one of the main ways to inactivate Bt toxins in waters. In addition, physical and chemical parameters such as water temperature and pH will also affect the stability of Bt toxin. In high-temperature water bodies, Bt proteins may be denaturated and easily degraded by enzymes; under relatively high pH conditions (water alkalinity), Bt toxins are more likely to dissolve, but may also lose crystal structure stability due to excessive swelling (Zhou et al., 2015). 4.2 Contribution of aquatic microorganisms to the degradation of Bt toxins Although photodegradation plays a major role in the water environment, water microorganisms are also involved in the degradation process of Bt toxin. In waters or bottom sludge rich in organic matter, bacteria, fungi and other microorganisms can use Bt toxins as nitrogen or carbon sources to accelerate their decomposition. Studies have shown that some aquatic bacteria have catabolism of proteins, and they can gradually decompose Bt proteins that settle in the sediment (Helassa et al., 2011). Especially in eutrophied water or sewage environments, the number of microorganisms and strong enzyme activity will be more significant, and the biodegradation effect of Bt toxins will be more significant. Laboratory simulation studies found that the concentration of Bt toxin was significantly reduced after a few days when added to the culture system rich in organic bottom sludge, while the control sterile system had almost no changes. This shows that the water microbial community can degrade Bt protein to a certain extent. However, compared with soil, microorganisms in water may be slightly less effective in degrading Bt toxins. This is because most Bt proteins will settle to the bottom quickly, and can actually stay in the water column and be directly contacted by microorganisms for a shorter time. 4.3 The migration and degradation rules of Bt toxins in sediments After Bt toxin enters the water body, a large part of it will settle into the bottom sediment with particulate matter. This is because Bt protein is easily adsorbed on suspended silt and organic debris, and transfers to the bottom mud environment as these particles sink. Sediments thus become another important environmental medium for Bt toxins. In sediments, the behavior of Bt toxins is more similar to that in the soil: they are fixed by adsorption and sediment particles and organic matter, and gradually degraded and removed by benthic microorganisms. River and lake sediments are generally anaerobic or micro-oxygenic (Wang and Huang, 2025), and the rate of microorganisms decompose organic matter is slow, but the concentration of Bt toxins is usually not high. A study of streams near agricultural areas found that Bt corn field runoff may bring Cry1Ab toxic protein into stream sediments, but was measured at very low concentrations (grade ng) and did not accumulate continuously during subsequent monitoring (Saxena and Pushalkar, 2013). This shows that even if Bt toxin enters the sediments of water, it will not be infinitely enriched, but will be continuously degraded or diluted. The degradation of Bt toxins in the sediments mainly depends on the protease action of anaerobic microorganisms, but the protein degradation rate is relatively slow under anaerobic conditions. Therefore, Bt toxin may exist in the sediment for a little longer than in the water column. However, due to water flow and biological perturbation, the Bt protein in the sediment may be released again into the overlying water and continue to degrade (Figure 1) (Bangaru, 2025). 5 Degradation of Bt Toxins in Plant Residues and Organic Matter Environment 5.1 Release and degradation of Bt toxins in plant residues After the Bt genetically modified crops are harvested, a large number of crop residues will be left in the field, which contains a certain amount of Bt toxic protein. These Bt toxins will be released into the environment as the plant residues decompose. The initial concentration of Bt protein in plant residues depends on the crop variety and growth period, generally reaching the highest during the maturity and harvest period, and then gradually decreases during the residual decomposition process. Taking Bt rice as an example, all parts of the plant (stem and leaves, root systems) contain Cry toxins during the maturity period. The Bt protein in the rice root system and fallen leaves that are left after harvest will be slowly released into the soil. A long-term localization experiment
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