Adsorption of the insecticidal protein of Bacillus thuringiensis subsp. kurstaki by soil minerals: Effects of organic acid ligands

The introduction of the toxin of Bacillus thuringiensis (Bt) into soil, as a result of the rapid increase in the planting of commercial Bt-transformed crops worldwide, may constitute a hazard to the soil ecosystem. The present investigation is concerned with the effect of low molecular-weight organic acid ligands (acetate, oxalate and citrate) on the adsorption of the toxin of B. thuringiensis subsp. kurstaki by kaolinite, montmorillonite, goethite, and silicon dioxide (SiO ₂) and its desorption from the surface of these minerals. At the same time, we measured the desorption of bound toxin by NaCl and phosphate buffer. Low concentrations (< 10 mmol L ^{− 1}) of organic acid anions inhibited toxin adsorption by kaolinite, goethite, and silicon dioxide, whereas high concentrations promoted adsorption. For montmorillonite, however, an increase in the concentration of oxalate or citrate inhibited the adsorption of toxin. Less than 15% of the adsorbed amount was desorbed by each of the three organic acid anions. A small proportion (< 11%) of the toxin was adsorbed via electrostatic forces and ligand exchange, and this percentage increased when organic acid ligands were present. Our results indicate that low molecular-weight organic acid ligands can markedly influence the adsorption of Bt toxin by soil minerals, and result in the binding of the toxin on the minerals getting looser.