Interaction of phthalates with model microbial membranes. Regrettable vs informed substitution

Soil contamination with microplastic particles (MP) is an emerging environmental threat. The biodegradation of MP is facilitated by the formation of a soil plastisphere, i.e., the colonization of the MP surface by soil microorganisms, primarily bacteria and fungi. The toxicity of MP is largely due to the release of toxic plastic additives, including plasticizers. The plasticizers used in the largest quantities are o-phthalic acid diesters, primarily di(ethylhexyl) phthalate (DEHP). This compound is toxic, hence its replacement with other substances, primarily other phthalates. Replacing legacy chemicals with other derivatives raises a significant dilemma – whether it is regrettable or informed substitution, meaning whether the substitute is actually less toxic. Phthalates damage the plasma membranes of microorganisms, so in our studies, we used Langmuir lipid monolayers whose composition mimics the membranes of Gram-negative, Gram-positive bacteria, and fungi. We examined how the incorporation of DEHP and two of its typical substitutes, di(isodecyl) phthalate (DIDP) and di(tridecyl) phthalate (DTP), affected the physical properties of the model membranes. The study utilized the Langmuir technique, Brewster angle microscopy, and Grazing Incidence X-ray Diffraction. The study demonstrated that the phthalates used disrupted the mechanical properties of the membranes, their morphology, and crystalline structure. Phthalates had the weakest effect on models of Gram-negative bacterial membranes, while the strongest effect was on models of Gram-positive bacterial membranes. It was concluded that, from the perspective of the biodegradation of MP in soil, replacing DEHP with DIDP or DTP can be considered an example of regrettable substitution.