Adsorption potential of microplastics for extracellular nucleic acids in natural and synthetic waters

Microplastics represent a potential pathway for the environmental transport of extracellular nucleic acids (eNAs), including genetically engineered constructs released from agroecosystems (e.g., antibiotic resistance genes and small interfering RNAs, siRNAs). Although concerns surrounding the environmental impacts of these constructs persist, their fate beyond the soil environment remains largely unexamined. To quantify how microplastics interact with eNAs during transport from terrestrial to aquatic compartments, we conducted adsorption experiments using pristine HDPE microspheres across freshwater, rainwater, and wastewater matrices, applying isotherm and kinetic models to describe observed behavior. eNAs readily adsorbed to microplastics in freshwater and rainwater, with capacities reaching ∼60 ng/mg microplastics in natural freshwater, but exhibited minimal measurable adsorption in wastewater. Adsorption capacity was generally lower in solutions with lower ionic strength, and for siRNAs (<20 ng/mg in natural freshwater). Most adsorption occurred rapidly, with >60% of total uptake occurring within the first 5 min in pristine synthetic freshwater, although initial rates were slowed by natural organic matter. Although eNA adsorption to microplastics was less than that previously estimated for soil components, rapid and measurable adsorption suggests that HDPE microplastics may potentially impact the persistence and transport of genetic material, posing an especially significant risk if bound eNAs remain available for transfer to environmental bacteria.