Modulation of Hydrophobic Organic Contaminant Toxicity to Soil Protists Via Co-Exposure with Microplastics

With the increasing demand for disposable and low-cost goods, the use of plastics and thereby the amount of microplastics in the environment have been growing. Microplastics absorb contaminants, thus, as microplastics are transported through the environment they carry absorbed contaminants with them. The impact of this pollution displays itself all throughout the environment, affecting the various microorganisms of the soil ecosystem, including soil protists. Protists regulate the soil bacterial population through grazing and transport which in turn also modulates soil fertility and plant productivity. Soil protists are also a significant food source for larger microorganisms, and as hydrophobic contaminants are known to bioaccumulate up the food chain, these contaminants can ultimately reach higher order predators such as humans while beginning at soil protists. Here we show the effect of microplastics to modulate the toxicity of a model hydrophobic organic contaminant (HOC) to a model organism. We measure lethal as well as sub-lethal toxicity endpoints including protist growth rate and motility. Naphthalene was selected as a model HOC because its hydrophobicity (quantified as Log K_ow) is in the range of many common HOCs likely to sorb to microplastics under environmental conditions, also because naphthalene can be readily quantified in aqueous solution by optical methods, as well as its known toxicity to soil protists. Colpoda Steinii, a large soil ciliate, was selected as the model soil protist. The microplastics (MPs) selected are small spheres of polyethylene (size ranging between 35 and 45 micrometers), the most commonly used plastic in the world, being lightweight and versatile. The lethal endpoint of the protists when exposed to the contaminants was determined, by utilizing an aqueous solution of naphthalene as well as an aqueous solution of naphthalene treated with MPs. Replicate chambers were set up with equal number of viable protists, and then various concentrations of the two naphthalene solutions. The protists were left in these solutions for 24 hours, after which the fractions of living protists were determined, and plotted to measure the percentage of protists killed versus total naphthalene added (both MP-free and MP-containing) to demonstrate the effect of toxicity from co-exposure of HOC with microplastics. Going further, sub-lethal effects of lower naphthalene concentrations will be studied, such as the impact on growth rate and protist motility. Through modeling the impact of HOCs on the soil ecosystem, specifically on soil protists, we can better understand their detrimental effects on the environment, particularly on their conjunction with MPs, an environmental contaminant not yet fully understood. Thereby ultimately being able to predict and mitigate these effects on a larger environmental scale.