Microplastic Size-Dependent Toxicity, Oxidative Stress Induction, and p‑JNK and p‑p38 Activation in the Monogonont Rotifer (Brachionus koreanus) Chang-Bum Jeong, †,‡ Eun-Ji Won, †,§ Hye-Min Kang, † Min-Chul Lee, † Dae-Sik Hwang, † Un-Ki Hwang, ∥ Bingsheng Zhou, ⊥ Sami Souissi, # Su-Jae Lee, ∇ and Jae-Seong Lee* ,† † Department of Biological Science, College of Science, Sungkyunkwan University, Suwon 16419, South Korea ‡ Department of Chemistry, College of Natural Sciences, Hanyang University, Seoul 04763, South Korea § Marine Chemistry and Geochemistry Research Center, Korea Institute of Ocean Science and Technology, Ansan 15627, South Korea ∥ Marine Ecological Risk Assessment Center, West Sea Fisheries Research Institute, National Fisheries Research & Development Institute, Incheon 46083, South Korea ⊥ State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan 430072, China # Universite ́ de Lille, CNRS, Universite ́ Littoral Cote d’Opale, UMR 8187, LOG, Laboratoire d’Oce ́ anologie et de Ge ́ osciences, F 62930 Wimereux, France ∇ Department of Life Sciences, College of Natural Sciences, Hanyang University, Seoul 04763, South Korea * S Supporting Information ABSTRACT: In this study, we evaluated accumulation and adverse effects of ingestion of microplastics in the monogonont rotifer (Brachionus koreanus). The dependence of microplastic toxicity on particle size was investigated by measuring several in vivo end points and studying the ingestion and egestion using 0.05-, 0.5-, and 6-μm nonfunctionalized polystyrene microbeads. To identify the defense mechanisms activated in response to microplastic exposure, the activities of several antioxidant-related enzymes and the phosphorylation status of mitogen-activated protein kinases (MAPKs) were determined. Exposure to polystyrene microbeads of all sizes led to significant size- dependent effects, including reduced growth rate, reduced fecundity, decreased lifespan and longer reproduction time. Rotifers exposed to 6-μm fluorescently labeled microbeads exhibited almost no fluorescence after 24 h, while rotifers exposed to 0.05- and 0.5-μm fluorescently labeled microbeads displayed fluorescence until 48 h, suggesting that 6-μm microbeads are more effectively egested from B. koreanus than 0.05- or 0.5-μm microbeads. This observation provides a potential explanation for our findings that microbead toxicity was size- dependent and smaller microbeads were more toxic. In vitro tests revealed that antioxidant-related enzymes and MAPK signaling pathways were significantly activated in response to microplastic exposure in a size-dependent manner. ■ INTRODUCTION Since its first description in 1972, plastic pollution has become one of the most concerning environmental problems due to its detrimental effects on ecosystems. 1−3 In the marine environ- ment, one of the important problems is associated with degradation of plastic debris into smaller particles, which are difficult to collect and remove from the aquatic environment. Size degradation can mainly occur by weathering through biodegradation, thermooxidative degradation, thermal degrada- tion, hydrolysis, and photodegradation. 4 Microplastics (less than 5 mm) resulting from the degradation of larger plastic debris have been shown to constitute up to 60−80% of all marine garbage. 2,4−6 The deleterious effects of plastics on marine organisms are evidently increasing; the relatively small plastic debris are easier to be absorbed into biological processes due to their large surface/volume ratio. Also, the capacity to adsorb persistent organic pollutants allows their ingestion and the accumulation of plastics in the aquatic food web to threaten the ecosystem further. 7 However, little attention has been paid Received: March 25, 2016 Revised: June 12, 2016 Accepted: July 20, 2016 Published: July 20, 2016 Article pubs.acs.org/est © 2016 American Chemical Society 8849 DOI: 10.1021/acs.est.6b01441 Environ. Sci. Technol. 2016, 50, 8849−8857