Uptake and depuration kinetics of microplastics with different polymer types and particle sizes in Japanese medaka (Oryzias latipes).

Microplastic (MP) pollution and the related impacts on aquatic species have drawn worldwide attention. However, knowledge of the kinetic profiles of MPs in fish remains fragmentary. In this study, we conducted exposure and depuration tests of the following fluorescent-labeled MPs: polyethylene (PE; sphere with 200 or 20 µm diameter) and polystyrene (PS; sphere with 20 or 2 µm diameter) using juvenile Japanese medaka (Oryzias latipes). The distribution and concentration of MPs in medaka were directly determined in-situ after tissue transparency. During the 14-day exposure, MPs was mainly detected in the gastrointestinal tract, while some MPs at the size of ≤ 20 µm were located in the area of the gills and head. The bioconcentration factor (BCF; L/kg) for MPs in medaka was estimated as 74.4 (200 µm PE), 25.7 (20 µm PE), 16.8 (20 µm PS), and 139.9 (2 µm PS). Within the first five days of depuration, MPs were exponentially eliminated from the fish body, but 2 µm PS-MPs could be still detected in the gastrointestinal tract at the end of the 10-day depuration phase. Our results suggest that MPs 2 µm in diameter may pose ecological risks to aquatic species due to their relatively higher BCF and the potential for long-term persistence in the body.

Bioaccumulation and reproductive effects of fluorescent microplastics in medaka fish.

This study was performed to elucidate the uptake and bioaccumulation of polystyrene microplastics (PS-MPs) in Japanese medaka (freshwater fish) and Java medaka (marine fish), and to assess its impacts on the survival, reproduction, and gene expression of Japanese medaka. Both species were exposed to 2-µm fluorescent PS-MPs (107 beads/L) for 3 weeks. The bioaccumulation factor of PS-MPs for Java medaka was calculated at about 4 × 102, higher than that for Japanese medaka (about 1 × 102). The exposure had no significant effects on the survival and reproduction of Japanese medaka. The mRNA sequencing analysis showed that the expression of a few genes involved in the cell adhesion, xenobiotic metabolic process, brain development, and other functions in medaka intestines significantly changed after exposure. These results suggest that virgin PS-MPs can potentially accumulate in medaka intestines, but has limited toxicity to Japanese medaka at the concentration up to 107 beads/L.