Pollution characteristics of microplastics in the wild tropical sea cucumber Holothuria atra (Echinodermata: Holothuroidea) and its habitat at different seasons

Microplastics (MPs), as a long-term micropollutant source, are widely present in the marine environment, necessitating investigations into their temporal variations in representative organisms and surrounding ecosystems. Here, we used the tropical sea cucumber Holothuria atra as a model organism and conducted a four-season MPs sampling survey from summer 2019 to spring 2020 in the coral reef area of Yunlong Bay, China. Results showed a 100 % detection rate of MPs across all samples, with an initial decrease followed by an increase in MPs abundance in both environmental and tissue samples over the investigation period. MPs abundance in sediments ranged from 404 to 548 particles/kg (dry weight), while surface seawater exhibited 56 to 89 particles/L across seasons. In H. atra, MPs abundance ranged from 7.95 to 13.97 particles/g in the gut and 3.63-7.27 particles/g in the respiratory tree. The elevated MPs abundance in the environmental samples during summer 2019 may be attributed to the extreme weather events. The composition characteristics of MPs indicated that tourism and fishing activity were the primary sources of MPs in the surveyed area. Transparent MPs dominated across all seasons, with a seasonal reduction in color variety attributed to plastic weathering and fading. After standardizing measurement units, the abundance of MPs from the tissue of sea cucumbers was significantly higher than that in the surrounding environment, indicating MPs were enriched within the sea cucumbers. Furthermore, the high correlation between MP characteristics in H. atra tissues and those in the environment further suggested that H. atra could serve as a reliable biological indicator for monitoring MPs in marine reef ecosystems.