Evaluation of bioavailable ¹³⁷Cs transfer from forest litter to Scarabaeidae beetle (Protaetia orientalis) through a breeding experiment in Fukuhshima

by Jaeick Jo, Yumiko Ishii, Rie Saito, Asuka Tanaka, Seiji Hayashi

Following the Fukushima Daiichi Nuclear Power Plant accident in 2011, most of the released ¹³⁷Cs remained in the litter and surface soil of the adjacent forest floor. However, ¹³⁷Cs absorption by large soil invertebrates near this site has not been estimated. The aim of this study was to understand the role of soil macroinvertebrates in ¹³⁷Cs uptake from forest litter into forest ecosystems. Breeding experiments were conducted using scarab beetle larvae (Protaetia orientalis). Dissection experiments revealed that 85% of the total ¹³⁷Cs was concentrated in the digestive tract of larvae, while a low proportion was absorbed into the skin and muscle tissues. The ¹³⁷Cs absorption rate, indicating the transfer of ¹³⁷Cs from consumed litter to larval tissue, was low (0.39%). ¹³⁷Cs concentrations decreased to one-fourth from larva to imago, possibly due to excretion from the digestive tract and during eclosion. In the elimination experiment, biological half-lives were 0.26–0.64 and 0.11–0.47 days and 3.35–48.30 and 4.01–17.70 days for the digestive tract and muscle/skin tissues in the fast and slow components, respectively, corresponding to ¹³⁷Cs discharge from the gastrointestinal tract and physiological clearance. In the sequential extraction experiment, litter digestion by flower chafer larvae significantly reduced the bioavailable fraction of ¹³⁷Cs including water-soluble, exchangeable, oxidized, and organic forms, from 23.2% in litter to 17.7% in feces. Residual ¹³⁷Cs was not reduced by digestion, probably because it was fixed in soil clay. Our study on breeding experiments of the Scarabaeidae beetle confirmed the low bioavailability of ¹³⁷Cs in the litter in Fukushima. However, litter feeders may play an important role in transferring ¹³⁷Cs to higher trophic levels in the forest ecosystem by extracting the bioavailable fraction of the vast stock of ¹³⁷Cs on the forest floor.