Accumulation mechanisms of radiocaesium within lichen thallus tissues determined by means of microscale localisation observation

Dohi, Terumi   ; Iijima, Kazuki  ; Machida, Masahiko  ; Suno, Hiroya*; Omura, Yoshihito*; Fujiwara, Kenso ; Kimura, Shigeru*; Kanno, Futoshi*

Many lichens are well known to accumulate radiocaesium and, thus acting as biomonitors of contamination levels. Here, we use autoradiography and demonstrate for the first time microscale localisation of radiocaesium within thallus tissues to investigate the radiocaesium forms and their accumulation mechanism. The radiocaesium was localised in the brown pigmented parts i.e., melanin-like substances, in the lower cortex of lichen thallus. Quantum chemical calculations showed that functional group of melanin-like substances can chelate Cs ion, which indicates that the Cs ions form complexes with the substances. Based on these findings, we suggest that radiocaesium ions may be retained stably in melanin-like substances for long periods (two to six years) due to steric factors, such as those seen in porphyrin-like structures and multimer formation in the lower cortex.