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Micro- and nanoscopic X-ray techniques were used to investigate the relationship between uranium (U) tissue distributions and adverse effects to the digestive tract of aquatic model organism Daphnia magna following uranium nanoparticle (UNP) exposure. X-ray absorption computed tomography measurements of intact daphnids exposed to sublethal concentrations of UNPs or a U reference solution (URef) showed adverse morphological changes to the midgut and the hepatic ceca. Histological analyses of exposed organisms revealed a high proportion of abnormal and irregularly shaped intestinal epithelial cells. Disruption of the hepatic ceca and midgut epithelial tissues implied digestive functions and intestinal barriers were compromised. Synchrotron-based micro X-ray fluorescence (XRF) elemental mapping identified U co-localized with morphological changes, with substantial accumulation of U in the lumen as well as in the epithelial tissues. Utilizing high-resolution nano-XRF, 400–1000 nm sized U particulates could be identified throughout the midgut and within hepatic ceca cells, coinciding with tissue damages. The results highlight disruption of intestinal function as an important mode of action of acute U toxicity in D. magna and that midgut epithelial cells as well as the hepatic ceca are key target organs.

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Abstract. Closed and semi-closed water bodies, such as lakes and ponds, are important water resources in Fukushima area and they are the most sensitive environments to radioactive contamination after the Fukushima Dai-ichi nuclear power plant accident. Wakiyama et al. (2017) investigated 137Cs activity in water and bottom sediment in four ponds; Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), within 10 km zone from the FDNPP during 2015–2016. This study follows up their observation to address longer time trends of 137Cs activity concentration in pond waters and to show speciation of 137Cs in soil and bottom sediment. Mean total 137Cs activity concentration in water ranged from 2.5 to 29 Bq L−1. There was not found steady trend in the activity concentration of total and particulate 137Cs for four ponds. The concentration of dissolved 137Cs was usually low in winter during the entire observation period in all four ponds. A tendency to a decrease in the 137Cs activity concentration in suspended sediments was found for four ponds and the decreasing rate constants, including radiological decay, on SU, FS, IZ and KM were 0.33, 0.53, 0.29 and 0.25 yr−1, respectively. The results of sequential extractions of soil and bottom sediment samples showed higher proportion of bioavailable 137Cs, i.e., exchangeable and organic bound 137Cs, in bottom sediment than in the soil.