Iodine distribution and volatilization in contrasting forms of forest humus during a laboratory incubation experiment.

Radionuclides 129I (t1/2 = 15.7 × 106 years) and 131I (t1/2 = 8.02 days) are both introduced into the environment as a result of nuclear human activities. Environmental transfer pathways and fluxes between and within ecosystems are essential information for risk assessment. In forest ecosystems, humus degradation over time could result in re-mobilization and then downward migration and/or volatilization of intercepted 129I. In order to estimate the scale of these processes, humus (mull and moder forms) sampled under deciduous and coniferous forests were spiked with 125I- (t1/2 = 59.4 days), as a surrogate for 129I, in order to study the evolution of its water-soluble and organic fractions as well as the volatilization rate during humus degradation at laboratory scale. To our knowledge, this is the first time that interactions between iodine and contrasting forms of forest humus have been investigated. The evolution of native stable iodine (127I) pools in unspiked humus was also studied. The nature of the humus' organic matter appears to be a factor that impacts on the proportions of water-soluble and organic fractions of iodine and on their evolution. Iodine-125 was mainly organically bound (fraction for mulls and moders: ∼54-59 and 41-49%, respectively) and no clear evolution was observed within the 4-month incubation period. A large decrease in 125I water-solubility occurred, being more marked for mull (from ∼14-32 to 3-7%) than for moder (from ∼21-37 to 7-19%) humus. By contrast, a significant fraction was not extractible (∼38-43%) and varied in inverse proportion to the water-soluble fraction, suggesting a stabilization of iodine in humus after wet deposit. The nature of the humus organic matter also impacted on 125I volatilization. Although of the same order of magnitude, the total volatilization of 125I was higher for moders (∼0.039-0.323%) than for mulls (∼0.015-0.023%) within the 4-month incubation period. Volatilization rates for mulls were correlated with the water-soluble fraction, implying that volatilization of 125I could occur from the humus solution. Our results showed that humus is thus a zone of iodine accumulation by association with organic matter and that potential losses by lixiviation are significantly more important compared to volatilization.

Assessing tritium internalisation in zebrafish early life stages: Importance of rapid isotopic exchange.

"It appeared that OBT content in organisms was low with an OBT/TFWT ratio of about 8% for both stages (24 hpf and 96 hpf)." Should be read as "It appeared that OBT content in organisms was low with an OBT/TFWT ratio of about 8% and 14% at 24 hpf and 96 hphf respectively".

Disruption of AtHAK/KT/KUP9 enhances plant cesium accumulation under low potassium supply.

Understanding the molecular mechanisms that underlie cesium (Cs+ ) transport in plants is important to limit the entry of its radioisotopes from contaminated areas into the food chain. The potentially toxic element Cs+ , which is not involved in any biological process, is chemically closed to the macronutrient potassium (K+ ). Among the multiple K+ carriers, the high-affinity K+ transporters family HAK/KT/KUP is thought to be relevant in mediating opportunistic Cs+ transport. Of the 13 KUP identified in A. thaliana, only HAK5, the major contributor to root K+ acquisition under low K+ supply, has been functionally demonstrated to be involved in Cs+ uptake in planta. In the present study, we showed that accumulation of Cs+ increased by up to 30% in two A. thaliana mutant lines lacking KUP9 and grown under low K+ supply. Since further experiments revealed that Cs+ release from contaminated plants to the external medium is proportionally lower in the two kup9 mutant alleles, we proposed that KUP9 disruption could impair Cs+ efflux. By contrast, K+ status in kup9 mutants is not affected, suggesting that KUP9 disruption does not alter substantially K+ transport in experimental conditions used. The putative primary role of KUP9 in plants is further discussed.

Assessing tritium internalisation in zebrafish early life stages: Importance of rapid isotopic exchange.

Tritium (3H) is mainly released into the environment in the form of tritiated water (HTO) by nuclear power plants and nuclear fuel reprocessing plants. To better understand how organisms may be affected by contamination to 3H it is essential to link observed effects to a correct estimation of absorbed dose rates. Due to quick isotopic exchanges between 3H and hydrogen, 3H measurement is difficult in small organisms such as zebrafish embryo, a model in ecotoxicological assay. This work aimed to optimise tritium measurement protocol to better characterise internalisation by early life stages of zebrafish. Zebrafish eggs were exposed at one HTO activity concentration of 1.22 × 105 Bq/mL. This activity was calculated to correspond to theoretical dose rates of 0.4 mGy/h, where some deleterious effects are expected on young fish. A protocol for the preparation of biological samples was adapted from the method classically used to segregate the different forms of tritium in organisms. To deal with very quick isotopic exchanges of 3H with hydrogen, the impacts of washing by non-tritiated water as well as the bias induced by absorbed tritium around organisms on the measured activity concentration were studied. We managed to develop protocols to perform total tritium and total organically bound tritium (OBT) activity concentrations measurements in zebrafish eggs and larvae. The measurement of these both forms allowed the calculation of tissue-free-water-tritium (TFWT). To better understand total tritium internalisation, a study of total tritium kinetics from 4 hpf (hour post-fertilization) to 168 hpf was performed. OBT and TFWT were also assessed to complete the total tritium internalisation kinetics. The internalisation is a rapid phenomenon reaching a steady-state within 24 h after the beginning of contamination for total tritium and TFWT, with concentration factors and TFWT/HTO close to unity. OBT formation seemed to be slower. It appeared that OBT content in organisms was low with an OBT/TFWT ratio of about 8% for both stages (24 hpf and 96 hpf). To verify absorbed dose rates at key developmental stages (24 hpf eggs and 96 hpf larvae), they were calculated from total tritium activity concentrations after exposure at 1.22 × 105 and 1.22 × 106 Bq/mL, as these two activity concentrations were used to assess effects of tritium in another part of the study. Dose rates calculated from total tritium activity concentrations measured in 24 hpf eggs and 96 hpf larvae were consistent with the nominal ones, which validates the robustness of the protocol developed in the present study.

Impact of drying and wetting cycles on 137Cs ageing in forest soils contaminated with different input forms.

Water and acetate extractable radiocesium (137Cs) concentrations were monitored for >400 days in soils that were amended with aqueous 137Cs or solid organic sources of 137Cs (plant litter or fragmented organic materials) and subjected to a series of wet-dry cycles. The soils were collected from broadleaf and cedar forests in Fukushima, Japan. In soils amended with aqueous 137Cs, the water extractable 137Cs fraction was very low (<1%) and decreased over time while it was below the detection limit in soils amended with solid organic sources of 137Cs. The acetate extractable 137Cs in soil amended with aqueous 137Cs also exhibited an exponential decrease over time (∼55%-30%) but, remained higher than in soils amended by solid organic sources of 137Cs which remained stable (ranging from 2% to 15%). These results collectively indicate that: (1) drying and wetting cycles have little impact on 137Cs availability, possibly due to the relatively short observation period; (2) 137Cs ageing (increased binding to soil) was apparent only when 137Cs was applied in the aqueous form; and (3) both the water and acetate-extractable 137Cs fractions were greater for aqueous amended than for solid organic amended soils. More acetate extractable 137Cs was observed in soils contaminated with broadleaf materials compared to their cedar counterparts, which may be linked to the nature of the organic material itself. For natural conditions, such kind of information is useful to improve our understanding of the evolution of 137Cs availability with time from different contamination sources.

Uranium transfer and accumulation in organs of Danio rerio after waterborne exposure alone or combined with diet-borne exposure.

Uranium (U) toxicity patterns for fish have been mainly determined under laboratory-controlled waterborne exposure conditions. Because fish can take up metals from water and diet under in situ exposure conditions, a waterborne U exposure experiment (20 µg L-1 , 20 d) was conducted in the laboratory to investigate transfer efficiency and target organ distribution in zebrafish Danio rerio compared with combined waterborne exposure (20 µg L-1 ) and diet-borne exposure (10.7 µg g-1 ). 233 Uranium was used as a specific U isotope tracer for diet-borne exposure. Bioaccumulation was examined in the gills, liver, kidneys, intestine, and gonads of D. rerio. Concentrations in the organs after waterborne exposure were approximately 500 ng g-1 fresh weight, except in the intestine (> 10 µg g-1 fresh wt) and the kidneys (200 ng g-1 fresh wt). No significant difference was observed between waterborne and diet-borne conditions. Trophic U transfer in organs was found but at a low level (< 10 ng g-1 fresh wt). Surprisingly, the intestine appeared to be the main target organ after both tested exposure modalities. The gonads (57% at 20 d) and the liver (41% at 20 d) showed the highest accumulated relative U burdens. Environ Toxicol Chem 2019;38:90-98. © 2018 SETAC.

The impact of radiocesium input forms on its extractability in Fukushima forest soils.

The effects of 137Cs deposit forms on its ageing in soil have not yet been reported. Soluble and Solid 137Cs input forms were mixed with the mineral soils collected under Fukushima's coniferous and broadleaf forests, incubated under controlled laboratory, and examined the evolution of 137Cs availability over time. Results show that the extracted 137Cs fraction with water was less than 1% for the soluble input form and below detection limit for the solid input forms. Likewise, with an acetate reagent, the extracted 137Cs fraction ranged from 46 to 56% for the soluble input and from 2 to 15% for the solid input, implying that the nature of the 137Cs contamination strongly influences its extractability and mobility in soil. Although the degradation of organic materials was apparent, its impact on the 137Cs extractability was found to be weak. Nevertheless, more Ac-available 137Cs was obtained from broadleaf organic material mixes than the coniferous counterparts, suggesting that the lignified nature of latter tend to retain more 137Cs. When extrapolated to a field context, more available 137Cs fraction may be expected from wet-derived contaminated forest soils than contaminated via solid-derived inputs. Such information could be helpful for radioecological management schemes in contaminated forest environments.