Soil microorganisms determine the sorption of radionuclides within organic soil systems.

The potential of soil microorganisms to enhance the retention of (137)Cs and (85)Sr in organic systems was assessed in a series of experiments. A biologically active, 'mineral-free', organic material, produced under laboratory conditions from leaves, was used as the uptake matrix in all experiments to minimise potential interference from competing clay minerals. Biological uptake and release were differentiated from abiotic processes by comparing the sorption of radionuclides in sterilised organic material with sterile material inoculated with soil extracts or single fungal strains. Our results show conclusively that living components of soil systems are of primary importance in the uptake of radionuclides in organic material. The presence of soil microorganisms significantly enhanced the retention of Cs in organic systems and approximately 70% of the Cs spike was strongly (irreversibly) bound (remained non-extractable) in the presence of microorganisms compared to only approximately 10% in abiotic systems. Sorption of (85)Sr was not significantly influenced by the presence of soil microorganisms. A non-linear temperature response was observed for the retention in biotic systems with increased uptake at between 10 and 30 degrees C and lower retention at temperatures above or below the optimum range. The optimum temperatures for biological uptake were between 15 and 20 degrees C for Cs, and 25 and 30 degrees C for Sr. Our results indicate that single strains of soil and saprotrophic fungi make an important contribution to the sorption of Cs and Sr in organic systems, but can only account for part of the strong, irreversible binding observed in biotic systems. Single strains of soil fungi increased the amount of non-extractable (137)Cs (by approximately 30%) and (85)Sr (by approximately 20%) in the organic systems as compared to abiotic systems, but the major fraction of (137)Cs and (85)Sr sorbed in systems inoculated with saprotrophic fungi remained extractable.

Immunochemical characterization of the pyruvate dehydrogenase complex in adult Ascaris suum and its developing larvae.

Polyclonal antibody was prepared against the pyruvate dehydrogenase complex purified from adult Ascaris suum body wall muscle. The antibody reacted with the E2, X, alpha E1 and beta E1 subunits of the complex in immunoblots of mitochondrial supernatant fractions and homogenates of adult muscle. In addition, the same subunits were observed in immunoblots of homogenates of L3 and L4 ascarid larvae, suggesting that a similar enzyme complex was present in all developmental stages despite their marked differences in energy metabolism. The phosphorylated and dephosphorylated alpha E1 peptides migrated differently during sodium dodecylsulfate polyacrylamide gel electrophoresis and both forms of the enzyme were recognized by the antibody. These results and those obtained with ELISA suggest that both phosphorylated and dephosphorylated forms of the alpha E1 subunit react equally well with the antibody. In immunoblots of adult body wall muscle, the phosphorylated alpha E1 peptide predominated, while immunoblots of L3 larvae contained predominantly the dephosphorylated form. These results reflect the in vivo activity state of the pyruvate dehydrogenase complex in these two stages and suggest that this technique may be useful for determining the activity state of enzyme complex directly from immunoblots of homogenates A. suum and other helminths.

Radiocaesium activity concentrations in the fruit-bodies of macrofungi in Great Britain and an assessment of dietary intake habits.

Radiocaesium activity concentrations in the fruit-bodies of some species of macrofungi are higher than in many other foodstuffs. The consumption of fruit-bodies contributes significantly to radiocaesium intake of humans in some countries. In the United Kingdom, the collection of wild fungi has generally been considered to be of minor importance and there are few data on consumption rates or radiocaesium activity concentrations in most edible species. Samples of commonly eaten species in Great Britain have been collected to assess radiocaesium contamination levels and geographical variation. Concurrently, surveys of consumption habits were conducted. A total of 425 samples representing 37 different species were collected. Significantly higher radiocaesium activity concentrations occurred in mycorrhizal compared to saprotrophic or parasitic species. The highest 137Cs activity concentration of 30.5 kBq kg-1 dry wt. was determined in a sample of Hydnum repandum collected in Wales. The transfer of radiocaesium from soil to fungal fruit-bodies was highly variable, ranging over three orders of magnitude within individual species. A number of approaches to quantifying radiocaesium transfer from soil to fungal fruit-bodies were used. Although these were in general agreement with previously measured values in other countries, all the approaches gave variable results. Over 200 people responded to the dietary habits questionnaire. The median intake rate was 0.75 kg year-1 (fresh wt.) and 60% of respondents consumed only one species (generally Agaricus campestris). However, intakes of up to 26 kg year-1 were recorded and a total of 82 species were consumed. The intake of 137Cs was determined by the amount of mycorrhizal fungi in the diet rather than the total intake of fungi. Assuming median recorded 137Cs activity concentrations in each fungal species, the estimated annual committed effective dose for over 95% of respondents was < 1 microSv. Hence, currently, the consumption of wild fungi in the UK would not be expected to significantly increase the dose above that attributable to the normal diet of most consumers. However, the results of this study demonstrate that, in the event of any future accidental release of radiocaesium, the potential ingestion dose received from the consumption of wild fungi would need to be considered.

Modelling the role of humic acid in radiocaesium distribution in a British upland peat soil.

The significance of exchange sites on organic matter in the retention of radiocaesium in highly organic soils remains unclear. To quantify this retention, we measured the binding of 134Cs to a humic acid isolated from a British upland peat soil, under a range of chemical conditions. We interpreted our results using Humic Ion Binding Model V, a model of humic substance chemistry which simulates ion exchange by non-specific accumulation of cations adjacent to the humic molecules. Model V could simulate the humic acid-solution partitioning of Cs under all the solution conditions used. The model was used to estimate the contribution of organic matter to Cs sorption by the whole soil composite. An estimate of Cs sorption by illite frayed edge sites was also made. These simulations show that organic matter may play only a minor role in binding Cs. even in highly organic soils.