Influence of alkali elements on the accumulation of radiocesium by mushrooms

Spatial distribution of 137Cs concentrations in mushrooms (Boletus hiratsukae) and their relationship with soil exchangeable cation contents

The activity concentrations of radiocesium (¹³⁷Cs) in wild mushrooms are reported to vary according to species, genus or ecological types. In addition, the concentration fluctuates among the same species collected within the same area. Therefore, we investigated whether the ¹³⁷Cs concentration of wild mushrooms would be (1) spatially biased, or (2) influenced by the ¹³⁷Cs or exchangeable potassium concentrations in the soils below. We set two survey plots 300 m apart in a Himalayan cedar forest in Tsukuba, Japan, where ca. 30 kBq/m² of ¹³⁷Cs was deposited after the Fukushima Nuclear Power Plant accident. From these plots, we collected fruit-bodies (fungal structures for spore production) of co-occurring Boletus hiratsukae, a mycorrhizal species, as well as from the soil below. The mean ¹³⁷Cs concentrations in the fruit-bodies were significantly different between the two plots, but no difference was observed in the soil ¹³⁷Cs concentration between the plots. Significant spatial autocorrelation was observed in the ¹³⁷Cs concentration in the organic layer for both sites, but no significant spatial autocorrelation was observed in the ¹³⁷Cs of fruit-bodies. Therefore, the variation in the ¹³⁷Cs concentrations of co-occurred B. hiratsukae was not explained by spatial bias or radioactivity in the below soil. In contrast, the exchangeable potassium concentration in the soil was negatively correlated with the ¹³⁷Cs in the fruit-bodies. Our results suggest that the ¹³⁷Cs absorption of wild mushrooms would be suppressed by the competitive effect of exchangeable potassium in the surrounding soils.

Factors affecting mushroom Pleurotus spp

Pleurotus genus is one of most extensively studied white-rot fungi due to its exceptional ligninolytic properties. It is an edible mushroom and it also has several biological effects, as it contains important bioactive molecules. In basidiomycete fungi, lignocellulolytic enzymes are affected by many typical fermentation factors, such as medium composition, ratio of carbon to nitrogen, pH, temperature, air composition, etc. The survival and multiplication of mushrooms is related to a number of factors, which may act separately or have interactive effects among them. Out that understanding challenges in handling Pleurotus species mushroom requires a fundamental understanding of their physical, chemical, biological and enzymatic properties. This review presents a practical checklist of available intrinsic and extrinsic factors, providing useful synthetic information that may help different users. An in-depth understanding of the technical features is needed for an appropriate and efficient production of Pleurotus spp.

Influence of alkali and alkaline earth elements on the uptake of radionuclides by Pleurototus eryngii fruit bodies

In the literature, there are many data available on radionuclide contents and their transfer to different species of mushrooms. There are some variables, however, which affect the transfer but are very difficult to observe in collected wild mushrooms. An example is the effect of different concentrations of alkali and alkaline earth elements in the soil. Modification of these concentrations in the soil solution has traditionally been used as a countermeasure to deal with radioactively contaminated areas. In the present work, fruiting bodies of Pleurotus eryngii, a saprophytic mushroom, were grown under controlled laboratory conditions, varying the content of alkali (potassium and cæsium) and alkaline earth (calcium and strontium) elements. The transfer of (134)Cs, (85)Sr, and (60)Co (added to the cultures) and of natural (210)Pb was analysed by increasing the content of each stable element considered. A significant, but nonlinear, enhancement of stable cæsium and (134)Cs was observed with increasing content of stable cæsium in the substrate/mycelium. The transfer of (85)Sr decreased with the addition of each stable cation, whereas the (60)Co and (210)Pb transfers were unaffected.

Correlations between potassium, rubidium and cesium ((133)Cs and (137)Cs) in sporocarps of Suillus variegatus in a Swedish boreal forest

An analysis of sporocarps of ectomycorrhizal fungi Suillus variegatus assessed whether cesium ((133)Cs and (137)Cs) uptake was correlated with potassium (K) or rubidium (Rb) uptake. The question was whether intraspecific correlations of Rb, K and (133)Cs mass concentrations with (137)Cs activity concentrations in sporocarps were higher within, rather than among, different fungal species, and if genotypic origin of sporocarps within a population affected uptake and correlation. Sporocarps (n = 51) from a Swedish forest population affected by the fallout after the Chernobyl accident were studied. The concentrations were 31.9 ± 6.79 g kg(-1) for K (mean ± SD, dwt), 0.40 ± 0.09 g kg(-1) for Rb, 8.7 ± 4.36 mg kg(-1) for (133)Cs and 63.7 ± 24.2 kBq kg(-1) for (137)Cs. The mass concentrations of (133)Cs correlated with (137)Cs activity concentrations (r = 0.61). There was correlation between both (133)Cs concentrations (r = 0.75) and (137)Cs activity concentrations (r = 0.44) and Rb, but the (137)Cs/(133)Cs isotopic ratio negatively correlated with Rb concentration. Concentrations of K and Rb were weakly correlated (r = 0.51). The (133)Cs mass concentrations, (137)Cs activity concentrations and (137)Cs/(133)Cs isotopic ratios did not correlate with K concentrations. No differences between, within or, among genotypes in S. variegatus were found. This suggested the relationships between K, Rb, (133)Cs and (137)Cs in sporocarps of S. variegatus is similar to other fungal species.

Effect of potassium and phosphorus on the transport of radiocesium by arbuscular mycorrhizal fungi

Potassium, a chemical analogue of cesium, and phosphorus, an essential macronutrient transported by arbuscular mycorrhizal fungi (AMF), have been suggested to influence the transport of radiocesium by AMF. However, no study investigated the effects of increasing concentrations of both elements on the importance of this transport. Here, the arbuscular mycorrhizal-plant (AM-P) in vitro culture system associating Medicago truncatula plantlets with Glomus intraradices was used to evaluate this effect. Using three concentrations of K (0, 1, 10 mM) and two concentrations of P (30 and 3000 microM) added to a compartment only accessible to the AMF, we demonstrated that K and P individually and in combination significantly influenced radiocesium transport by AMF. Whilst increased concentration of K decreased the amount of radiocesium transported, the opposite was observed for P. Although the exact mechanisms involved need to be assessed, both elements were identified as important factors influencing the transport of radiocesium by AMF.

Accumulation of potassium, rubidium and caesium (133Cs and 137Cs in various fractions of soil and fungi in a Swedish forest

Radiocaesium ((137)Cs) was widely deposited over large areas of forest in Sweden as a result of the Chernobyl accident in 1986 and many people in Sweden eat wild fungi and game obtained from these contaminated forests. In terms of radioisotope accumulation in the food chain, it is well known that fungal sporocarps efficiently accumulate radiocaesium ((137)Cs), as well as the alkali metals potassium (K), rubidium (Rb) and caesium (Cs). The fungi then enhance uptake of these elements into host plants. This study compared the accumulation of these three alkali metals in bulk soil, rhizosphere, soil-root interface, fungal mycelium and sporocarps of mycorrhizal fungi in a Swedish forest. The soil-root interface was found to be distinctly enriched in K and Rb compared with the bulk soil. Potassium concentrations increased in the order: bulk soil<rhizosphere<fungal mycelium<soil-root interface<fungal sporocarps; and Rb concentration in the order: bulk soil<rhizosphere<soil-root interface<fungal mycelium<fungal sporocarps. Caesium was more or less evenly distributed within the bulk soil, rhizosphere and soil-root interface fractions, but was actively accumulated by fungi. Fungi showed a greater preference for Rb and K than Cs, so the uptake of (137)Cs could be prevented by providing additional Rb or K at contaminated sites. The levels of K, Rb, and Cs found in sporocarps were at least one order of magnitude higher than those in fungal mycelium. These results provide new insights into the use of transfer factors or concentration ratios. The final step, the transfer of alkali metals from fungal mycelium to sporocarps, raised some specific questions about possible mechanisms.

Pilot study of bioaccumulation and distribution of cesium, potassium, sodium and calcium in king oyster mushroom (Pleurotus eryngii) grown under controlled conditions

This pilot study presents preliminary results on interrelations between alkali and alkaline earth elements during their transfer to mycelium and fruitbodies of saprophytic fungi. The accumulation and distribution of four elements (cesium, potassium, sodium, and calcium) was evaluated in king oyster mushroom (Pleurotus eryngii) cultivated under controlled conditions. Elemental composition of caps, stipes, and the substrate was analyzed by atomic absorption/emission spectroscopy to evaluate discrimination, concentration, and transfer factors. The transfer factors determined for all the investigated elements were different and can be put in the following order: Cs > K > Na > Ca. There has been a higher accumulation of cesium in caps than in stipes. Distribution of cesium in fruitbodies depended on the presence of other ions in the substrate. The addition of Ca2+ limited the transport of cesium and potassium from stipes to caps. Sodium and calcium were mainly accumulated in the stipes. In a control experiment, without supplementation with K+, Na+, and Ca2+, approximately 62% of the cesium present in the substrate was extracted by mycelium and transported to the fruitbodies. Possible applications of fruiting saprophytic fungi in bioremediation are discussed.

Accumulation and localization of cesium in edible mushroom (Pleurotus ostreatus) mycelia

The characteristics of Cs accumulation and localization in edible mushrooms were examined using the mycelia of Pleurotus ostreatus-Y1. Scanning electron microscope images revealed the existence of white spots, and energy dispersive X-ray microanalyzer analysis indicated the presence of larger amounts of Cs and P in these spots in mycelia cultured on medium containing 25 mM CsCl. The (137)Cs activities in the mycelia were approximately 4-6 times higher than those in water used for (137)Cs elution. Higher Cs concentrations in the sediment fraction including vacuolar pellets were obtained compared to the upper fractions. It was observed that yellowish spots caused by the fluorescence of 4',6-diamidino-2-phenylindole (DAPI)-stained polyphosphate were localized in the mycelia. The higher fluorescence intensity of the yellowish-grained spots was measured in comparison with other regions in the mycelium. These results suggested that Cs in the mycelia was trapped by polyphosphate in vacuoles or other organelles.

Role and influence of mycorrhizal fungi on radiocesium accumulation by plants

This review summarizes current knowledge on the contribution of mycorrhizal fungi to radiocesium immobilization and plant accumulation. These root symbionts develop extended hyphae in soils and readily contribute to the soil-to-plant transfer of some nutrients. Available data show that ecto-mycorrhizal (ECM) fungi can accumulate high concentration of radiocesium in their extraradical phase while radiocesium uptake and accumulation by arbuscular mycorrhizal (AM) fungi is limited. Yet, both ECM and AM fungi can transport radiocesium to their host plants, but this transport is low. In addition, mycorrhizal fungi could thus either store radiocesium in their intraradical phase or limit its root-to-shoot translocation. The review discusses the impact of soil characteristics, and fungal and plant transporters on radiocesium uptake and accumulation in plants, as well as the potential role of mycorrhizal fungi in phytoremediation strategies.

Influence of the nutritional mechanism of fungi (mycorrhize/saprophyte) on the uptake of radionuclides by mycelium

Field studies have shown that the uptake of radionuclides by fungi depends on its nutritional mechanism (mycorrhizal or saprophytic), but this fact is only fully demonstrated for radiocaesium. To extend this conclusion to other radionuclides likely to be released in semi-natural ecosystems, we carried out a series of experiments under controlled laboratory conditions of the growth of mycelium of mycorrhizal and saprophytic fungi (Pleurotus eryngii and Hebeloma cylindrosporum respectively) on liquid culture media containing known added activities of 60Co, 85Sr, and 134Cs. The radionuclide incorporated most efficiently into Pleurotus eryngii was 134Cs, and into Hebeloma cylindrosporum was 60Co. For 134Cs and 85Sr, we also analysed the influence that the content of the chemically analogue elements K and Ca respectively in the culture medium had on its uptake. For both species, the uptake of 134Cs is not correlated with the content of K in the media, but it is the uptake of 85Sr increasing with it. These results seem to be independent of the nutritional mechanism. The influence of linearity in the uptake of 134Cs and 85Sr with increasing concentrations of Cs and Sr in the culture medium was also studied. For Pleurotus eryngii it was observed a decrease in the uptake of 134Cs with increasing Cs content in the medium, and an increase in the uptake of 85Sr with increasing Sr content in the medium.

Transport of radiocaesium by arbuscular mycorrhizal fungi to Medicago truncatula under in vitro conditions

The capacity of arbuscular mycorrhizal (AM) fungi to take up and translocate radiocaesium (Cs) to their host has been shown using the root-organ culture (ROC) system. However, the absence of photosynthetic tissues, lack of a normal root hormonal balance and incomplete source-sink relationships may bias the bidirectional transfer of elements at the symbiotic interface and complicate transport studies. Accordingly, we developed a novel culture system [i.e. the Arbuscular Mycorrhizal-Plant (AM-P) in vitro culture system], where AM fungi and an autotrophic host plant develop under strict in vitro conditions. With this system, we unambiguously demonstrated the capacity of AM fungi to transport Cs. The extraradical fungal hyphae took up 21.0% of the initial supply of 134Cs. Translocation to the plant represented 83.6% of the 134Cs taken up. Distribution of 134Cs in the host plant was 89.8% in the mycorrhizal roots and 10.2% in the shoot. These results confirm that AM fungi can take up, translocate and accumulate Cs. They further demonstrate unambiguously and for the first time that Cs can be transferred from AM fungi to host tissues. These results suggest a potential involvement of AM fungi in Cs biogeochemical cycle and in plant Cs accumulation.

Accumulation of radiocesium in wild mushrooms collected from a Japanese forest and cesium uptake by microorganisms isolated from the mushroom-growing soils

Mushrooms and soils samples collected from a sub-alpine forest of Mt. Fuji in Japan were measured for 137Cs and stable Cs. The ranges of 137Cs specific activities and stable Cs concentrations in the mushrooms were 291-7950 Bq kg(-1) dry weight and 4.69-58.1 mg kg(-1) dry weight, respectively. Both 137Cs specific activities and stable Cs concentrations in the mushrooms were higher than those in common agricultural plants. The 137Cs specific activities and stable Cs concentrations in the soils were 3.18-149 Bq kg(-1) dry weight and 0.618-2.18 mg kg(-1) dry weight, respectively. The appearance frequencies of filamentous actinomycetes and planktonic bacteria from the soils decreased according to increasing Cs contents in the medium. No relationship was observed between the appearance frequencies of those and the stable Cs concentrations in the soils. The filamentous actinomycetes from any soil sample could not grow in the presence of 25 mM Cs, although the planktonic bacteria from the soil samples could grow with up to 50 mM Cs in YM agar. In addition, the planktonic bacteria from approximately 70% of the soil samples could grow even in the presence of 100 mM Cs. Filamentous actinomycetes were more sensitive to Cs than planktonic bacteria. In in vitro experiments, Cs uptake by these strains of filamentous actinomycetes and planktonic bacteria was high in the presence of 5 mM CsCl and the strains accumulated Cs, the same as in mushrooms. Our results indicate that filamentous actinomycetes in the soils have higher sensitivity to Cs than planktonic bacteria, and several strains of filamentous actinomycetes have a high Cs accumulation in the presence of 5 mM Cs.