**Cesium (137Cs and 133Cs), Potassium and Rubidium in Macromycete Fungi and**  *Sphagnum* **Plants**

Mykhailo Vinichuk1, 3, Anders Dahlberg2 and Klas Rosén1 *1Department of Soil and Environment, Swedish University of Agricultural Sciences, 2Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences, 3Department of Ecology, Zhytomyr State Technological University, 1,2Sweden 3Ukraine* 

#### **1. Introduction**

278 Radioisotopes – Applications in Physical Sciences

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#### **1.1 Cesium (137Cs and 133Cs), potassium and rubidium in macromycete fungi**

Radiocesium (137Cs) released in the environment as result of nuclear weapons tests in the 1950s and 1960s, and later due to the Chernobyl accident in 1986, is still a critical fission product because of its long half-life of 30 years and its high fission yield. The study of the cesium radioisotope 137Cs is important, as production and emission rates are much higher than other radioisotopes. This chapter comprises results obtained in several experiments in Swedish forest ecosystems and aims to discuss the behavior of cesium isotopes (137Cs and 133Cs) and their counterparts potassium (K) and rubidium (Rb) in the "soil-fungi-plants transfer" system. The chapter consists of two parts: one mainly dealing with 137Cs, 133Cs, K and Rb in forest soil and macromycete fungi, and the other with the same isotopes in separate segments of *Sphagnum* plants.

The bioavailability of radionuclides controls the ultimate exposure of living organisms and the ambient environment to these contaminants. Consequently, conceptually and methodologically, the understanding of bioavailability of radionuclides is a key issue in the field of radioecology. Soil-fungi-plants transfer is the first step by which 137Cs enters food chains.

#### **1.1.1 The role of fungi in 137Cs transfer in the forest**

The availability of radionuclides (137Cs in particular) in soils of different ecosystems is to a large extent regulated by various vascular plants and fungal species. Thus, the behavior of 137Cs in forest ecosystems differs substantially from other ecosystems, foremost due to the abundance of fungal mycelia in soil, which contribute to the persistence of the Chernobyl radiocesium in the upper horizons of forest soils (Vinichuk & Johanson, 2003). Both saprotrophic and mycorrhizal fungi have key roles in nutrient and carbon cycling processes in forest soils. The mycelium of soil fungi has a central role in breaking down organic matter

Cesium (137Cs and 133Cs), Potassium

same forest (Vinichuk et al., 2010b).

Muramatsu, 1998).

and Rubidium in Macromycete Fungi and *Sphagnum* Plants 281

than in plants remains unclear (Kuwahara et al., 1998; Bystrzejewska-Piotrowska & Bazala, 2008). In addition to radiocesium, fungi effectively accumulate potassium (K), rubidium (Rb) and stable cesium (133Cs) (Gaso et al., 2000) and the concentrations of 137Cs, 133Cs and Rb in fungal sporocarps can be one order of magnitude higher than in plants growing in the

The chemical behavior of the alkali metals, K, Rb and 133Cs, can be expected to be similar to 137Cs, due to similarities in their physicochemical properties, e.g. valence and ion diameter (Enghag, 2000). Potassium is a macronutrient and an obligatory component of living cells, which depend on K+ uptake and K+ flux to grow and maintain life. In radioecology cesium is assumed to behave similarly to potassium. At the cellular level, K is accumulated within cells and is the most important ion for creating membrane potential and excitability. Myttenaere et al. (1993) summarize the relationship between radiocesium and K in forests and suggest the possible use of K as an analogue for predicting radiocesium behavior. Generally, 137Cs is positively associated with K concentration across plant species in an undisturbed forest ecosystem, which suggests 137Cs, stable 133Cs and K are assimilated in a similar way and the elements pass through the biological cycle together (Chao et al., 2008). Cs influx into cells and its use of K transporters is reviewed by White & Broadley (2000) and

Rubidium is another rarely studied alkali metal, which may be an essential trace element for organisms, including fungi. However, there is scarce information on the concentrations and distribution of Rb in fungi and its behavior in food webs originating in the forest. Rubidium is often used in studies on K uptake and appears to emulate K to a high degree (Marschner, 1995): both K and Rb have the same uptake kinetics and compete for transport along concentration gradients in different compartments of soil and organisms (Rodríguez-Navarro, 2000). The concentrations of K, Rb and 133Cs have been analyzed in fungal sporocarps (Baeza et al., 2005; Vinichuk et al., 2010b; 2011) and a relation between the uptake of Cs and K has been found (Bystrzejewska-Piotrowska & Bazal, 2008). Cesium uptake in fungi is affected by the presence of K and Rb and the presence of 133Cs (Gyuricza et al., 2010; Terada et al., 1998). Although in fungal sporocarps, the relationships between these alkali metals and 137Cs when taken up by fungi and their underlying mechanisms are insufficiently understood, as Cs does not always have high correlation with K and it is suggested there is an alternative pathway for Cs uptake into fungal cells (Yoshida &

The correlations between 137Cs and these alkali metals suggest the mechanism of fungal uptake of 133Cs and 137Cs is different from K and that Rb has an intermediate behavior between K and 133Cs (Yoshida & Muramatsu, 1998). However, this interpretation is based on a few sporocarp analyses from each species, and comprised different ectomycorrhizal and saprotrophic fungal species. Although fungal accumulation of 133Cs is reported as speciesdependent, there are few detailed studies of individual species (Gillet & Crout, 2000). The variation in 137Cs levels within the same genotype of fungal sporocarps can be as large as the

Another way to interpret and understand the uptake and relations between 137Cs, 133Cs, K and Rb in fungi is to use the isotopic (atom) ratio 137Cs/133Cs. Chemically, 133Cs and 137Cs are the same, but the atom abundance and isotopic disequilibrium differ. Among other factors, uptake of 133Cs and 137Cs by fungi depends on whether equilibrium between the two isotopes is achieved. An attainment of equilibrium between stable 133Cs and 137Cs in the

potassium transport in fungi is reviewed by Rodríguez-Navarro (2000).

variation among different genotypes (Dahlberg et al., 1997).

and in the uptake of nutrients from soil into plants via the formation of symbiotic mycorrhizal associations (Read & Perez-Moreno, 2003). The fungi facilitate nutrient uptake into the host plant, both as a consequence of the physical geometry of the mycelium and by the ability of the fungi to mobilize nutrients from organic substrates through the action of extracellular catabolic enzymes (Leake & Read, 1997). In addition to acquiring essential macronutrients, mycorrhizal fungi are efficient at taking-up and accumulating microelements (Smith & Read, 1997), this ability results in the accumulation of non-essential elements and radionuclides, particularly 137Cs and can have important consequences for the retention, mobility and availability of these elements in forest ecosystems (Steiner et al., 2002).

Although fungal biomass, in comparison to plant biomass, is relatively low in forest soil (Dighton et al., 1991; Tanesaka et al., 1993), many fungal species accumulate more 137Cs than vascular plants do and 137Cs activity concentrations in many fungi are 10 to 100 times higher than in plants (Rosén et al., 2011). Fungi (particularly sporocarps) accumulate 137Cs against a background of low 137Cs activity concentrations, thus, the contribution of fungi to 137Cs cycling in forest systems is substantial.

Fungi are important in radiocesium migration in nutrient poor and organic rich soils of forest systems (Rafferty et al., 1997). In organic matter, the presence of single strains of saprotrophic fungi considerably enhances the retention of Cs in organic systems (Parekh et al., 2008): ≈ 70% of the Cs spike is strongly (irreversibly) bound (remains non-extractable) compared to only ≈ 10% in abiotic (sterilized) systems. Fungal mycelium may act as a sink for radiocesium (Dighton et al., 1991; Olsen et al., 1990), as it contains 20–30% 137Cs in soil inventories, and as much as 40% of radiocesium can leached from irradiated samples compared to control samples (Guillitte et al., 1994). Mycelium in upper organic soil layers may contain up to 50% of the total 137Cs located within the upper 0-10 cm layers of Swedish and Ukrainian forest soils (Vinichuk & Johanson 2003). In terms of the total radiocesium within a forest ecosystem, fungal sporocarps contain a small part of activity and may only account for about 0.5 % (McGee et al., 2000) or even less − 0.01 to 0.1% (Nikolova et al., 1997) of the total radiocesium deposited within a forest ecosystem. However, these estimates are based on the assumption radionuclide concentration in fungal sporocarps is similar to that of the fungal parts of mycorrhizae (Nikolova et al., 1997). The activity concentration in sporocarps is probably higher than in the mycelium (Vinichuk & Johanson, 2003, 2004) and sporocarps constitute only about 1% of the total mycelia biomass in a forest ecosystem. Due to the high levels of 137Cs in sporocarps, their contribution to the internal dose in man may be high through consumption of edible mushrooms (Kalač, 2001). Consequently, the consumption of sporocarps of edible fungi (Skuterud et al., 1997) or of game animals that consumed large quantities of fungi with high 137Cs contents (Johanson & Bergström, 1994) represents an important pathway by which 137Cs enters the human food system.

The 137Cs activity concentration in edible fungi species has not decreased over the last 20 years (*Suillus variegatus*) or significantly increased (*Cantharellus* spp*.)* (Mascanzoni, 2009; Rosén et al., 2011).

#### **1.1.2 137Cs, 133Cs and alkali metals in fungi**

Although fungi are important for 137Cs uptake and migration in forest systems and since the Chernobyl accident, fungal species may contain high concentrations of radiocesium, the reasons and mechanisms for the magnitude higher concentration of radiocesium in fungi

and in the uptake of nutrients from soil into plants via the formation of symbiotic mycorrhizal associations (Read & Perez-Moreno, 2003). The fungi facilitate nutrient uptake into the host plant, both as a consequence of the physical geometry of the mycelium and by the ability of the fungi to mobilize nutrients from organic substrates through the action of extracellular catabolic enzymes (Leake & Read, 1997). In addition to acquiring essential macronutrients, mycorrhizal fungi are efficient at taking-up and accumulating microelements (Smith & Read, 1997), this ability results in the accumulation of non-essential elements and radionuclides, particularly 137Cs and can have important consequences for the retention, mobility and availability of these elements in forest ecosystems (Steiner et al.,

Although fungal biomass, in comparison to plant biomass, is relatively low in forest soil (Dighton et al., 1991; Tanesaka et al., 1993), many fungal species accumulate more 137Cs than vascular plants do and 137Cs activity concentrations in many fungi are 10 to 100 times higher than in plants (Rosén et al., 2011). Fungi (particularly sporocarps) accumulate 137Cs against a background of low 137Cs activity concentrations, thus, the contribution of fungi to 137Cs

Fungi are important in radiocesium migration in nutrient poor and organic rich soils of forest systems (Rafferty et al., 1997). In organic matter, the presence of single strains of saprotrophic fungi considerably enhances the retention of Cs in organic systems (Parekh et al., 2008): ≈ 70% of the Cs spike is strongly (irreversibly) bound (remains non-extractable) compared to only ≈ 10% in abiotic (sterilized) systems. Fungal mycelium may act as a sink for radiocesium (Dighton et al., 1991; Olsen et al., 1990), as it contains 20–30% 137Cs in soil inventories, and as much as 40% of radiocesium can leached from irradiated samples compared to control samples (Guillitte et al., 1994). Mycelium in upper organic soil layers may contain up to 50% of the total 137Cs located within the upper 0-10 cm layers of Swedish and Ukrainian forest soils (Vinichuk & Johanson 2003). In terms of the total radiocesium within a forest ecosystem, fungal sporocarps contain a small part of activity and may only account for about 0.5 % (McGee et al., 2000) or even less − 0.01 to 0.1% (Nikolova et al., 1997) of the total radiocesium deposited within a forest ecosystem. However, these estimates are based on the assumption radionuclide concentration in fungal sporocarps is similar to that of the fungal parts of mycorrhizae (Nikolova et al., 1997). The activity concentration in sporocarps is probably higher than in the mycelium (Vinichuk & Johanson, 2003, 2004) and sporocarps constitute only about 1% of the total mycelia biomass in a forest ecosystem. Due to the high levels of 137Cs in sporocarps, their contribution to the internal dose in man may be high through consumption of edible mushrooms (Kalač, 2001). Consequently, the consumption of sporocarps of edible fungi (Skuterud et al., 1997) or of game animals that consumed large quantities of fungi with high 137Cs contents (Johanson & Bergström, 1994)

represents an important pathway by which 137Cs enters the human food system.

The 137Cs activity concentration in edible fungi species has not decreased over the last 20 years (*Suillus variegatus*) or significantly increased (*Cantharellus* spp*.)* (Mascanzoni, 2009;

Although fungi are important for 137Cs uptake and migration in forest systems and since the Chernobyl accident, fungal species may contain high concentrations of radiocesium, the reasons and mechanisms for the magnitude higher concentration of radiocesium in fungi

2002).

Rosén et al., 2011).

**1.1.2 137Cs, 133Cs and alkali metals in fungi** 

cycling in forest systems is substantial.

than in plants remains unclear (Kuwahara et al., 1998; Bystrzejewska-Piotrowska & Bazala, 2008). In addition to radiocesium, fungi effectively accumulate potassium (K), rubidium (Rb) and stable cesium (133Cs) (Gaso et al., 2000) and the concentrations of 137Cs, 133Cs and Rb in fungal sporocarps can be one order of magnitude higher than in plants growing in the same forest (Vinichuk et al., 2010b).

The chemical behavior of the alkali metals, K, Rb and 133Cs, can be expected to be similar to 137Cs, due to similarities in their physicochemical properties, e.g. valence and ion diameter (Enghag, 2000). Potassium is a macronutrient and an obligatory component of living cells, which depend on K+ uptake and K+ flux to grow and maintain life. In radioecology cesium is assumed to behave similarly to potassium. At the cellular level, K is accumulated within cells and is the most important ion for creating membrane potential and excitability. Myttenaere et al. (1993) summarize the relationship between radiocesium and K in forests and suggest the possible use of K as an analogue for predicting radiocesium behavior.

Generally, 137Cs is positively associated with K concentration across plant species in an undisturbed forest ecosystem, which suggests 137Cs, stable 133Cs and K are assimilated in a similar way and the elements pass through the biological cycle together (Chao et al., 2008). Cs influx into cells and its use of K transporters is reviewed by White & Broadley (2000) and potassium transport in fungi is reviewed by Rodríguez-Navarro (2000).

Rubidium is another rarely studied alkali metal, which may be an essential trace element for organisms, including fungi. However, there is scarce information on the concentrations and distribution of Rb in fungi and its behavior in food webs originating in the forest. Rubidium is often used in studies on K uptake and appears to emulate K to a high degree (Marschner, 1995): both K and Rb have the same uptake kinetics and compete for transport along concentration gradients in different compartments of soil and organisms (Rodríguez-Navarro, 2000). The concentrations of K, Rb and 133Cs have been analyzed in fungal sporocarps (Baeza et al., 2005; Vinichuk et al., 2010b; 2011) and a relation between the uptake of Cs and K has been found (Bystrzejewska-Piotrowska & Bazal, 2008). Cesium uptake in fungi is affected by the presence of K and Rb and the presence of 133Cs (Gyuricza et al., 2010; Terada et al., 1998). Although in fungal sporocarps, the relationships between these alkali metals and 137Cs when taken up by fungi and their underlying mechanisms are insufficiently understood, as Cs does not always have high correlation with K and it is suggested there is an alternative pathway for Cs uptake into fungal cells (Yoshida & Muramatsu, 1998).

The correlations between 137Cs and these alkali metals suggest the mechanism of fungal uptake of 133Cs and 137Cs is different from K and that Rb has an intermediate behavior between K and 133Cs (Yoshida & Muramatsu, 1998). However, this interpretation is based on a few sporocarp analyses from each species, and comprised different ectomycorrhizal and saprotrophic fungal species. Although fungal accumulation of 133Cs is reported as speciesdependent, there are few detailed studies of individual species (Gillet & Crout, 2000). The variation in 137Cs levels within the same genotype of fungal sporocarps can be as large as the variation among different genotypes (Dahlberg et al., 1997).

Another way to interpret and understand the uptake and relations between 137Cs, 133Cs, K and Rb in fungi is to use the isotopic (atom) ratio 137Cs/133Cs. Chemically, 133Cs and 137Cs are the same, but the atom abundance and isotopic disequilibrium differ. Among other factors, uptake of 133Cs and 137Cs by fungi depends on whether equilibrium between the two isotopes is achieved. An attainment of equilibrium between stable 133Cs and 137Cs in the

Cesium (137Cs and 133Cs), Potassium

**1.2.2 Study design** 

area and sampling see Dahlberg et al. (1997)**.**

concentrations of 133Cs, K and Rb were determined.

by Dahlberg et al. (1997), were used.

**1.2.3 Methods** 

Rb and 133Cs.

and Rubidium in Macromycete Fungi and *Sphagnum* Plants 283

Sporocarps of ectomycorrhizal fungi *Suillus variegatus* was studied in an area located about 40 km north-west of Uppsala in central Sweden (N 60°08'; E 17°10'). The forest is located on moraine and is dominated by Scots pine (*Pinus sylvestris*) and Norway spruce (*Picea abies*), with inserts of deciduous trees, primarily birch (*Betula pendula* and *Betula pubescens*). The field layer consisted mainly of the dwarf shrubs bilberry (*Vaccinium myrtillus* L.), lingonberry (*Vaccinium vitis-idaea* L.) and heather *Calluna vulgaris* L.): for details about the

For studies of K, Rb and 133Cs concentrations in soil fractions and fungal compartments, samples of soil and fungal sporocarps were collected from 10 sampling plots during September to November 2003. Four replicate soil samples were taken, with a cylindrical steel tube with a diameter of 5.7 cm, from around and directly underneath the fungal sporocarps (an area of about 0.5 m2) and within each 10 m2 area to a depth of 10 cm. Soil cores were divided horizontally into two 5-cm thick layers. Sporocarps of 12 different fungal species were collected and identified to species level, and the 137Cs activity concentration in fresh material was determined. The sporocarps were dried at 35°C to constant weight and

A selection of dried sporocarps of *S. variegatus* (n*=*51), retained from a study by Dahlberg et al. (1997) on the relationship between 137Cs activity concentrations and genotype identification, was used. The sporocarps were collected once a week during sporocarp season (end of August through September) in 1994 and were taken from five sampling sites (100 to 1600 m2 in size) within an area of about 1 km2. Eight genotypes with 2 to 8 sporocarps each were tested (in total 32 sporocarps) and are referred to here as individual genotypes. Sporocarps within genotypes were spatially separated by up to 10-12m. All genotypes were used for the estimation of correlation coefficients, but only genotypes with at least four sporocarps were included in the alkali metal analysis. In addition, 19 individual sporocarps with unknown genotype (i.e. not tested for genotype identity) were included: these sporocarps consisted of both the same and different genotypes. The combined set of sporocarps refers to all sporocarps: for further details about the sampling and identification of genotypes see Dahlberg et al. (1997). The 137Cs activity concentration values corrected to sampling date and expressed as kBq kg−1 dry weight (DW) for each sporocarp, as reported

For the studies of K, Rb and 133Cs concentrations in soil fractions and fungal compartments, fungal mycelia were separated from the soil samples (30–50 g, 0–5 cm layer depth) under a dissection microscope (magnification X64) with forceps and by adding small amounts of distilled water to disperse the soil. The prepared fraction of mycelium (30−60 mg DW g–1 soil) was not identified to determine of the mycelia extracted from the soil samples and the sporocarps belonged to the same species, as it assumed a majority of the prepared mycelia belonged to the same species as the nearby sporocarps. The method for mycelium preparation is described in Vinichuk & Johanson (2003). Mycelium samples were dried at 35°C to constant weight for determination of K,

bioavailable fraction of soils within forest ecosystems is reported Karadeniz & Yaprak (2007) but in cultivated soils, equilibrium between fallout 137Cs and stable 133Cs among exchangeable, organic bound and strongly bound fractions has not reached, even though most 137Cs was deposited on the soils more than 20 years before (Tsukada, 2006).

The important roles fungi play in nutrient uptake in forest soils, in particular its role in 137Cs transfer between soil and fungi, requires better understanding of the mechanisms involved. Although transfer of radioactive cesium from soils to plants through fungi is well researched, there is still limited knowledge on natural stable 133Cs and other alkali metals (K and Rb) and the potential role as a predictor for radiocesium behavior, and less is known about the relationships between 133Cs and other alkali metals (K and Rb) during uptake by fungi.

To explore mechanisms governing the uptake of radionuclides (137Cs) data on uptake of stable isotopes of alkali metals (K, Rb, 133Cs) by fungal species, and the behavior of the three alkali metals K, Rb and 133Cs in bulk soil, fungal mycelium and sporocarps are required. Therefore, an attempt was made to quantify the uptake and distribution of the alkali metals in the soil–mycelium–sporocarp compartments and to study the relationships between K, Rb and 133Cs in the various transfer steps. Additionally, the sporocarps of ectomycorrhizal fungi *Suillus variegatus* were analyzed to determine whether i) Cs (133Cs and 137Cs) uptake was correlated with K uptake; ii) intraspecific correlation of these alkali metals and 137Cs activity concentrations in sporocarps was higher within, rather than among different fungal species; and, iii) the genotypic origin of sporocarps affected uptake and correlation.

Substantial research in this area has been conducted in Sweden after the fallout from nuclear weapons tests and the Chernobyl accident. Some results are published in a series of several articles in collaboration with Profs K.J. Johanson, H. Rydin and Dr. A. Taylor (Vinichuk et al., 2004; 2010a; 2010b; 2011).

This chapter aims to summarize the acquired knowledge from studies in Sweden and to place them in a larger context. The results are summarized and discussed and address the issues of K, Rb and 133Cs concentrations in soil fractions and fungal compartments (Section 1.3.); concentration ratios of K, Rb and 133Cs in soil fractions and fungi (Section 1.4); relationships between K, Rb and 133Cs in soil and fungi (Section 1.5); the isotopic (atom) ratios 137Cs/K, 137Cs/Rb and 137Cs/133Cs in fungal species (Section 1.6); K, Rb and Cs (137Cs and 133Cs) in sporocarps of a single species (Section 1.7); mechanisms of 137Cs and alkali metal uptake by fungi (Section 1.8); Cs (137Cs and 133Cs), K and Rb in *Sphagnum* plants (Section 2); distribution of Cs (137Cs and 133Cs), K and Rb within *Sphagnum* plants (Section 2.3); mass concentration and isotopic (atom) ratios between 137Cs, K, Rb and 133Cs in segments of *Sphagnum* plants (Section 2.4); relationships between 137Cs, K, Rb and 133Cs, in segments of *Sphagnum* plants (Section 2.5); mechanisms of 137Cs and alkali metal uptake by *Sphagnum* plants (Section 2.6); and conclusions from the Swedish studies (Section 3). Before presenting and discussing results a short description of study area, study design and methods used is presented (section 1.2).

#### **1.2 Study area, study design and methods for results presented 1.2.1 Study area**

The K, Rb and 133Cs concentrations in soil fractions and fungal compartments were studied in an area located in a forest ecosystem on the east coast of central Sweden (60°22′N, 18°13′E). The soil was a sandy or clayey till and the humus mainly occurred in the form of mull. A more detailed description of the study area is presented by Vinichuk et al. (2010b).

Sporocarps of ectomycorrhizal fungi *Suillus variegatus* was studied in an area located about 40 km north-west of Uppsala in central Sweden (N 60°08'; E 17°10'). The forest is located on moraine and is dominated by Scots pine (*Pinus sylvestris*) and Norway spruce (*Picea abies*), with inserts of deciduous trees, primarily birch (*Betula pendula* and *Betula pubescens*). The field layer consisted mainly of the dwarf shrubs bilberry (*Vaccinium myrtillus* L.), lingonberry (*Vaccinium vitis-idaea* L.) and heather *Calluna vulgaris* L.): for details about the area and sampling see Dahlberg et al. (1997)**.**
