Plant and Soil226: 227–234, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Metal-binding capacity of arbuscular mycorrhizal mycelium

Erik J Joner1,∗, Roberto Briones2 & Corinne Leyval11Department of Biotechnological Sciences, Agricultural University of Norway, P.O. Box 5040, N-1432 Aas,Norway. Present address: Centre de Pedologie Biologique - CNRS, 17, rue N.D. des Pauvres, B.P. 5, F-54501Vandoeuvre-les-Nancy Cedex, France.2Centre de Pedologie Biologique – CNRS, 17, rue N.D. des Pauvres, B.P. 5,F-54501 Vandoeuvre-les-Nancy Cedex, France

Received 25 June 1999. Accepted in revised form 20 December 1999

Key words:Biomass, biosorption, heavy metals, fungi, rhizosphere, roots

Abstract

Experiments with excised mycelium of severalGlomusspp. with different histories of exposure to heavy metalswere carried out to measure their capacities to bind Cd and Zn. Cd sorption was followed for up to 6 h ofincubation to determine its time course relationships. Controls treated with a metabolic inhibitor were includedto evaluate whether sorption was due to active uptake or passive adsorption. The effect of ion competition (effectsof Ca or Zn on Cd sorption) and general measurements of cation exchange capacity (CEC) of roots and hyphaewere also performed. The results showed that AM mycelium has a high metal sorption capacity relative to othermicroorganisms, and a CEC comparable to other fungi. Metal sorption was rapid (<30 min) and appeared mainlyto be due to passive adsorption. Adsorption was highest in a metal-tolerantG. mosseaeisolate and intermediate fora fungus isolated from a soil treated with metal-contaminated sludge. The former adsorbed up to 0.5 mg Cd per mgdry biomass, which was three times the binding capacity of non-tolerant fungi, and more than 10 times higher thanreported values for, e.g., the commonly used biosorption organismRhizopus arrhizus.The implications of theseresults for AM involvement in plant protection against excess heavy metal uptake are discussed.

Abbreviations:AM – arbuscular mycorrhiza, CEC – cation exchange capacity, Ce – ion concentration in solutionat equilibrium, qe – ion quantity adsorbed at equilibrium.

Introduction

Arbuscular mycorrhizal (AM) fungi form ubiquitousroot–fungus symbioses that have the capacity to re-duce excess plant uptake of certain heavy metals(Leyval et al., 1997). The first interaction in this con-text is the contact between metal ions and AM hyphaein soil, and the processes that take place at the hyphalsurface are the first to influence the fate of any ion inquestion. A large range of micro-organisms have beenexamined for their capacity to fix metal ions, eitheras living or dead cells. Such biosorption phenomenahave been explored for cleaning industrial effluentsor recovering precious metals or radionuclides (Gadd,1990). Biosorption also has an implication for metal

∗ E-mail: joner@cpb.cnrs-nancy.fr

uptake in plants as the rhizosphere and the rhizoplaneare particularly densely inhabited by microorganismswhich may adsorb metals on their wall material or onextracellular slime (Denny and Ridge, 1995; Scot andPalmer, 1990).

The mechanisms underlying reduced metal uptakein AM plants (when observed) are largely unknown.Selective fungal transport that impairs non-essentialelements may be one such mechanism. Such selectivetransport may be regulated at different stages, namelyuptake, translocation or transfer. Reduced transfer, asindicated by enhanced root/shoot Cd ratios in AMplants, has been suggested as a barrier in metal trans-port (Joner and Leyval, 1997). This may occur, e.g.,due to intracellular precipitation of metallic cationswith PO4. Uptake into hyphae may be influenced byadsorption on hyphal walls as chitin has an important

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metal-binding capacity (Zhou, 1999). More indirecteffects of AM fungi on rhizosphere characteristics in-clude, e.g., changes in pH (Li et al., 1991), microbialcommunities (Olsson et al., 1998) and root exudationpatterns (Laheurte et al., 1990), factors that are knownto influence metal mobility or availability.

The objective of the present report was to obtainsome first indications as to whether the AM myceliumhave ion-binding capacities that could influence fungaltransport of metals in mycorrhizal plants, and if sorp-tion was affected by fungal metal tolerance to heavymetals.

Materials and methods

A series of experiments was carried out using excisedmycelium of one or moreGlomusspecies producedindividually in compartmented pots. The culture con-ditions and media in these cultures were modifiedduring the course of this study as the materials used inthe experiments withG. mosseaestrain ‘Gm’ were notavailable when the experiments with the other fungiwere prepared.

Hyphal production

Hyphal cultures in two-compartment pots were estab-lished with the fungiGlomus mosseaeGm (non-metaltolerant strain, BEG 12; exp 1–4),G. mosseaeP2(metal-tolerant strain from soil with a 60 -year his-tory of industrial metal pollution (Weissenhorn et al.,1993), BEG 69; exp 2),Glomussp. [German isol-ate from soil given 100 m3 ha−1 year−1 of metal-containing sludge for 10 years (see e.g. Chaudri etal., 1993), DAOM 225952; exp 2] orG. lamellosum(Finnish isolate, V43a, from non-contaminated soil,isolated by M. Vestberg; exp 2).Glomus mosseaeGm was grown on subterranean clover (Trifolium sub-terraneum, cv. Mount Barker) and the other fungion ryegrass (Lolium perenne, cv. Barclay). Seedsand surface sterilized (5% Chloramin T) spores wereplaced in 200 ml bags of nylon mesh (20µm) filledwith a 1:1:1 (v/v/v) mixture of expanded clay ag-gregates (Lecar), sand and pieces of mineral wool(Grodaniar) (experiments withG. mosseaeGm) ora 1:1 (v/v) mixture of attapulgite clay (Terragreenr;Oil-dri III R, Wisbech, Cambs, UK) and vermiculite(experiment with the other fungi). The mesh bagswere placed centrally in 1.5-l pots lined with plasticbags and the space outside the mesh bag was filled

with washed quartz sand to serve as a hyphal com-partment. All growth media were heat sterilized tokeep the presence of foreign microorganisms at a lowlevel. The root-free hyphal compartment was coveredwith non-transparent plastic to prevent air-borne con-tamination and algal growth. Plants were kept in agreenhouse (G. mosseaeGm) or a growth chamber(fungi exceptG. mosseaeGm). Both growth facilitiesprovided 16 h day−1of light at a minimum photon fluxdensity of 350µmol m−2 s−1 PAR, and a temperatureof 22–16◦C (day–night). A nutrient solution (1 mMCa(NO3)2 4H2O; 1 mM NH4NO3; 1.0 mM K2SO4;0.8 mM MgSO4� 7H2O; 70µM Na2HPO4 2H2O; 25µM Fe(III) NaEDTA; 25µM H3BO3; 5µM MnSO4�H2O; 2µM ZnSO4 7H2O; 0.5µM CuSO4 5H2O; 0.1µM Na2MoO4 2H2O; 4 nM CoCl2 6H2O) was sup-plied every second day to maintain the equivalent to60% of the water holding capacity of the media.

Hyphal extraction

Hyphae were extracted when plants were 6–12 weeksold. Prior to harvesting the hyphae, the sand in the hy-phal compartment had been renewed to obtain hyphaeof similar age, and hyphae were harvested with 7–14 day intervals. Extraction was done by flotation ina large tray, washing out remaining sand in a beakerand finally cleaning the mycelium under a dissect-ing microscope. Cleaned mycelium was divided intoapproximately equal portions for adsorption measure-ments. The process of hyphal extraction and cleaningtook 1–3 h and was carried out at room temperature.After incubation in metal ion solutions the myceliumwas washed onto a membrane filter (1.2µm, mixedester cellulose, Millipore), examined for purity undera dissecting microscope, peeled off using fine forceps,dried at 40◦C for 24 h and weighed.

Sorption studies

In exp. 1, the mycelium was incubated (20◦C) in a buf-fered (20 mM HEPES, pH 7.2) or unbuffered solutioncontaining 1 mg l−1 Cd as Cd(NO3)2 4H2O (this Cdsalt was used in all experiments). The time course ofCd sorption on AM mycelium ofG. mosseaeGm wasfollowed as affected by the presence/absence of bufferand presence/absence of a metabolic inhibitor (20 mMNaN3) by sampling the incubation solutions after 30min, 1, 2, 3 and 5 h, following centrifugation.

In exp. 2, Cd and Zn sorption by mycelia of fourand two AM fungi, respectively, was measured as a

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function of initial metal concentration in the incub-ation solution [0.1 (G. mosseaeGm. only); 1.0; 10and 100 mg Cd l−1, or 1.0, 10, 20 or 100 mg Znl−1, [Zn(NO3)2 unbuffered, 20◦C, 2.5 h incubation].1 mM NaNO3 was used as supporting electrolyte forall fungi exceptG. mosseaeGm. Results are expressedusing the equilibrium concentration (Ce) and adsorbedquantity (qe) of Cd and Zn after incubation.

In exp. 3 mycelium ofG. mosseaeGm was incub-ated in solutions containing 10 mg Cd l−1 (0.089 mM)in the presence or absence of 5 mM Ca(NO3)2 4H2Oor 10 mM ZnSO4 7H2O.

In exp. 4, non-mycorrhizal roots, mycorrhizalroots or extraradical hyphae (both the latter:G.mosseaeGm) were incubated in solutions with initialCd concentrations of 1, 10, 100 and 1000 mg Cd l−1

(unbuffered, 20◦C, 3h incubation).All experiments had three replicates, and exp. 2

was repeated once for the three latter fungi, poolingthe data.

Hyphal lengths were measured according to thegrid-line intersect method (Hanssen et al., 1974) asmodified by Abbott et al. (1984), and a hyphallength/biomass relationship calculated after countingtrypan blue-stained hyphae from three 1-ml aliquotsof G. mosseaeGm mycelium blended in a Waringblender (250 ml, 2×30 s.), filtering, drying and weigh-ing remaining hyphae as above. Hyphal diameter wasmeasured on>200 randomly selected non-squashedhyphae ofG. mosseaeGm andG. lamellosumusinga root two-progression dot reticule at×200 magni-fication, calculating hyphal surface for the individualhyphal diameter classes.

Cation exchange capacity was measured on young(<14 day) non-mycorrhizal roots of ryegrass grownin sand, AM fungal hyphae as described for exp. 2,and washed, dried and ground biomass ofRhizopusarrhizus DSM 905, cultivated without excessive ex-posure to metals according to Deneux-Mustin et al.(1994). Roots and hyphae were dried at 45◦C for 48 hand hyphae were ground in an agate mortar with liquidN2. CEC was quantified by potentiometric titration(Deneux-Mustin et al., 1994; Gran, 1952) (n=1). Cdand Zn concentrations were measured by inductivelycoupled plasma atomic emission spectrography.

Differences in adsorption to fungi and roots ex-posed to increasing concentrations of metals weretested with Tukey’s test after multiple comparisonsamong simple linear regression slopes atP=0.05(Zar, 1984). Differences in Cd sorption in the pres-ence/absence of NaN3, buffer or competing ions was

Figure 1. Time course of Cd sorption to extraradical hyphae ofGlomus mosseaeGm incubated in a solution containing 10 mg Cdl−1 and either 20 mM Hepes buffer, pH 7.2, or 20 mM NaN3.Bars represent standard error of the mean (SEM,n=3) and asterisksignificant difference (P<0.05, Tukey’s test) within incubation time.

compared by single factor ANOVA and Tukey’s testfor multiple comparisons.

Results

Experiment 1

Cd sorption to AM hyphae was fast and maximumsorbed Cd was achieved already at the first sampling(30 min) with no additional sorption during 6 h in-cubation (Figure 1). This time-course was similar forhyphae incubated in buffered and unbuffered solu-tions, as well as for metabolically inactivated hyphae.Addition of NaN3 to the incubation solution resultedin a pH increase of 0.5–1.0 units, and the Hepes bufferincreased pH by 0.8–1.3 units after 6 h incubation, re-lative to solutions only containing Cd(NO3)2 (resultsnot shown). Hyphae adsorbed similar amounts of Cdfrom buffered and unbuffered solutions, but approx.50% more Cd in the presence of NaN3 (Figure 1).

Experiment 2

Cd sorption as affected by solution Cd concentrationincreased from 1.5 mg g−1 at an equilibrium concen-tration (Ce) of 0.007 mg l−1 (initial Cd conc., 0.1 mgl−1, G. mosseaeGm; thus 93% sorbed to hyphae) to475 mg g−1 at aCe of 91.27 mg l−1 (G. mosseaeP2).The sorption was 3-5 times higher forG. mosseaeP2compared to the other fungi (Figure 2). Zn sorptionranged from 5.6 mg g−1 at a Ce of 0.12 mg l−1 –76 mg g−1 (Figure 3), thus generally showing lower

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Figure 2. Cd sorption (quantity at equilibrium;qe) to extraradicalhyphae of fourGlomusspp. at initial solution concentrations from0.1 to 100 mg Cd l−1 at resulting equilibrium concentrations (Ce).When larger than the symbol, bars represent SEM (n=3), and capitalletters fungi for which calculated regression curves are significantlydifferent regarding slope and elevation (P<0.05, Tukey’s test).

Figure 3. Zn sorption (qe) to extraradical hyphae of twoGlomusspp. at initial solution concentrations from 1 to 100 mg Zn l−1 atresultingCe. Symbols as in Figure 2.

Figure 4. Sorption of Cd to extraradical hyphae ofGlomus mosseaeGm incubated in a solution containing 10 mg Cd l−1 (0.089 mM)in the presence or absence of Ca (5 mM Ca(NO3)2 4H2O) or Zn(10 mM ZnSO4 7H2O). Bars represent SEM and different letterssignificant difference (P<0.05, Tukey’s test).

sorption than for Cd even if compared on a molarbasis.

Experiment 3

The presence of the competing cations Ca and Zn atconcentrations 56 and 112 times higher, respectively,than that of Cd decreased Cd sorption by 87 and 92%(Figure 4). Mean Zn sorption in the latter case was68 mg g−1 or 24 times higher than the simultaneousCd sorption (results not shown).

Experiment 4

Cd sorption to mycorrhizal roots (G. mosseaeGm)and non-mycorrhizal roots was not significantly differ-ent, but root sorption was inferior to hyphal sorptionat all tested Cd concentrations (Table 1). Heat-killedroots (100◦C, 1 min) displayed an approx. 50% higherCd sorption capacity (results not shown), an increasecorresponding to that of NaN3-treated hyphae.

For the sorption measurements in general, eachharvest gave 1–10 mg dry weight of AM myceliumper pot. A ratio of hyphal length to biomass of 46m mg−1 dry weight was estimated using mycelia fromG. mosseaeGm. Hyphal surface area was calculatedto be 0.566 and 0.591 m2 g−1 for G. mosseaeGm andG. lamellosum, respectively.

Measurements of CEC of fresh and dried roots andfresh and dried/ground fungal biomass are presented inTable 2. Fungi had approx. 10 times higher CEC thanroots, with small differences between fungi. Dryingenhanced CEC of roots considerably, while it led to asmall reduction for hyphae ofG. lamellosum.

Discussion

The rapid initial biosorption of Cd (≤30 min) withno significant additional sorption during the following5.5 h indicate that surface adsorption (and related pro-cesses like ion-exchange, complexation, precipitationand crystallisation on and within the multilaminate,microfibrillar cell wall) on AM hyphae was the dom-inating mechanism behind removal of Cd from solu-tion. A similar time-course of biosorption has beendemonstrated with a range of metals using liveStrep-tomyces noursei(Mattuschka and Straube, 1993) andBacillus subtilis(Cotoras et al., 1992), whereas liv-ing mycelia of R. arrhizusand Trichoderma viridereached Cd, Zn and Cu sorption equilibria after>3h (Morley and Gadd, 1995). Non-metabolic binding

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Table 1. Cd sorption to non-mycorrhizal (NM) clover roots, mycorrhizal (Myc) rootscolonized byG. MosseaeGm and extraradical hyphae ofG. mosseaeGm as influencedby initial Cd concentration in incubation solution. SEM, standard error of mean (n=3).

Cd in solution NM roots Myc roots Hyphae

(mg 11) mg g−1 SEM mg g−1 SEM mg g−1 SEM

1 5.8 1.0 3.8 0.3 22 1.6

10 18 1.3 19 1.6 42 3.7

100 51 8.9 34 2.9 114 5.7

1000 138 54 111 26 816 465

Regression slope∗ a a b

Regression elevation a a b

∗Different letters indicate significant difference (P<0.05) according to Turkey’smultiple comparison test.

Table 2. Cation exchange capacity (CEC) of rootsand fungal biomass (n=1).

Species Form CEC

(µmol g−1)

T. subterraneum Fresh 71

T. subterraneum Dried 307

Glomussp. Fresh 2538

G. lamellosum Fresh 2334

G. lamellosum Dried/ground 2198

R. arrhizus Dried/ground 2280

of metals to cell walls is commonly rapid comparedto metabolism-dependent absorption, and the formerprocess is quantitatively the most important (Gadd,1990). Future experiments should employ consider-ably shorter time spans to determine the metal sorptiondynamics of AM hyphae. The relative contribution ofadsorption and absorption could further be addressedby including desorption treatments after sampling overlonger time spans, and compare excised mycelium toa functional symbiotic mycelium, as the excision mayinfluence absorption.

The amounts of Cd sorbed on AM hyphae wereconsiderable and increased with solution Cd concen-tration. At the highest solution concentrations theamounts seem too high to obey a mono-layered Lang-muir adsorption model, and precipitation probablycontributed substantially. At comparable solution con-centrations, sorption to AM fungal biomass was farhigher than reported values for a range of soil microor-ganisms (2–6 mg Cd g−1 at pH 7–7.2 with 10 mg Cdl−1; Krantz-RŸlcker et al., 1996; Kurek et al., 1982;Mullen et al. 1992) andR. arrhizus, (5.5 mg Cd g−1

biomass at pH 6.5 with 112 mg Cd l−1; Morley andGadd, 1995), a commonly used fungus in biosorp-tion schemes. The specific surface area of bacteriaand evenR. arrhizus(1.7 m2 g−1; Morley and Gadd1995) are far higher than that ofG. mosseae(0.5–0.6 m2 g−1), a fact that points to AM mycelium asan unusually strong Cd biosorbent. The measured Znsorption was also higher than biosorption values repor-ted in the literature (1.6 mg Zn g−1 at 65 mg Zn l−1

for S. noursei; Mattuscka and Straube, 1993; 20 mgZn g−1 at 65 mg Zn l−1 for R. arrhizus; Tobin et al.,1984; 14 mg Zn g−1 at 20 mg Zn l−1 for R. arrhizus;Zhou, 1999).

Dead microorganisms (Aspergillus, ParacoccusandSerratiaspp.) have previously been shown to ad-sorb more metals (Cd, Pb and Cu) than correspondingliving cells (Kapoor and Viraraghavan, 1998; Kureket al., 1982), a phenomenon that has been ascribedto cell surface modification during killing with chem-ical agents (Huang and Huang, 1996). Though thechemical agents used in the reports cited above didnot include NaN3, it seems likely that correspond-ing modifications may have taken place that can ac-count for the enhanced binding of Cd to NaN3 treatedhyphae in our experiment.

Apart from increasing the solubility of metals insoil, reduced pH generally decreases the rate and ex-tent of metal biosorption (Gadd 1990). We did notobserve this relationship for Cd with pH differencesof one unit between pH 6 and 7. This result is basedon a comparison of a buffered and an unbuffered treat-ment, and it is likely that the concentration of free Cdions was reduced by complexation with phosphate inthe buffer (Tobin et al., 1984). This would inhibit orretard Cd sorption and act in the opposite directionof the anticipated effect of an increased pH. Testing

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metal sorption using solutions adjusted to different pHwithout the interference of a buffer thus seems to bethe appropriate way to obtain a pH response curvefor sorption of Cd or other metals. Chelator-bufferedsystems, used for testing metal toxicity to microorgan-isms and plants (see, e.g., Angle and Chaney, 1989;Ibekwe et al., 1996), offer little advantage in thiscontext. Such buffers were designed to ensure con-stant and defined concentrations of free metal ions ingrowth media with high concentrations of complex-ing organic and inorganic constituents. This techniqueis not useful for sorption studies by measurement ofmetals remaining in solution after sorption. The al-ternative, sorption measured after mineralization offungal biomass, would require 10–1000 times higherbiomass. Our measurements are carried out in a simplenon-complexing electrolyte during a short period oftime where no growth of the fungus takes place, mak-ing such buffers superfluous. Experiments to assessmetal sorption when the fungus is symbiotically con-nected may be approached with compartmented RiT-DNA transformed root cultures where AM hyphaeproliferate in a liquid medium (Maldonado-Mendozaand Harrison, 1999). Washing the hyphal compart-ment and replacing the complex medium [M medium(Bécard and Fortin, 1988) without sucrose] with ametal-containing electrolyte may permit short-term(hours) experiments. Longer experiments (days) con-cerning adsorption do not seem feasible. Transportand toxicity studies may however profit from chelatorbuffering.

Competition between Cd, Ca and Zn ions for ad-sorption sites on AM hyphae seemed to favour Cd overCa and Zn, as Cd sorption was proportionally higherthan for the competing ions relative to the solutionconcentration ratios. The Zn sorption was also lowcompared to Cd sorption in experiments where com-parable molar concentrations of Cd were employed(68 mg Zn g−1 at 10 mM Znversus816 mg Cd g−1

at 8.9 mM Cd; Table 1). If the binding sites on AMhyphae are ion specific, related to ionic radius, or otherphysical parameters remains to be elucidated in exper-iments with a wider range of cations. Some sorptionstudies have ranked Cd biosorption affinity (S. nour-sei, R. arrhizus; molar basis) slightly below that ofZn and attributed this to electrochemical properties ofthe metal ions (Mattuschka and Straube, 1993; To-bin et al., 1984) while others have found up to twotimes higher biosorption (R. arrhizus, T. viride andB. subtilis; molar basis) of Cd relative to Zn (Cotoraset al., 1992; Morley and Gadd, 1995). Adsorption to

model mineral surfaces like hydrous oxide gels showhigher adsorption of Zn compared to Cd (Kinniburghet al., 1976). All the cited differences between Cd andZn sorption were obtained without competing cationspresent and show relatively small differences relativeto our competition measurements. Until the nature andquantity of charged groups on the walls of AM hyphaerelative to other biosorption agents are known, onemay only speculate that since AM fungi have an es-sential role in plant Zn uptake (e.g., Thompson, 1996)they may have experienced an evolutionary selectionpressure against wall components that bind Zn andthus reduce its transport to plants.

The differences between roots and hyphae regard-ing Cd sorption expressed on a dry weight basis wererelatively small (2–4 times more for hyphae), andwould probably be higher for roots if calculated on asurface basis. Plant roots do, however, possess proper-ties that enhance their surface and ion binding capacityabove that predicted by a simple consideration of rootcircumflex. Intercellular spaces in the epidermis andcortex may comprise 10% of the root volume, andtheir large and negatively charged surfaces preferablybind divalent cations (Marschner, 1986). Differencesin the quantitative and qualitative aspects of chargedgroups available for ion adsorption on the surfacesof roots and hyphae are also likely to be differentand need to be characterized. One aspect of sucha characterization is the involvement of extracellularpolysaccharides which are found on a range of mi-croorganisms as well as on roots, a matrix that maybind large quantities of metals on negatively chargedgroups (Scot and Palmer, 1990).

Previously reported values for CEC of roots andfungal hyphae are in the range of 100–700µmol g−1

(Crooke, 1964) and 100–3000µmol g−1 (Marschneret al., 1998; McKnight et al., 1990), respectively. TheCEC values we measured for extraradical AM hyphaeare thus in accordance with previously reported val-ues for fungi. Unfortunately we have not been ableto measure CEC ofG. mosseaeP2, so we cannot sayif CEC is determining fungal Cd sorption or not. Fu-ture studies will show this, as well as the quantitativeimportance of surface-exposed functional groups withdifferent acidity.

Metal resistance in an organism may often be as-sociated with both enhanced internal sequestrationand decreased metal uptake (Gadd, 1990). Decreaseduptake may be regulated by efflux and decreased mem-brane transport or impermeability, but surface bindingon cell walls, pigments or excreted metabolites consti-

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tute an important mechanism in fungal tolerance andsurvival when exposed to high metal concentrations(Gadd, 1993). The AM fungus that clearly had thehighest metal sorption capacity in the present studywas a metal tolerant strain ofG. mosseae. It thus seemsthat one trait conferring metal tolerance in this fungusis its enhanced capacity of extracellular metal adsorp-tion. The only other fungus we used that had a historyof metal exposure wasGlomussp. This fungus did notaccumulate significantly more Cd or Zn relative to theother non-tolerant fungi, and it has not been checkedfor metal tolerance. The latter may give indications asto which exposure time is required for AM fungi todevelop this or other adaptive traits.

Microbial biosorption has a large potential in treat-ments of metal-containing effluents from mining in-dustry, electroplating, nuclear fuel processing plants,etc. Metal-polluted soil also contains an aqueous solu-tion, the soil solution, that contains metals in equilib-rium with adsorptive surfaces of the soil. Metals insoil are mainly adsorbed on the surface of mineralsand organic matter, as they have high adsorption ca-pacities and as their total surfaces are commonly farhigher than those of microbial cells. Microorganismsare however particularly numerous in the rhizosphereand the rhizoplane, and may play a quantitatively sig-nificant role in reducing excess metal uptake by plants.Mycorrhizal fungi are found both in the rhizosphereand beyond it, and they constitute the only group ofmicroorganisms that are capable of transporting min-eral elements from the soil solution to plants. Assuch, they may have a unique possibility to ‘filter’ions during uptake. Fungal adaptations that enhancesuch filtering, for instance extracellular adsorptionor intracellular fungal sequestration, may be import-ant mechanisms to reduce the exposure of herbivoresand man to toxic metals as such anthropogenic loadsincrease.

Acknowledgements

The authors wish to thank G. Belgy for ICP analyses.This work was financed by grants from the Norwe-gian Research Council to C.L and from CONACYT,Mexico, to R.B.

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