Kurt Haselwandter

Effect of heavy metal pollution on mycorrhizal colonization and function: physiological, ecological and applied aspects

Abstract High concentrations of heavy metals in soil have an adverse effect on micro-organisms and microbial processes. Among soil microorganisms, mycorrhizal fungi are the only ones providing a direct link between soil and roots, and can therefore be of great importance in heavy metal availability and toxicity to plants. This review discusses various aspects of the interactions between heavy metals and mycorrhizal fungi, including the effects of heavy metals on the occurrence of mycorrhizal fungi, heavy metal tolerance in these micro-organisms, and their effect on metal uptake and transfer to plants. Mechanisms involved in metal tolerance, uptake and accumulation by mycorrhizal hyphae and by endo- or ectomycorrhizae are covered. The possible use of mycorrhizal fungi as bioremediation agents in polluted soils or as bioindicators of pollution is also discussed.

The role of fungi in weathering

No rock at the Earths surface escapes weathering. This process is the primary source of all the essential elements for organisms, except nitrogen and carbon. Since the onset of terrestrial life, weathering has been accelerated under the influence of biota. The study of biological weathering started at the end of the 19th century. Although the role of bacteria (Eubacteria, Archaea) has attracted a lot of interest, until recently the role of fungi has largely been neglected. More recently, however, fungal weathering has become an increasingly important focus of biogeochemical research.

Red list plants: colonization by arbuscular mycorrhizal fungi and dark septate endophytes

Since information concerning the mycorrhization of endangered plants is of major importance for their potential re-establishment, we determined the mycorrhizal status of Serratula tinctoria (Asteraceae), Betonica officinalis (Lamiaceae), Drosera intermedia (Droseraceae) and Lycopodiella inundata (Lycopodiaceae), occurring at one of two wetland sites (fen meadow and peat bog), which differed in soil pH and available P levels. Root colonization by arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) was quantified. Colonization by AMF appeared to be more frequent in the fen meadow than in the peat bog, and depended on the host plant. Roots of S. tinctoria and B. officinalis were well colonized by AMF in the fen meadow (35–55% root length) and both arbuscules and vesicles were observed to occur in spring as well as in autumn. In the peat bog, L. inundata showed a low level of root colonization in spring, when vesicles were found frequently but no arbuscules. In roots of D. intermedia from the peat bog, arbuscules and vesicles were observed, but AMF colonization was lower than in L. inundata. In contrast, the amount of AMF spores extracted from soil at the peat bog site was higher than from the fen meadow soil. Spore numbers did not differ between spring and autumn in the fen meadow, but they were higher in spring than in autumn in the peat bog. Acaulospora laevis or A. colossica and Glomus etunicatum were identified amongst the AMF spores extracted from soil at the two sites. S. tinctoria and B. officinalis roots were also regularly colonized by DSE (18–40% root length), while L. inundata was only rarely colonized and D. intermedia did not seem to be colonized by DSE at all.

Arbuscular mycorrhizal fungi, an essential component of soil microflora in ecosystem restoration

A wide range of natural factors such as lightning-caused fires and geomorphic or palaeotectonic processes may affect the stability of natural ecosystems (Herrera et al. 1993). Additionally, human activities causing pollution of air, water and soil, and overuse of resources like grasslands or clear-cutting of forests have a strong impact on a wide range of ecosystems. They may become degraded to such an extent that spontaneous recovery is strongly limited, especially if the damaging agent is continuously present. In general, successful restoration requires the reconstruction of adequate biological, physico-chemical, hydrological and morphological conditions. Moreover, the presence of hazardous substances can necessitate chemical or bioremediated clean-up. A common reason for the failure of many restoration attempts is the neglect of the fact that the plant root systems are associated with a diverse community of active soil micro-organisms. It is well known that a functioning association between plants and rhizosphere micro-organisms can modify the substratum, facilitate plant establishment under hostile conditions, and counteract the stagnation of the succession.

Effect of the arbuscular mycorrhizal symbiosis upon uptake of cesium and other cations by plants

Abstract Pot experiments were set up to determine the species-specific uptake of cesium (Cs) by mycorrhizal (AM) and non-mycorrhizal (non-AM) plants. Using stable Cs and K application, side-effects of mineral fertilization (K) on AM development and uptake of Cs and the other cations Na, Ca and Mg were investigated. AM colonization by the fungus Glomus mosseae led to a significant decrease in shoot Cs content of Agrostis tenuis from the first (4 weeks) to the third harvest (8 weeks). With regard to the root system, statistically significant differences were observed from the first (4 weeks) to the second harvest (6 weeks). Supply of additional K produced a significant decrease in Cs uptake by both AM and non-AM plants over a 10-week period. In the case of AM plant shoots, K fertilization did not very effectively reduce Cs uptake by A. tenuis. Cs contents of fertilized AM roots were similar to non-AM controls. Potassium application resulted in an increase in K content and a slight reduction in Na and Mg contents of shoots and roots. Without K fertilization, the Na content of non-AM controls was significantly enhanced over AM shoots. Shoot and root Ca contents were generally higher without than with K addition. Negative side-effects of K fertilization as a countermeasure to Cs uptake were not observed in relation to AM development. The intensity of colonization by G. mosseae was not significantly depressed by K treatment. AM development in plants appeared to decrease Cs uptake, at least at moderate nutrient levels. It is possible that Cs is sequestered by AM extraradical fungal hyphae and consequently not transferred to the plant to the extent found in non-AM roots.

Seasonal fluctuations of airborne fungal allergens

A Burkard jet spore sampler was set up as a volumetric ‘viable’ spore trap at two alpine sites of lower (582/872 m) and two sites of higher altitude (1905/1960 m). A total of 46 fungal genera or groups were identified, and the seasonal fluctuations of eighteen of the most frequent fungi at low altitude plotted as a ‘spore calendar’. Total yearly colony counts at high altitudes were only 23·1% of those at low altitude. The dominant genera at the four sites indicate an ‘alpine type’ of airspora with Epicoccum Aureobasidium and Phoma predominating, with Alternaria and Botrytis of minor importance. Genera, such as Gonatobotrys, Spiniger, Syncephalastrum and Wallemia , which have never previously been detected in aerobiological surveys, were recorded. The results provide valuable information for diagnosis, therapy and prophylaxis of allergic diseases.

Indoor and outdoor incidence of airborne fungal allergens at low- and high-altitude alpine environments

Air samples of three indoor alpine environments at different altitudes were taken by means of volumetric sampling and compared with outdoor air at the same altitude for viable fungal spores. At low altitude (582 m) closed rooms usually provided lower concentrations of potential fungal allergens than outdoor environments. By contrast buildings at a higher altitude (1905 m) showed more than double the concentration of fungal spores than the nearby alpine outdoor atmosphere. In all indoor environments the spore counts decreased in the following order: Cladosporium > yeasts > mycelia sterilia > Penicillium > Epicoccum > Botrytis > Aureobasidium > Alternaria . At all sampling sites the total airborne propagules, of Cladosporium , mycelia sterilia, Alternaria, Botrytis and Epicoccum showed seasonal distribution and, in general, significant correlation between both the indoor and outdoor environment. Penicillium indoor counts did not correlate with those outdoors nor with those of Aspergillus indoors or outdoors.

Ferricrocin: an ectomycorrhizal siderophore of Cenococcum geophilum

The ectomycorrhizal fungus Cenococcum geophilum was grown in low-iron medium and the excreted siderophores were extracted, purified and analyzed by HPLC. The principal hydroxamate siderophore produced, was identified as ferricrocin as confirmed by analytical HPLC, FAB-mass spectrometry and 1H- and 13C-NMR spectra. Although the occurrence of ferricrocin has been shown earlier to occur in the ericoid mycorrhizal fungi, this is the first report of ferricrocin in a true ectomycorrhizal fungus which is taxonomically related to the ascomycetes.

Molecular studies on terricolous microfungi reveal novel anamorphs of two Tuber species

This study reports novel terricolous mitosporic fungal morphs nested in the genus Tuber according to molecular phylogenetic analysis. Fungal DNA was amplified directly from field-collected anamorph samples. Nuclear ribosomal DNA (nrDNA) sequences including the ITS regions and the D1 and D2 domains of the LSU identify the anamorphs as mitosporic Tuber borchii and Tuber oligospermum. The link of the novel anamorphs to the genus Tuber is confirmed by the comparative analysis of five collections from four sampling sites. Ectomycorrhizas with characteristic features of Tuber borchii ectomycorrhizas were found in the soil volume collected with one of the mitosporic T. borchii collections. A nrDNA sequence amplified from these ectomycorrhizae is identical with the corresponding anamorph sequence. The possible role of the newly discovered anamorphs in the Tuber life-cycle and the potential significance of anamorphs for the propagation of ectomycorrhizal fungi are discussed.

Isolation and identification of hydroxamate siderophores of ericoid mycorrhizal fungi

Three ericoid mycorrhizal fungi were grown in pure culture under iron deprivation: (i) the ascomyceteHymenoscyphus ericae, a characteristic endophyte of ericaceous plants on acid soils; (ii) the hyphomyceteOidiodendron griseum, an ericoid mycorrhizal fungus which is also a soil-borne fungus able to colonize wood; and (iii) an endophyte of the calciculous ericaceous plantRhodothamnus chamaecistus. All three fungi produced several hydroxamate siderophores which were isolated in the ferric form by adsorption to Amberlite XAD-2, gel chromatography on Sephadex LH20 and by HPLC on a C18 reversed-phase column. Siderophores were identified by (i) co-chromatography with known fungal siderophores, (ii) ion spray mass spectrometry after semi-preparative HPLC and (iii) analyzing their electrophoretic behavior. WhileH. ericae andO. griseum were similar in producing ferricrocin as their principal siderophore, the endophyte ofR. chamaecistus produced mainly fusigen.