Molecular diversity of arbuscular mycorrhizal fungi in relation to soil chemical properties and heavy metal contamination
Introduction
Contamination of soils by heavy metals (HM), resulting from anthropogenic activities such as ore mining and smeltering, is a serious problem in many areas around the world (Gadd, 1993). In addition to physical or chemical soil properties, the quality of HM polluted soils depends on the diversity and activity of biota present (Doran and Linn, 1994, Jeffries et al., 2003). In this context, the arbuscular mycorrhizal fungi (AMF), belonging to the phylum Glomeromycota and associating with native plants, may be important for assessing soil quality in polluted areas (Gaur and Adholeya, 2004).
Glomeromycotan fungi are ubiquitous soil organisms (Treseder and Cross, 2006) and have been repeatedly described in HM polluted sites (e.g. Sambandan et al., 1992, Turnau et al., 2001). Although arbuscular mycorrhizal symbiosis is widespread, the symbiotic functions of AMF species are not equivalent and vary according to the specific AMF isolates, host plants and soil properties (e.g. Hempel et al., 2009, Hildebrandt et al., 1999, Kelly et al., 2005, Redon et al., 2008, Redon et al., 2009). Therefore, the identification of specific phylotypes of AMF and their relationship with soil properties are crucial and provide an important step to understand the ecology of AMF at HM contaminated ecosystems (Gaur and Adholeya, 2004).
Several surveys on the effect of HM on diversity and abundance of AMF have been conducted in HM polluted areas around the globe (e.g. Chao and Wang, 1990, del Val et al., 1999, Regvar et al., 2003, Sambandan et al., 1992, Vallino et al., 2006). Although other soil parameters, such as soil nutrient concentration and pH, may also be highly correlated to AMF distribution (del Val et al., 1999), they have been rarely used to interpret patterns of AMF community composition.
Our study area, adjacent to an open pit mine, is the main source of soil HM pollution in the Anguran region in Iran. In a first survey on AMF, the spore abundance and root colonization parameters of indigenous plants were analyzed on 35 plots along a transect of decreasing HM soil concentration away from the mine (Zarei et al., 2008b). These data showed a strong negative correlation between HM soil concentrations and both AMF spore abundances and root colonization intensities. Morphological spore identification to the genus level also suggested that there was a reduced AMF species diversity in highly HM contaminated plots. To test this hypothesis, the AMF diversity in the roots of the indigenous plant species Veronica rechingeri was assessed in four selected plots along the same transect using molecular DNA based markers for AMF sequence type identification (Zarei et al., 2008a). The results of this second study clearly showed a decline in the number of AMF sequence types colonizing the roots of V. rechingeri, ranging from six types in unpolluted plots to only two types in highly HM polluted plots. The AMF distribution data also revealed AMF sequence types that are adapted to high HM concentrations.
In the present study, we extend our approach by describing the molecular diversity of AMF associated to different dominant plant species (except V. rechingeri) in relation to important soil parameters at the Anguran Zn and Pb mining region. These results will give a clearer insight into the combined impact of HM and soil chemical properties on AMF community composition in the Anguran mining region.
Section snippets
Sampling area, sample collection, and analysis
The Anguran Zn-(Pb) deposit, Zanjan Province, NW Iran (36°40′ N, 47°20′ E; 2950 m a.s.l.), is located within the central Sanandaj-Sirjan Zone of the Zagros orogenic belt. Anguran deposits may represent a new type of low-temperature, carbonate-hosted Zn–Pb ore that is distinct from the Mississippi Valley type and sedimentary-exhalative deposits (Gilg et al., 2006).
In order to study AMF diversity in soil with different levels of Zn and Pb concentrations, the sampling region was divided into areas
AMF spore, and root colonization data
Soil chemical properties showed some variation between the studied areas (Table 1). AMF spore abundance and root colonization parameters were significantly different between the areas according to the Tukey HSD test (Table 2).
Molecular and phylogenetic data
Amplicons with the expected size were obtained from total root DNA extracts of the three respective plant species at each level of HM pollution after nested PCR, i.e. 12 PCR products. After pooling the three products within each HM level and cloning, we obtained 30
Discussion
Our previous studies in the Anguran Zn and Pb mining region showed a strong negative effect of soil HM contamination on AMF abundance and diversity. As soil Zn and Pb contents increase with decreasing distance from the mine pit, AMF spore numbers, mycorrhizal colonization parameters, and AMF diversity decreased (Zarei et al., 2008a, Zarei et al., 2008b), this result is also clearly visible in Table 2 in the current study. The variations in the colonization rate should not be over-interpreted as
Conclusion
Soil HM concentration was found to impact the AMF sequence type distribution in the roots of several dominant plant species in the Anguran mining region. However other soil chemical parameters, which showed variation between the sampling sites in the study area, might also contribute to soil feedbacks on AMF diversity, as detected by multivariate analysis. The distribution of AMF sequence types among the different levels of HM pollution was found to be largely in accordance with surveys on
Acknowledgements
We would like to thank the members of the Soil Ecology Department at the UFZ, especially Sabine Jarzombski and Stephan König for help during lab work and RFLP analysis, respectively. We are in debt to Prof Dr. Nahid Saleh Rastin for support and to Dr. Jessica Gutknecht for improving the manuscript. We also would like to thank Dr. Tancredi Caruso for helpful comments on the statistical analyses and two anonymous reviewers for their constructive suggestions.
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Both authors contributed equally.