Mycorrhizal function under heavy metal pollution

Heavy metal contamination of soils poses a major environmental and human health problem. Polluted soils may be recuperated by various strategies of which on-site bioremediation is a preferable and low-cost strategy with a low impact on the ecosystem. Phytoremediation makes use of plants either to immobilise heavy metals in the soil or to decontaminate the soil from these elements via extraction into the above-ground plant biomass. Plants used in phytoremediation have to tolerate high external and internal heavy metal concentrations, traits typical for metal- hyperaccumulating plants. Low biomass production of the majority of these plants has so far been a major limitation to phytoremediation. Recently discovered clones of fast-growing poplars and willows that accumulate heavy metals beyond the hyperaccumulation threshold may overcome this limitation. Rhizosphere processes play an important role in heavy metal bioavailability and uptake by plants. Mycorrhizal fungi are a major component of the rhizosphere microbial community. Most plants form symbiotic associations with mycorrhizal fungi. The majority of heavy metal accumulating plants are notable exceptions. Mycorrhizal associations have therefore often been considered to be of minor importance in phytoremediation. Poplars and willows do however form mycorrhizal associations. Mycorrhizal characteristics such as the large amount of external hyphae, which stabilise soil structure and very effectively exploit soil nutrients, and the internal structures, which efficiently transfer nutrients directly to root cells, give these associations an important role in mediating plant uptake of heavy metals. Mycorrhizal fungi are known to affect heavy metal bioavailability, and uptake of toxic metals is normally reduced. These results were exclusively obtained in studies with non-accumulating plants. The functional compatibility of a mycorrhizal association however depends on both partners. Preliminary research indicates a potentially substantial contribution of mycorrhizas to accumulator willow and poplar heavy metal uptake. The general aims of the proposed study are to examine and quantitatively describe (i) the distribution and transport of heavy metals in mycorrhizal associations of poplars and (ii) the effect of mycorrhizal fungi on the bioavailability of heavy metals in soil. Experimental work will be conducted in the field and under controlled conditions to meet the following objectives of the project: 1. Characterisation of the functional compatibility of various mycorrhizal associations of heavy metal accumulator versus non-accumulator trees. 2. Identification of the diversity of function in mycorrhizal fungi with respect to heavy metal transport processes within fungal mycelia and heavy metal transfer to the host plant. 3. Quantification of the effect of mycorrhizal fungal mycelia on solution heavy metal concentrations and labile soil heavy metal pools. The projected is expected to yield important information with respect to the potential use of mycorrhizal fungi in phytoremediation and will contribute to a better understanding of the ecological significance of mycorrhizal associations in heavy metal polluted soils.

Heavy metal contamination of soils poses a major environmental and human health problem. Polluted soils may be recuperated by various strategies of which on-site bioremediation is a preferable and low-cost strategy with a low impact on the ecosystem. Phytoremediation makes use of plants either to immobilise heavy metals in the soil or to decontaminate the soil from these elements via extraction into the above-ground plant biomass. Plants used in phytoremediation have to tolerate high external and internal heavy metal concentrations, traits typical for metal- hyperaccumulating plants. Low biomass production of the majority of these plants has so far been a major limitation to phytoremediation. Recently discovered clones of fast-growing poplars and willows that accumulate heavy metals beyond the hyperaccumulation threshold may overcome this limitation. Rhizosphere processes play an important role in heavy metal bioavailability and uptake by plants. Mycorrhizal fungi are a major component of the rhizosphere microbial community. Most plants form symbiotic associations with mycorrhizal fungi. The majority of heavy metal accumulating plants are notable exceptions. Mycorrhizal associations have therefore often been considered to be of minor importance in phytoremediation. Poplars and willows do however form mycorrhizal associations. Mycorrhizal characteristics such as the large amount of external hyphae, which stabilise soil structure and very effectively exploit soil nutrients, and the internal structures, which efficiently transfer nutrients directly to root cells, give these associations an important role in mediating plant uptake of heavy metals. Mycorrhizal fungi are known to affect heavy metal bioavailability, and uptake of toxic metals is normally reduced. These results were exclusively obtained in studies with non-accumulating plants. The functional compatibility of a mycorrhizal association however depends on both partners. Preliminary research indicates a potentially substantial contribution of mycorrhizas to accumulator willow and poplar heavy metal uptake. The general aims of the proposed study are to examine and quantitatively describe (i) the distribution and transport of heavy metals in mycorrhizal associations of poplars and (ii) the effect of mycorrhizal fungi on the bioavailability of heavy metals in soil. Experimental work will be conducted in the field and under controlled conditions to meet the following objectives of the project: 1. Characterisation of the functional compatibility of various mycorrhizal associations of heavy metal accumulator versus non-accumulator trees. 2. Identification of the diversity of function in mycorrhizal fungi with respect to heavy metal transport processes within fungal mycelia and heavy metal transfer to the host plant. 3. Quantification of the effect of mycorrhizal fungal mycelia on solution heavy metal concentrations and labile soil heavy metal pools. The projected is expected to yield important information with respect to the potential use of mycorrhizal fungi in phytoremediation and will contribute to a better understanding of the ecological significance of mycorrhizal associations in heavy metal polluted soils.