Using Hyperaccumulators to Restore Polluted Soils

Hyperaccumulator plants are capable of accumulating exceptionally high levels of trace elements (TEs) in their above-ground biomass. These plants can achieve TE concentrations in their dry biomass that are up to 100 times higher than those found in non-hyperaccumulators growing in the same soil. Typically, a plant is classified as a hyperaccumulator if it accumulates TEs at concentrations of 0.1% or more. For specific elements such as zinc and manganese, the threshold concentration is 1%, while for cadmium, it is 0.01%. Currently, over 400 species of hyperaccumulators have been identified, with new discoveries continually expanding this list. These species may accumulate various TEs, including nickel, manganese, zinc, cadmium, thallium, copper, cobalt, and arsenic.

The trait of hyperaccumulation has evolved independently across several plant families, with many hyperaccumulators belonging to the Brassicaceae family. The precise advantage of TE hyperaccumulation remains a topic of ongoing research, with several theories proposed:

1. Tolerance to or Disposal of TEs: Hyperaccumulation might help plants tolerate or remove TEs from their systems.

2. Drought Resistance: It could be a strategy to enhance drought resistance.

3. Competitive Advantage: Accumulating TEs may reduce competition from less TE-tolerant plants.

4. Unintentional Uptake: Hyperaccumulation might result from inadvertent uptake of TEs.

5. Defence Against Herbivores or Pathogens: The high levels of TEs might serve as a defence mechanism against herbivores or pathogens.

6. Despite the intriguing hypothesis that TE accumulation might provide protection against herbivores, research has often shown that this is not always the case.

Hyperaccumulator species are commonly found in ultramafic (serpentine) or calamine soils, which typically have elevated concentrations of heavy metals. Their presence can thus serve as bioindicators of soil mineralisation and contamination. Hyperaccumulators hold significant potential in the mining industry for applications such as phytoremediation, phytomining, and phytomanagement.

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