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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Hyperaccumulator | 1/2 | https://en.wikipedia.org/wiki/Hyperaccumulator | reference | science, encyclopedia | 2026-05-05T07:18:38.913323+00:00 | kb-cron |
== Metallophytes == A metallophyte is a type of plant capable of surviving in metal-rich soil. Metallophytes are classified as metal indicators, excluders, or hyperaccumulators. Such plants range between obligate metallophytes and facultative metallophytes. Obligate metallophytes can only survive in the presence of heavy metals while facultative metallophytes can tolerate such conditions but are not confined to them.
== Hyperaccumulators ==
A hyperaccumulator is a category of metallophyte that is capable of growing in soil or water with a higher concentration of metals, absorbing the metals through its roots and storing it in its foliage. The metals are concentrated at levels that are toxic to closely related species not adapted to growing on the metalliferous soils. Approximately 85-90% of hyperaccumulators are obligate metallophytes. Compared to non-hyperaccumulating species, hyperaccumulator roots extract the metal from the soil at a higher rate, transfer it more quickly to their shoots, and store large amounts in leaves and roots. The ability to hyperaccumulate toxic metals compared to related species has been shown to be due to differential gene expression and regulation of the same genes in both plants. Hyperaccumulators are regularly discussed within the context of phytoremediation, although their commercialization remains aspirational. 450 plant species, including the model organisms Arabidopsis and Brassicaceae, have demonstrated the capacity to uptake and sequester metals such as Arsenic (As), Cobalt (Co), Iron (Fe), Copper (Cu), Cadmium (Cd), Lead (Pb), Mercury (Hg), Selenium (Se), Manganese (Mn), Nickel (Ni), Zinc (Zn), and Molybdenum (Mo) in 100–1000 times the concentration found in sister species or populations.
=== Table on Hyperaccumulators ===
Note that it is under debate as to whether Allium, Amaranthus, Iris, Lonicera, Rorippa, Salsola and Solanum are truly hyperaccumulators or metallophytes at all as their hyperaccumulation was recorded in labs, not nature.
=== Applications of Hyperaccumulators ===
==== Phytoremediation ==== Hyperaccumulating plants are of interest in the context of phytoremediation: to detoxify contaminated soils. Phytoextraction is a subprocess of phytoremediation in which plants remove metal ions from soil or water. Phytoextraction could in principle be used to remove contaminants from an ecosystem. For example, water hyacinth have been demonstrated to remove arsenic from water. Cadmium accumulation has also received attention as this metal is usually toxic. Caesium-137 and strontium-90 were removed from a pond using sunflowers after the Chernobyl accident. The remediation of metal-contaminated soils recognizes that metals cannot be degraded, they must be removed. Organic pollutants can be, and are generally the major targets for phytoremediation. Field trials support the feasibility of using plants for environmental cleanup.
==== Phytomining ====
Phytomining, sometimes called agromining, is the concept of extracting heavy metals from the soil using hyperaccumulating plants. Once the hyperaccumulation has proceeded to some extent, the metals are collected from the plant matter and then refined for sale or disposed of. Phytomining typically follows three steps: 1) Phytoextraction, where metals are sequestered from soil into plants; 2) Enrichment, where plant biomass is eliminated and heavy metals are enriched as solids; 3) Extraction, where the solid remains are processed into more accessible forms. Phytomining would, in principle, minimize environmental effects compared to conventional mining. Phytomining could also remove low-grade heavy metals from mine waste. A 2021 review concluded that the commercial viability of phytomining was "limited" because it is a slow and inefficient process. Its purpose is either: (i) gathering the metals for economic use (ii) gathering toxic metals to improve the soil. Phytomining was proposed in 1983 by Rufus Chaney, a USDA agronomist. He and Alan Baker, a University of Melbourne professor, first tested it in 1996. They, as well as Jay Scott Angle and Yin-Ming Li, filed a patent on the process in 1995 which expired in 2015. Several startups are investigating the process for mining surface-available heavy metals. In 2025, Genomines received 45 million dollars of Series A funding to commercialize nickel phytomining from mine tailings. The French company Econick and the Albanian company MetalPlant both have nickel phytomining projects. As of mid-2024, MetalPlant had extracted less than a kilo of usable nickel, using Odontarrhena plants.
=== Physiological advantage for hyperaccumulation === The biological advantage of hyperaccumulation may be that the toxic levels of heavy metals in leaves deter herbivores or increase the toxicity of other anti-herbivory metabolites. The plant defense hypothesis, "the elemental defense hypothesis", provided by Poschenrieder, suggested that the expression of these genes assist in antiherbivory or pathogen defenses by making tissues toxic to organisms attempting to feed on that plant. Another hypothesis, "the joint hypothesis", provided by Boyd, suggests that expression of these genes assists in systemic defense. The benefit for a plant to hyperaccumulate may be that root-to-shoot transport system drives hyper-accumulation by creating a metal deficiency response in roots.
=== T. caerulescens === As a hyperaccumulator variously of Cd, Pb, and Zn, T. caerulescens, pennycress, has received particular attention. Its leaves accumulate up to 380 mg/kg Cd. On the other hand, the presence of copper seems to impair its growth. It is found mostly in Zn/Pb-rich soils, as well as serpentines and non-mineralized soils. When grown on mildly polluted soils, a closely related species, Thlaspi ochroleucum, is a heavy metal-tolerant plant, but it accumulates much less Zn in the shoots than T. caerulescens. Thus, T. ochroleucum is a non-hyperaccumulator and of the same family T. caerulescens is a hyperaccumulator. The transfer of Zn from roots to shoots varied significantly between these two species. T. caerulescens had much higher shoot/root Zn concentration levels than T. ochroleucum, which always had higher Zn concentrations in the roots. When Zn was withheld, the amount of Zn previously accumulated in the roots in T. caerulescens decreased even more than in T. ochroleucum, with a concomitantly greater rise in the amount of Zn in the shoots. The decreases in Zn in roots may be mostly due to transport to shoots, since the volume of Zn in shoots increased during the same time span.