Have you ever heard of locoweed? As a child, I watched a lot of Bgrade movie Westerns and one thing I learned was that, if you are a rancher, you don’t want your cattle grazing on locoweed! Locoweed is the common name given to several species of Astragalus, a genus in the Fabaceae or pea family. Also known as milk vetch or poison vetch, many species of Astragalus take up unusually large quantities of selenium from the alkaline soils of the western plains.The high selenium content contributes to a disease known as alkali poisoning or “blind staggers” in cattle unfortunate enough to graze on this plant—the cattle literally behave as though they are crazy. There are many regions where natural geochemical processes have produced soils that are rich in metals such as nickel, chromium, gold, cadmium, selenium, and arsenic. Normally high levels of heavy metals would be toxic to plants, just as they are to humans, yet many plants actually thrive on soils rich in such metals. For some plants, the metals are not a problem simply because the cell membranes surrounding the root cells prevent the metals from entering the root. Other plants actually take up the metals and accumulate them to levels that would be toxic to most other plants.
In Astragalus, for example, selenium may account for as much as 10% of the dry weight of the seeds. In soils that are rich in nickel, some plants may contain 200,000 times more nickel than plants growing in normal soils. Many years ago, such plants were known as “indicator species,” and prospectors would take the presence of such plants as an early indication that the soils may have contained a mineral of interest, such as gold. This was called phytoprospecting. We now call these plants accumulator species, which are not injured by high concentrations of heavy metals because they sequester (isolate) the metals with small proteins called phytochelatins.
The sequestered metals are then stored in the large central vacuole of the plant cell, where they cannot interfere with the cell’s metabolism. There has recently been a renewed interest in accumulator species because these plants may have the potential to assist in cleaning up soils contaminated with heavy metals as a result of twentieth-century industrial activities. Using plants to clean up soils is called phytoremediation (phyto meaning “plant” and remediation meaning “to correct a fault”).
The idea is to grow accumulator species on mine tailings and wastes from paper mills, for example, where they would extract the heavy metals. Plants will naturally take longer to do the job, but plants are much more cost-effective and would not create even more ecological problems as engineering-based technologies often do.
Phytoremediation would also help to stabilize contaminated sites because the plants help to control erosion. An additional benefit of accumulator species is that they begin the revegetation of barren industrial sites and assist in the recovery of useful metals. Phytomining, as it is called, has proven effective in the recovery of both nickel and thallium in demonstrations. Inother trials, various species of willows (Salix) have shown promise for extraction of heavy metals from soils treated with sewage sludge. The advantage of using plants is that they can be harvested and burned. The heavy metals remain concentrated in the ash, which makes their disposal much easier.
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