It has been said that humans try to understand plants for two reasons: Either to kill them better – e.g. if they are considered to be weeds that compete for resources with our crop plants – or to exploit them for human advantage. In the latter category using plants to clean up the mess that people make is an understandable ambition.
One of the ways that (hu)mankind has polluted the planet is in its attempts to extract the Earth’s mineral resources. Such overly-enthusiastic extractive endeavours have helped us mine considerable amounts of metals from the planet’s crust, the lithosphere. However, a consequence of that has often been the accompanying enrichment of potentially harmful metals – so-called heavy metals (HMs)* – in the environment, with potential to harm humans, and other living things. Elevated concentrations of HMs in the soil often condemn that land to being of little, or no, use for growing crops that could be used as food for man – or his domestic animals – because many plants have the ability to absorb HMs from the soil. In that way HMs may accumulate to levels that can be harmful to humans if the HM-enriched plants or their parts are consumed. Nevertheless, and whilst many plants will be killed by exposure to such HM-contaminated soils, some highly-specialised species survive (so-called metallophytes) in such situations.
Indeed, some even thrive in such challenging environments despite accumulating extremely high concentrations of HMs within their tissues**. And it’s that remarkable ability that can be exploited by humanity to help clean-up such contaminated sites. By repeatedly removing the bodies of HM-accumulating plants that have grown in HM-contaminated soils, and allowing a new ‘crop’ of metal-accumulating plants to grow in the same soil, such plants could be used to remove – or reduce to safe levels – HM amounts in the soil. Known as phytoremediation, this technique may be used on HM-contaminated soils to reduce the HMs to concentrations that are safe enough for crop production.
Plants that are particularly useful in this phytoremediation context are termed hyper-accumulators, because they accumulate metals to concentrations many times greater than those found in the soil. So useful are these plants that a catalogue of these beneficial botanics has been produced by Roger Reeves et al.. The database – which resides at http://hyperaccumulators.smi.uq.edu.au/collection/ – contains such information as taxonomy, distribution, ecology, and analytical data on the species. And, when this piece was written, the repository listed 721 hyperaccumulator species***. Although this list should only grow as more of these intriguing plants are discovered, Reeves et al. caution that, “in many parts of the world, by virtue of their existence solely or significantly on metalliferous soils, hyperaccumulator plants are threatened by habitat loss, especially from mining and minerals extraction”. “Timely identification of hyperaccumulator species, along with other metal-tolerant plants, is therefore necessary to preserve them to study their unique physiological mechanisms, and to take advantage of their unique properties”. Bibulous botanics cleaning-up humanity’s mess – if we treat them respectfully…
*The phrase ‘heavy metals’ is one of those curious terms; it’s so often used in a negative way that the binomial usually elicits a knee-jerk reaction that all such elements must by definition be harmful. But, many heavy metals – e.g. manganese, zinc, nickel, iron, copper – are essential for the well-being of living organisms. As ‘Swiss physician and alchemist’ Paracelsus might say, “it’s the dose that makes the poison”. In small amounts HMs can be life-giving and life-sustaining (and termed micronutrients); in larger amounts they can be toxic and life-terminating…
**And some of the metal-accumulating abilities of these plants are impressive, e.g. the fern Pteris vittata can contain up to 2.3% of arsenic (which is strictly a metalloid rather than a true HM, but included within the database as historically and by convention it’s usually listed amongst the true heavy metals), Noccaea caerulescens – 5.4% zinc, Virotia neurophylla – 5.5% manganese, and a record-breaking 7.6% nickel by Berkheya coddii…
***For those unable to get hold of the New Phytologist paper, the breakdown of those 721 spp. is: 532 nickel (per Table 1, but shown in-text as 523…), 53 copper, 42 cobalt, 42 manganese, 41 selenium, 20 zinc, 8 lead, 7 cadmium, 5 arsenic, 2 thallium, 2 rare earth elements (lanthanum and cerium), and 1 chromium hyper-accumulator. Which list totals 755, i.e. more than 721 – because some spp. hyper-accumulate more than one metal(!). In some cases the metal concentrations are so high that such plants may be economically-viable as a source from which to extract the metals – as an alternative to digging the stuff out of the ground – in the technique known as phytomining.
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