Facultative hyperaccumulation of heavy metals and metalloids

Our plain-English summary of Facultative hyperaccumulation of heavy metals and metalloids, by A.J. Pollard, R.D. Reeves, A.J. Baker (Plant Science, 2014).

The study

• Authors: A.J. Pollard, R.D. Reeves, A.J. Baker
• Journal: Plant Science
• Published: 2014;217-218:8-17
• Type of article: Review
• Cited by: 133 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Approximately 500 species of plants are known to hyperaccumulate heavy metals and metalloids. The majority are obligate metallophytes, species that are restricted to metalliferous soils. However, a smaller but increasing list of plants are "facultative hyperaccumulators" that hyperaccumulate heavy metals when occurring on metalliferous soils, yet also occur commonly on normal, non-metalliferous soils. This paper reviews the biology of facultative hyperaccumulators and the opportunities they provide for ecological and evolutionary research. The existence of facultative hyperaccumulator populations across a wide edaphic range allows intraspecific comparisons of tolerance and uptake physiology. This approach has been used to study zinc and cadmium hyperaccumulation by Noccaea (Thlaspi) caerulescens and Arabidopsis halleri, and it will be instructive to make similar comparisons on species that are distributed even more abundantly on normal soil. Over 90% of known hyperaccumulators occur on serpentine (ultramafic) soil and accumulate nickel, yet there have paradoxically been few experimental studies of facultative nickel hyperaccumulation. Several hypotheses suggested to explain the evolution of hyperaccumulation seem unlikely when most populations of a species occur on normal soil, where plants cannot hyperaccumulate due to low metal availability. In such species, it may be that hyperaccumulation is an ancestral phylogenetic trait or an anomalous manifestation of physiological mechanisms evolved on normal soils, and may or may not have direct adaptive benefits.

Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.

In plain English

This article was a review of plant research, not an original study, and it did not discuss asbestos. The authors reviewed published research on "facultative hyperaccumulators". These are plants that build up very high concentrations of heavy metals or metalloids in their leaves when growing on metal-rich soils, such as serpentine (ultramafic) soils, but that also grow on ordinary soils. The authors reported that approximately 500 plant taxa have been proposed as hyperaccumulators, mostly of nickel. They compared surveys of four serpentine regions (New Caledonia, Cuba, Turkey and Brazil) that together contained 281 hyperaccumulating taxa. In every region, plants restricted to metal-rich soils outnumbered facultative ones. The authors' table of 36 facultative species showed that controlled experiments comparing plants from metal-rich and ordinary soils had been published for only 10 species, 7 of them in one plant family. The authors concluded that earlier evolutionary explanations do not fit these plants well. They proposed several alternative hypotheses and said a broader base of research is required. They described estimates of how common these plants are as rough approximations.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review of published research on facultative hyperaccumulator plants, with a table classifying the literature and some previously unpublished data from one author
• Who was studied: Plant species that hyperaccumulate metals, including 36 facultative hyperaccumulator species listed in Table 1, and nickel hyperaccumulators from serpentine-soil surveys in New Caledonia, Cuba, Turkey and Brazil (281 taxa in total)
• Exposure: Heavy metals and metalloids in soil taken up by plants (mainly nickel, also zinc, cadmium, arsenic, manganese, cobalt and thallium), on metal-rich soils such as serpentine (ultramafic) soils compared with ordinary soils; asbestos is not studied or mentioned
• Main results:
  • Approximately 500 plant taxa have been proposed as hyperaccumulators of one or more metals.
  • Citing published estimates, the authors stated that as few as 10–15% of hyperaccumulators are facultative.
  • In all four surveyed serpentine regions, obligate hyperaccumulators outnumbered facultative ones, with large differences between regions.
  • Controlled experimental (Type I) studies had been published for only 10 of the 36 facultative species, and 7 of those 10 were in the Brassicaceae family.
  • Most publications in Table 1 (over 550 across all species) were Type III studies, which did not compare plants from metal-rich and ordinary soils.
  • In previously unpublished data by Pollard (Fig. 1), lettuce (Lactuca sativa) grown in high-zinc solution reached the hyperaccumulation threshold for zinc in the leaves, but its shoot mass was drastically reduced and it showed toxicity.
• The authors' conclusion: The authors concluded that previously published evolutionary models do not apply well to facultative hyperaccumulators. They put forward several hypotheses to explain how these plants evolved and stated that a broader base of research is required.
• Limitations the authors state:
  • The authors stated that estimates of how common facultative hyperaccumulation is are at best rough approximations, because of uncertain taxonomy, unknown species ranges, identification errors and leaf-analysis errors.
  • The authors stated that there were not enough data to draw confident conclusions about whether facultative hyperaccumulation differs between island and mainland floras.
  • The authors stated that the literature is concentrated on a few species and metals, with few detailed experimental studies of nickel hyperaccumulators.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Serpentine Group.

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