Heavy Metal Hype

Hyperaccumulators are plant species that have evolved the capacity to take up and accumulate large amounts of heavy metals such as zinc, manganese and nickel from the soil. Many heavy metals are essential for plants to grow and remain healthy but are required in only very small quantities. Any soil with high levels of such heavy metals would at best be bad for the plants and at worst toxic. Hence, growing in such spots is not a particularly good idea. Unless the plant in question is a hyperaccumulator.

Few places in the world are home to such “ultramafic” soils. Let’s visit New Caledonia, a French territory in the south-west Pacific, about 1,500 kilometres east of Australia. New Caledonia is renowned for its exceptional flora – in total about 3,500 vascular plant species, of which an astonishing 74% are endemic (i.e. occur nowhere else). New Caledonia was once part of the huge Gondwana super-continent, having split from Australia about 80 million years ago. Throughout the first half of the Tertiary, it was submerged, deep on the ocean floor, only re-emerging about 37 million years ago, in the early Eocene. At that time an “ophiolitic nappe” covered New Caledonia. It’s complicated but in essence this is a sheet of oceanic crust and upper mantle that’s been lifted to the top of the continental crust. The mineral composition of this ophiolitic nappe was crucial in creating these ultramafic soils, which today still cover about a third of Grande Terre, New Caledonia’s main island.

In the early Eocene, having just re-emerged from the deep ocean, New Caledonia was entirely plant-free. Plants needed to re-colonise it by means of long-distance dispersal from other land masses, predominantly Australia and, to a lesser extent, the island of New Guinea (to this day, there are strong phylogenetic links between the New Caledonian and Australian flora). Now imagine, after a long ocean journey, a plant (maybe in the form of a seed in a seabird’s belly) arrives in New Caledonia and finds ultramafic soils, which not only contain toxic levels of heavy metals but which also show very low levels of fertility (low nitrogen concentrations) and have low water-holding capacities. To put it mildly, not all arrivals could cope. Even 37 million years later, many plant groups are non-existent or heavily underrepresented in the New Caledonian flora, while the descendants of those immigrants which did cope are heavily over-represented. Among these latter plant groups (such as species belonging to the plant orders Celastrales, Oxalidales, Malpighiales, Sapindales, and Gentianales), many species are hyperaccumulators. It’s most likely that before these plants arrived in New Caledonia they had already developed some form of defence, combined with a tolerance for low nutrient conditions.

We don’t yet understand exactly how Pycnandra acuminata accumulates nickel but we do know it’s associated with organic acids such as citric acid, an important metabolite in all aerobic organisms. Citrate, its derivate, acts as the main “ligand” for nickel. Recent studies showed the presence of Ni-nicotianamine complexes. Nicotianamine is a very common molecule in higher plants. The biochemical process of binding nickel is comparable to the binding of magnesium in the chlorophyll molecule. However, scientists are only now learning to understand the genetic mechanism behind the biosynthesis of these nickel ligands.

Like many other hyperaccumulators, the blue latex plant grows very slowly, taking decades to produce flowers and seeds. On Grande Terre, it’s threatened by habitat loss and deforestation as a result of nickel mining.

Further research is necessary to check whether Pycnandra acuminata and similar hyperaccumulators can be used to clean soil contaminated with heavy metals – a process known as phytoremediation (or even “phytomining”) that can be also be effective cleaning up other contaminants such as oil and pesticides.

You can learn about more weird and wonderful plant types by visiting Plants Worldwide, the blog of our Vice Chair, Dr Jonas Muller.

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