Whereas most plants can take up metals from their environment on some level, there are a handful of plants species on this planet that are surprisingly good at it. We call these plants “hyperaccumulators,” and the levels of heavy metals in their tissues would be fatal to most organisms. It may seem strange that plants would willingly accumulate toxic levels of metal in their tissues until you consider both where these plants live and why they may be doing it.
Generally speaking, hyperaccumulators hail from regions of the world rich in metalliferous soils such as serpentine. These soils are difficult for plants to live in because of their naturally high metal content. The plants that do grow in metalliferous soils are often very restricted in their distribution and either cannot grow anywhere else or get out-competed in less toxic soils. Hyperaccumulators have been found to take up a variety of metals including nickel, zinc, cadmium, and many others. Some do this to such a degree that it actually changes the color of their sap.
One of the most famous examples of a hyperaccumulator species is a tree endemic to the island of New Caledonia called Pycnandra acuminata. New Caledonia is a hot spot for metalliferous soils so finding such a tree there is not terribly surprising. What is surprising is just how much metal this tree accumulates. One study found that its blue-green sap contains upwards of 25% nickel. A similar example can be seen in a different species of tree known to science as Phyllanthus balgooyi, which is native to Borneo. Not only is this tree strange thanks to the fact that its leaves are not leaves at all, but rather flattened photosynthetic stems, but it is also a hyperaccumulator of nickel. Recent work suggests that its sap can contain upwards of 16% nickel, which also gives it a distinctive blue-green hue.
Again, there are several examples of plants that do this. It is by no means restricted to just nickel nor the islands of New Caledonia and Borneo. That is not to say its a common trait either. Despite its occurrence across different plant lineages, hyperaccumulation is still quite rare. To date, it is estimated that only about 0.2% of all angiosperms are capable of this feat. Also, it appears to be most common in tropical regions of the world. What is most amazing is that it doesn’t appear to be limited by the amount of metal in the soil. Researchers have found that many hyperaccumulators are able to maintain high levels of metal in their tissues across a wide range of soil metal concentrations. How they deal with this biologically is a topic for another post but the question remains, why concentrate toxic levels of heavy metals in your tissues?
The answer is likely defense. Whereas the high concentrations of heavy metals in their tissues are not toxic to the plants themselves, they are certainly toxic to anything that may want to eat them. One way that hyperaccumulation can work as a defense mechanism is by deterring herbivores outright. Insects and other herbivores may be able to detect heavy metals within the tissues and will actively avoid feeding on those plants. If no other options are available, then eating such plants can straight up harm herbivores. One study found that locusts feeding on tissues containing high levels of heavy metals exhibited significant reductions in growth and development.
There is still a lot to learn about hyperaccumulation in plants. How this trait evolves, why we see it in some lineages and not others, and how plants are able to tolerate toxic levels of heavy metals are but a few of the questions that scientists are actively working on answering. One exciting avenue of research is understanding how some of these plants can be used to clean soils polluted by human activities such as mining. They call the process “phytomining” and it involves planting certain hyperaccumulators in polluted soils, allowing them to absorb metals, and then removing that biomass, taking all of the accumulated metals along with it. Certainly this needs a lot more work before it can be used effectively.
We need to act fast, however, as so many botanical hyperaccumulators are under threat of extinction. Because so many of these plants grow on restricted soil types in remote corners of the world, they are at great risk from habitat destruction. Places like New Caledonia are being strip mined at an unsustainable rate to get at the very metals that these plants have evolved to tolerate. If something is not done to protect these unique places and the flora they support, there is no telling what Earth stands to lose. This is yet another reason why we must support land conservation at all costs!