Green Goldmine: The Surprising Role of Plants in Rare Earth Element Accumulation

Imagine plants not just beautifying our gardens and purifying our air, but also acting as natural mines for critical resources like rare earth elements (REEs). These elements, vital for modern technologies, are scarce. But scientists have uncovered several new plant species with exceptional abilities to accumulate REEs from soil. By screening thousands of specimens across different lineages, Léo Goudard and colleagues discovered new REE-rich taxa that could aid bioprospecting and expand options for more sustainable mining of these crucial metals.

The study, published in the journal Annals of Botany, took a unique approach. Instead of scouring forests, researchers turned to the Herbarium of the Muséum national d’Histoire naturelle in Paris. Last author Antony van der Ent told Botany One in an email that the herbarium in Paris was particularly well suited for their research. “The Paris Herbarium has the largest collection of plant specimens in the world, and it has been fully digitized. We have a longstanding collaboration with the scientists of the Paris Herbarium who have supported this research. Partnering with taxonomists is absolutely key to undertaking this type of plant diversity-metallomics studies,” he said.

They used X-ray fluorescence to analyze over 4,000 plant specimens from two fern families and two flowering plant families housed in the museum’s herbarium. These lineages were selected based on taxa already proven to take up REEs. Initial screening for the element yttrium—used as a proxy for total REEs—revealed 268 specimens with concentrations from 49 micrograms per gram upwards of 1,424 micrograms per gram. 

Follow-up atomic emission spectroscopy of select specimens showed remarkably high total REE contents. Specimens of Dicranopteris linearis and Blechnopsis orientalis registered levels over 6,000 and 4,000 micrograms per gram respectively, among the highest on record. The analysis validated using yttrium as an indicator for REEs in herbarium scans. 

Some of the plants found to be “hyperaccumulators” of rare earth elements were outside the well-studied regions of China that are typically considered hotspots for this trait. Finding highly accumulating plants in other parts of the world significantly expands the known geographic distribution and genetic diversity of this exceptional ability in the plant kingdom. It shows rare earth hyperaccumulation may be more widespread than previously thought.

The XRF scanning results show that 147 specimens of B. orientalis and 56 specimens of D. linearis accumulate Y in their leaves up to concentrations of 1424 µg g-1 and 697 µg g-1, respectively. Dicranopteris linearis is a fern already widely documented for its REE (hyper)accumulation capabilities in the locations in China, but is in fact, polyphyletic, representing a species complex of cryptic species. This study reveals some specimens of D. linearis accumulating more than 1000 µg g-1 of REEs in their fronds are from Indonesia and the Philippines. 

Goudard et al. 2024

But there’s more. This study didn’t just identify new champions; it validated the use of XRF on herbarium specimens for REE detection. This opens up a treasure trove of past collections, potentially revealing hidden REE-rich plants waiting to be rediscovered. But the research is up against the threat of imminent habitat loss, meaning that the research is under pressure of a potentially unknown deadline.

“Most hyperaccumulator plants are extremely rare in nature and local endemics to mineralised outcrops,” says van der Ent. “Given that many hyperaccumulator plants are restricted to surfacing metal ore deposits, there is effectively a ‘race’ between mining companies destroying their habitat, and us trying to get there before to discover them and collect germplasm. Ironically, hyperaccumulators and other metal-tolerant plants (‘metallophytes’) are hugely beneficial in the rehabilitation of mine wastes, but that for application they need to be found and saved first.”

Finding these plants poses a tough challenge. “The natural habitats of hyperaccumulator plants are often very remote and difficult to reach. Intensive collaboration with local scientists is absolutely essential in conducting research in tropical environments. Key challenges are hazardous safety conditions (for example in the DR Congo famous for its unique copper-cobalt metallophytes) and insurmountable bureaucracy for research permits (a positive example in this respect is the Sabah Biodiversity Council in Malaysia which effectively manages the whole process in a single application assessed on its merit),” says van der Ent.

van der Ent’s work around the work was sparked by a comment when he was still in school. “When I was in high school I heard about a small plant (Noccaea caerulescens) that grows just over the border in Belgium that can hyperaccumulates zinc. It fascinated me how a plant could contain so much metal, and I went to look for it on the waste heaps of the long-closed zinc-lead mines near Kelmis. This fascination then instigated my global quest to find and study hyperaccumulator plants, including in Australia, Chile, Indonesia, New Caledonia and South Africa,” he says.

This study shines a light on the hidden potential of the plant kingdom and herbarium archives, revealing their ability to hold valuable resources and help us build a more sustainable future. We could harness these natural “biogeochemical pathfinders” to locate REE deposits buried underground. Even more exciting, these plants could become tools for “phytoremediation”, cleaning up contaminated sites by absorbing REEs from polluted soil and water. Now, imagine rows of hyperaccumulating plants, silently working to extract critical elements for our technological needs.

At a time when some universities are looking to shut down herbaria, the research shows that a well-maintained herbarium could be a gold (or other metal) mine. van der Ent believes that many more hyperaccumiltors are waiting to be discovered.

“Through our research we have more than doubled the number of hyperaccumulator plants known around the world in the last 10 years. However, I believe this is just the tip of the proverbial iceberg, and as this study in Annals of Botany shows, many more discoveries await in global herbaria. This research also exemplifies the value of herbaria as crucial resources for information on biodiversity globally, and shows why it is so important that these institutions receive continued funding and support.”

READ THE ARTICLE 

Goudard L., Blaudez D., Sirguey C., Purwadi I., Invernon V., Rouhan G., van der Ent A. “Prospecting for rare earth element (hyper)accumulators in the Paris Herbarium using X-ray fluorescence spectroscopy reveals new distributional and taxon discoveries” Annals of Botany. Available at: https://doi.org/10.1093/aob/mcae011

Cover image: Synchrotron µXRF elemental maps showing the distribution of Cesium in a frond of Dicranopteris linearis. Photo: Goudard et al. 2024.