Rare earth element (hyper)accumulation in some Proteaceae from Queensland, Australia

Background and aims Proteaceae species strongly acidify their rhizosphere to increase soil phosphorus (P) availability, which also inadvertently increases the availability of other elements, such as manganese and rare earth elements (REE). This study aimed to investigate REE (hyper) accumulation in the Proteaceae genus Helicia from Queensland, Australia, from a systematic assessment of REE concentrations in herbarium specimens and verification with field collected samples. Methods Herbarium X-ray fluorescence (XRF) scanning (using yttrium as a proxy for REEs) was undertaken on selected Helicia species (Proteaceae) at the Queensland Herbarium, followed by Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis and field collection of H. glabriflora samples to confirm the XRF findings. Results The herbarium XRF analysis revealed highly anomalous REE concentrations in some Helicia species, reaching similar to 1300 mu g Y g(-1) in H. australasica and H. glabriflora. The ICP-AES analysis of the herbarium specimens revealed total REE concentrations (REE) of up to 2300 mg mu g(-1) in H. australasica and H. glabriflora, with relatively higher light REEs (similar to 70%) and yttrium (similar to 20%) concentrations, compared to heavy REEs (similar to 10%). The field collected H. glabriflora material had relatively higher total REE concentrations in the mature leaves (similar to 850 mu g REE g(-1)) and stems (similar to 675 mu g REE g(-1)), compared to young leaves (similar to 130 mu g REE g(-1)), roots (similar to 220 mu g REE g(-1)) and soil (similar to 90 mu g REE g(-1)). Conclusion The discovery of REE (hyper)accumulation in these Helicia species suggests that the Proteaceae family may host several other REE hyperaccumulators that are hitherto undiscovered. This calls for systematic assessment of the Proteaceae using the same approach as used in this study. The findings have potential implications for (i) discovery of REE hyperaccumulator plants, (ii) investigation of REE uptake and accumulation in plants and (iii) biogeochemical exploration of buried REE ore deposits.