Locked up Inside the Vessels: Rare Earth Elements Are Transferred and Stored in the Conductive Tissues of the Accumulating Fern Dryopteris erythrosora

Abstract

Rare earth elements (REEs) are strategic metals strongly involved in low-carbon energy conversion. However, these emerging contaminants are increasingly disseminated into ecosystems, raising concern regarding their toxicity. REE-accumulating plants are crucial subjects to better understand REE transfer to the trophic chain but are also promising phytoremediation tools. In this analysis, we deciphered REE accumulation sites in the REE-accumulating fern Dryopteris erythrosora by synchrotron X-ray μfluorescence (μXRF). This technique allows a high-resolution and in situ analysis of fresh samples or frozen-hydrated cross sections of different organs of the plant. In the sporophyte, REEs were translocated from the roots to the fronds by the xylem sap and were stored within the xylem conductive system. The comparison of REE distribution and accumulation levels in the healthy and necrotic parts of the frond shed light on the differential mobility between light and heavy REEs. Furthermore, the comparison emphasized that necrotized areas were not the main REE-accumulating sites. Finally, the absence of cell-to-cell mobility of REEs in the gametophyte suggested the absence of REE-compatible transporters in photosynthetic tissues. These results provide valuable knowledge on the physiology of REE-accumulating ferns to understand the REE cycle in biological systems and the expansion of phytotechnologies for REE-enriched or REE-contaminated soils.

High-energy interference-free K-lines synchrotron X-ray fluorescence microscopy of rare earth elements in hyperaccumulator plants

“…However, this may be problematic in many plant samples as co-localization of La-Ce-Sm-Gd occurs in D. erythrosora . 18 Note also that the information depth into the sample (absorption length of characteristic X-ray fluorescence) is different for L-shell and K-shell excitation. The extinction length in hydrated samples is 150 µm for L-lines, but in the centimeter range for K-lines.…”

“… 17 Previously, we showed that it was possible to differentiate between REEs (La, Ce, Sm, Gd, Yb) in fronds of Dryopteris erythrosora , harboring a moderate/high accumulation of REEs (350 to 2000 mg kg −1 DW) but with relatively high concentrations of interfering elements (Fe, Mn). 18 Conversely, differentiation between La and Ce distribution was not possible in D. linearis samples with high prevailing concentrations of total REEs (tREEs) and interfering elements (Fe, Mn). 19 This is a consequence of the reliance on detectors [silicon drift detectors (SDDs) with an energy resolution 125 eV at 5.9 keV, increasing with count rate] that are typically not able to accurately determine the line intensities of the various REEs (due to their overlapping L-shell X-ray emission lines that are separated only by tens of eVs) in the presence of the K-lines of transition elements (such as Ti, Fe, Mn, Cr ).…”