Nanoscale distribution of potentially toxic elements in seaweeds revealed by synchrotron X-ray fluorescence

Assessing the impact of potentially toxic elements (PTEs) on coastal waters requires understanding their interaction with seaweeds, as they are foundational organisms in nearshore ecosystems. While seaweeds are known to accumulate PTEs, information on the mechanisms and locations of this accumulation is very limited. We employed synchrotron-based X-ray fluorescence (S-XRF) to map the distribution of various chemical elements at nanometric resolution in Fucus vesiculosus, a brown alga dominant in intertidal zones. Our results revealed that different PTEs have distinct subcellular accumulation patterns, which are largely consistent across different samples and cell layers. The distribution of Ba and U was predominantly located in the cell walls, while Co, Ni, Cu, and Zn were concentrated within physodes, specialized organelles containing phlorotannins. These findings suggest that F. vesiculosus regulates PTE uptake either by preventing their accumulation, or by sequestering them into specialized organelles. Physodes seem to play a key role in this regulation, concentrating divalent elements to minimize their toxicity and potentially storing them for intracellular functions. The use of S-XRF allowed us to map elemental distributions on seaweed cells with unprecedented detail, furthering our understanding of the subcellular structures responsible for PTE accumulation, and setting the foundation for studying their uptake mechanisms.