Mapping potentially toxic elements in seaweed cells using X-ray fluorescence

Seaweeds play a foundational role in marine coastal ecosystems, which are increasingly affected by pollution. Understanding how PTEs interact with seaweeds requires detailed knowledge of their cellular localization within seaweed cells, as compartmentalization determines toxicity levels.

Before the emergence of nano-imaging techniques, attempts to resolve this question were hindered by methodological limitations [1]. Previous studies using nano-imaging techniques have been limited to zinc and arsenic distributions in the kelp Laminaria digitata [2,3], leaving significant gaps in knowledge regarding other potentially toxic elements.

This present study addressed these gaps by analysing samples of the brown seaweed F. vesiculosus collected from eight sites along the coast of north-west Spain, representing a gradient of pollution. Cryofixation was used to preserve the localization of PTEs, and 200-nm-thick samples were analysed using nanoscale X-ray fluorescence imaging at beamline ID16B. This technique provided detailed maps of elemental concentrations across different cellular structures, as illustrated in Figure 1.

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Fig. 1: Images of the cells of F. vesiculosus with numbered subcellular structures. A) TEM image of cells from the meristoderm (upper cells) and cortex (lower cells). Images (A) and (C) were obtained from 70-nm ultrathin sections. B) Diagram highlighting and colour-coding the structures identified in image (A). C) TEM image of two meristoderm cells. D) Image obtained using beamline ID16B on 200-nm-thick unstained sections. (The colour of each pixel represents the concentration of osmium, added during the fixation of the samples. The numbers represent the following structures: 1: nucleus; 2: plastids; 3: physodes; 4: vacuoles; 5: mitochondria, 6: cell wall, 7: polysaccharides.)

The findings revealed that metabolically essential PTEs, such as cobalt, copper, nickel, and zinc, were primarily located within specialized organelles called physodes, which are unique to brown algae and contain phenolic compounds called phlorotannins (Figure 2D).

PTEs without metabolic functions, including barium and uranium, were largely excluded from the cells and localized in the cell walls and external polysaccharides. This was also the case for strontium, which, although not potentially toxic, does not serve significant metabolic functions (Figure 2F).

Elements such as manganese were concentrated in plastids due to their photosynthetic roles (Figure 2C), while calcium was distributed both intracellularly and in the cell wall, where it plays a structural role (Figure 2A). Iron and chromium were detected in mineral particles on the surface of the seaweed and in some cells, whose concentrations of these elements were much higher than those found in cellular structures (Figure 2B). Silver, when present, was attributed to the presence of an isolated silver nanoparticle – an emerging pollutant. 
 

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Fig. 2: Elemental distribution maps for (A) calcium, (B) iron, (C) manganese, (D) nickel, (E) osmium, and (F) strontium obtained using beamline ID16B at 300 nm-resolution. Colour represents the concentration of each pixel for the indicated element. The minimum limit is always 0 mg g^-1, while the maximum varies by element: 80 mg g^-1 for Ca, 4 mg g^-1 for Fe, 1.6 mg g^-1 for Mn, 6 mg g^-1 for Ni, 250 mg g^-1 for Os, and 4 mg g^-1 for Sr.

These findings suggest that brown seaweeds regulate PTE toxicity by preventing the internalization of non-metabolically essential PTEs and by sequestering essential ones within specific organelles. The study also provides evidence that physodes play a role in mitigating PTE toxicity and in offering UV protection, given the concentration of elements like zinc, which can absorb UV light in specific forms. Additionally, the higher internalization of copper compared to other non-essential PTEs likely explains its greater toxicity to seaweeds, since most toxic effects are restricted to the intracellular compartment.

This research offers the most comprehensive data to date on elemental distributions in seaweeds and contributes to understanding how pollution impacts seaweed physiology. It also highlights the need for further studies to confirm these patterns in other PTEs, such as lead and cadmium, and to investigate whether similar regulatory mechanisms are present in other seaweed groups, including red and green algae. Future research employing molecular techniques could help clarify the processes governing PTE internalization and compartmentalization, enhancing predictions about the resilience of seaweed communities to pollution and global environmental changes, and thus aiding in the conservation and management of coastal ecosystems. 

Principal publication and authors
Nanoscale distribution of potentially toxic elements in seaweeds revealed by synchrotron X-ray fluorescence, A. Vázquez-Arias (a), M.T. Boquete (a), B. Martín-Jouve (b), R. Tucoulou (c), C. Rodríguez-Prieto (d), J.Á. Fernández (a), J.R. Aboal (a), J. Hazard. Mater. 480, 136454 (2024); https://doi.org/10.1016/j.jhazmat.2024.136454 
(a) Universidade de Santiago de Compostela, Santiago de Compostela (Spain)
(b) National Centre for Biotechnology, Madrid (Spain)
(c) ESRF
(d) Universitat de Girona, Girona (Spain)

References
[1] A. Vázquez-Arias et al., Sci. Total Environ. 856, 159215 (2023).
[2] A. Mijovilovich et al., Metallomics 15, 58 (2023).
[3] E. Ender et al., J. Anal. At. Spectrom. 34, 2295 (2019).