Exposure pathways and species-specific rare earth element accumulation in a Mediterranean shallow hydrothermal system

Rare earth elements (REEs) are increasingly recognised as emerging contaminants, yet their transfer from sediment and water to biota remains poorly understood in coastal marine systems characterised by shallow-water hydrothermal vents. These vents release REEs at concentrations approaching those of anthropogenically impacted sites, providing a natural laboratory to investigate their transfer to marine biota. REE distribution and sediment-to-biota transfer were characterised along the pH gradient of the shallow CO₂-vent system of Levante Bay (Vulcano Island, southern Tyrrhenian, Italy), combining sediment analyses with those of three co-occurring, functionally distinct organisms representing complementary exposure pathways: the seagrass Cymodocea nodosa, the sea anemone Anemonia viridis, and the benthic goby Gobius incognitus. REE concentrations decreased overall along the sequence: sediment > C. nodosa > G. incognitus > A. viridis. Biota-Sediment Accumulation Factor (BSAF) values were below unity in all matrices at both areas, indicating limited transfer despite hydrothermal enrichment. Post-Archean Australian Shale (PAAS) normalised REE patterns, diagnostic anomalies and multivariate ordination revealed three distinct biotic signatures in species sharing the same hydrothermally enriched environment: a sediment-like, light REE-enriched profile in A. viridis; a relatively heavy REE-enriched, biologically fractionated profile in C. nodosa; and a low, dispersive profile in G. incognitus. The latter was the only matrix showing higher BSAF at both low-pH sites, possibly reflecting the dietary pathway. Differences in REE accumulation among matrices exceeded those between the two pH conditions. These results indicate that co-occurring species exhibit distinct REE signatures and that the choice of bioindicator embeds a specific exposure pathway.