Oyster-mussel differences in cadmium accumulation linked to molecular weight distribution of Cd-binding proteins

Marine bivalves are key seafood resources and biomonitors, yet closely related species show striking differences in cadmium (Cd) accumulation. While toxicokinetics suggest these differences are largely driven by Cd uptake, given consistently low elimination across species, the molecular mechanisms remain poorly understood. Here, we applied size-exclusion chromatography hyphenated to inductively coupled plasma-mass spectrometry (SEC-ICP-MS) and stable isotope tracing to characterize cytosolic Cd-binding proteins in three oysters (Crassostrea gigas, C. hongkongensis, C. angulata) and three mussels (Perna viridis, Mytilus coruscus, and M. galloprovincialis) common in Chinese coastal waters. Oysters generally exhibited higher tissue Cd (up to tenfold higher than mussels) but stored a smaller fraction in the cytosol (35-48 %) compared to mussels (66-74 %), with oyster C. angulata being an exception (70 %). The molecular size distribution of cytosolic Cd-binding proteins also differed markedly: oysters bound a large share of Cd to high molecular weight proteins (>44 kDa, 35-72 %), whereas mussels relied more on medium or low molecular weight proteins, with P. viridis showing over 60 % Cd bound to low molecular weight ligands (∼2 kDa). Stable isotope tracing further showed that newly accumulated Cd rapidly adopted species-specific binding patterns similar to pre-existing Cd, indicating fast intracellular equilibration. In contrast, essential metals such as Cu and Zn showed more dispersed binding across protein sizes, and Pb exhibited weaker, less specific associations. These results demonstrate that interspecific differences in Cd burdens are closely linked to protein size-dependent binding strategies. Such mechanistic insights strengthen cross-species assessments for food safety and environmental monitoring.