Heavy metal accumulation and interaction dynamics in Brachidontes pharaonis: a bioindicator study in the Red Sea

This work evaluates utilizing the native mussel Brachidontes pharaonis as a bioindicator and sentinel organism for monitoring heavy metals Cu, Zn, and Cd along the Red Sea coast of Egypt. Samples were collected from four coastal locations, and the concentrations of heavy metals in the mussels’ tissues, shells, seawater, and sediments were analyzed. Subsequently, bioassay experiments were conducted by exposing the organisms to single, binary, and tertiary metal mixtures, and the accumulation of heavy metals was determined to elucidate the dynamics of metal–metal interactions. Field samples revealed significant variations in heavy metal concentrations in the mussels’ soft tissues across different locations, with Zn ranging from 58.1 to 121.0 µg/g dw (dry weight), Cu ranging between 18.3 and 36.7 µg/g dw, and Cd ranging from 0.3 to 1.04 µg/g dw. Conversely, the shells exhibited minimal spatial variations, with much lower contents of Cu (ranging from 1.9 to 2.8 µg/g dw) and Zn (ranging from 1.8 to 1.9 µg/g dw). However, the shells accumulated Cd at higher levels (ranging from 1.4 to 2.1 µg/g dw) compared to the soft tissues. Following a 96-h bioassay experiment, the soft tissues displayed a linear accumulation of metals with increasing exposure dose, with Cd showing the highest accumulation rate (approximately threefold) followed by Zn (twofold) and Cu (1.7-fold). In binary and tertiary exposures, the metals exhibited a general antagonistic interaction, affecting each other’s accumulation. On the other hand, the accumulation of heavy metals in the shells after the 96-h bioassay exposure did not follow a consistent linear pattern, suggesting that accumulation during this short experimental period occurs primarily through adsorption rather than the biological pathway.