| Abstract
| - We examined the bioavailability of colloid-bound metals[Cd, Cr(III), and Zn] to two marine bivalves (mussel Pernaviridis and clam Ruditapes philippinarum) from subtropicaland tropical waters. Natural colloids (between 1 nm and 0.2μm) were isolated by cross-flow ultrafiltration. Bivalveswere then exposed to radiolabeled colloids, and the uptakeof colloid-bound metals was compared with the uptakeof metals associated with the low molecular weight fraction(LMW, <1 kDa). In general, the bioavailability of colloid-bound Zn to mussels was significantly inhibited as comparedto that of the LMW-bound Zn. Its uptake decreased withincreasing colloidal organic carbon (COC) concentration.There was no major difference in Cd bioavailability betweenthe LMW and the colloidal treatments, primarily becausemost of the radiolabeled colloidal Cd remained in the trulydissolved phase during the uptake period. In contrast,the bioavailability of colloid-bound Cr was enhanced in themussels. In clams, bioavailability of metals was notsignificantly influenced by colloidal binding, although Znuptake was slightly enhanced when it was associated withthe LMW fraction. The measured dry weight concentrationfactor (DCF) in bivalve tissue was the highest for Zn,followed by Cd and Cr. Furthermore, DCF was higher inthe mussels (20−340) than in the clams (10−35). Colloid-bound metals were mostly accumulated in mussel digestivegland and remaining soft tissue (64−87%), whereas alarge fraction (36−73%) of metals was found on the shellof the clams. Coagulation of radioactive tagged colloidalorganic matter was insignificant (<9%) for metals in theabsence of large suspended particles, indicating thatcoagulation effects on metal uptake were minimal underour experimental conditions. Thus, our study demonstratedthat colloid-bound metals were bioavailable to both themussels and the clams, but the influences of colloidal bindingon metal uptake varied among metals and between thetwo bivalves.
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