Environmental Context. Trace metals are both micronutrients and toxicants, depending on concentration, and in coastal waters they bind to natural organic matter including nanoparticles. The binding type affects trace metal bioavailability to bivalves such as oysters, which ingest metals through water and food particles. Bivalves are biomonitors because of the high trace metal concentrations, especially Cu and Zn, in their tissues. Here, the polysaccharide alginic acid is shown to protect against the assimilation and bioavailability of trace metals to American oysters. Abstract. Little is known about how colloidal macromolecular organic matter (COM) modifies the bioavailability of toxic metals to aquatic organisms. In order to understand the physical and chemical properties of COM on the bioavailability of some metals to estuarine bivalves used as biomonitors, American oysters (Crassostrea virginica) were exposed to natural COMs and model acid polysaccharides (APS, alginic acid (AA), kappa carrageenan (CAR), and latex particles), and natural colloidal organic carbon (COC), tagged with either radioactive Ag, Cd, Co, Cr, Fe, Hg or Zn, or 14C-labelled sugar OH groups. Filter-feeding oysters efficiently removed latex particles 0.04 m in diameter, with removal half-times of 2.55.5 h, equivalent to a filtration rate of approximately 3 L day1 g1. Thus, AA protects against metal uptake by oysters, which is confirmed by metal dry-weight concentration factors (DCFs) similar to, or lower than, those for 14C-labelled AA. However, metal-DCFs for CAR and COC were higher than for 14C-labelled counterparts, suggesting that in these treatments, metal uptake was enhanced over that of carbon. The 14C-labelled AA was taken up significantly more than other 14C-labelled organics, suggesting different behavior in the digestive tract. Bioavailabilty of metals bound to organic nanoparticles with different nutritional and physiological properties is not fully understood, and will require further experiments.