Effect of dissolved organic matter on the uptake of trace metals by American oysters

To examine the effects of dissolved organic matter on metal bioavailability , uptake of trace metals (Cd, Co, Hg, Cr, Ag, Zn) by American oysters (Cras sostrea virginica) was compared between treatments with different dissolved organic carbon (DOC) concentrations and contrasting low molecular weight ( LMW, 1 kDa) and high molecular weight (HMW, 1 kDa-0.2 mum) DOC fractions, u sing radiotracer techniques and short-term exposure experiments. Uptake rat e constants (mL g(-1) h(-1)) of metals, in general, increased with increasi ng DOC concentrations, with an initial decrease at lower DOC concentrations . Oyster dry weight concentration factors (DCF, mt g(-1)), determined at th e end of exposure experiments (8 h), also increased for Cd, Co, Cr, Ag, and Zn, but decreased for Hg, with increasing DOC concentrations. Changes of m etal uptake rate constants and DCF values with DOC concentration suggest th at metal uptake pathways by American oysters vary from predominantly uptake (by diffusion of neutral) of free ionic, inorganically complexed, and LMW organic ligand complexed metals at very low DOC concentration to direct ing estion and digestion of HMW or colloidally complexed metals at higher DOC c oncentrations. Measured partition coefficients (K-c) between dissolved and colloidal phases were comparable between metals, ranging from 10(5.12) to 1 0(5.75) mL g(-1). However, DCF values and uptake rate constants differed co nsiderably between metals, with the highest DCF Values and uptake rate cons tants found for B-type metals, e.g., Ag, Hg, Zn, and Cd, and the lowest one s for several intermediate-type metals (e.g., Co, Cr). Metal types and thus the interaction of metals with organic ligands, such as strong complexatio n of B-type metals with S-containing organic ligands, may play an important role in the bioavailability and toxicity of metals to aquatic organisms. D ifferences in metal uptake in contrasting LMW and HMW DOC treatments sugges t a generally depressed bioavailability of colloidally complexed metals at low DOC concentration (0.5 ppm) but a generally enhanced uptake at higher D OC concentrations.