Bioavailability of colloid-bound Cd, Cr, and Zn to marine plankton

The uptake of colloid-bound Cd, Cr, and Zn by marine plankton (including the diatom Thalassiosira pseudonana, the dinoflagellate Prorocentrum minimum, and the copepod Paracalanus aculeatus) was studied using the radiotracer technique. Natural organic colloids (operationally defined as particles between 1 kDa and 0.2 μm) were isolated from Hong Kong coastal seawater using cross-flow ultrafiltration, and radiolabeled with ¹⁰⁹Cd, ⁵¹Cr, and ⁶⁵Zn. The uptake of colloid-bound metals by plankton was then compared with the uptake of metals in the low molecular weight fraction (LMW, <1 kDa). Our results indicate that colloids enhanced the initial surface sorption of Cd and Cr onto the algal cells. The rates of subsequent uptake were comparable between the colloidal- and LMW-complexed Cr, whereas the uptake of colloidal Cd and Zn was significantly depressed by colloidal binding. The overall bioavailability of colloidal Cd and Cr to phytoplankton was therefore comparable to or higher than that of LMW metals. In contrast, the bioavailability of colloidal Zn to phytoplankton was significantly lower than that of the LMW Zn. Similar to that for phytoplankton, the bioavailability of colloidal Cr to copepods was significantly enhanced whereas the bioavailability of colloidal Cd to copepods remained relatively unchanged, and the bioavailability of Zn was significantly depressed compared to their respective LMW metals. Direct ingestion of colloidal metals was unlikely because no consistent trend between uptake and colloidal organic carbon concentration was found for Cd, Cr, and Zn. Our study therefore demonstrated that colloidal metals are available to marine plankton and could be actively involved in planktonic food webs. Thus, colloids are important not only in scavenging trace metals, but also in the interactions between metals and the biota in aquatic environments.