COPPER AND CADMIUM-BINDING TO FISH GILLS - MODIFICATION BY DISSOLVED ORGANIC-CARBON AND SYNTHETIC LIGANDS

Adult fathead minnows (Pimephales promelas) were exposed to 17 mug Cu . L-1 or 6 mug Cd . L-1 for 2 to 3 h in synthetic softwater solutions at pH 6.2 containing either naturally-occurring, freeze-dried dissolved organic carbon (DOC) or synthetic ligands such as EDTA. After exposures, gills were assayed for bound Cu or Cd. As a first approximation, lake of origin or molecular size fraction of DOC did not influence Cu binding to gills, while DOC concentration did. DOC concentrations greater-than-or-equal-to4.8 mg . L-1 prevented Cu from accumulating on fathead gills. At the relatively low concentrations used, neither Cu nor Cd interfered with binding of the other metal on gills, suggesting different gill binding sites. Cadmium accumulation on gills was more sensitive to increased concentrations of Ca and H+ than was Cu. Surprisingly, Cd bound to gills to the same or greater extent than did Cu: for synthetic ligands, Cd binds less well than Cu. This result corroborates previously published observations that Cd, unlike Cu, is taken up at gills through high affinity Ca channels. Accumulation of Cd on fish gills was never associated with C-14-labelled EDTA or C-14-citrate, indicating that free metal interacts with the gill while metal-ligand complexes usually do not.