Toxicity of Cd, Cu, Pb, Ni, and Zn to Chironomids, and Trophic Transfer of Cd from Chironomids to Zebrafish

Abstract

The toxicity of the metals: Cd, Cu, Pb, Ni, and Zn to the freshwater aquatic larvae of the midge fly Chironomus riparius was investigated using 24 h waterborne exposures. Even at the most sensitive life stage, first instar, the chironomids were extremely metal tolerant, with LC50 values for all metals being orders of magnitudes above both the CCME Canadian water quality guidelines for the protection of aquatic life, and the USEP A Water Quality Criteria. This high tolerance of C. riparius to metal toxicity, combined with an exceptional ability to accumulate and tolerate high internal metal burdens makes the chironomid an ideal organism to use in studies on factors affecting the trophic transfer of Cd. Zebrafish were fed with Cd-contaminated chironomids for 7 days, followed by 3 days of gut clearance with clean chironomids. Chironomids loaded with Cd by exposure to Cd-contaminated sediments exhibited a significantly higher trophic transfer efficiency (TTE) than did zebrafish 1 fed chironomids contaminated with Cd by waterborne exposure, although in both cases the TTE's were low (<2%). The majority ofCd transferred to zebrafish was stored in the gut and carcass, irrespective of ingestion of a natural diet (chironomids loaded with Cd), or a manufactured pellet diet of identical Cd concentrations. On a tissue concentration basis, the highest tissue accumulations in zebrafish were (in decreasing order): kidney> gut> liver> gill> carcass; this accumulation pattern w;ls also independent of concentration of Cd in the diet or of prey exposure route. Subcellular fractionation of chironomids found most of the Cd in the metal rich granule fraction followed by the organelle fraction. It also revealed that sediment exposed chironomids had significantly more Cd in the metallothionein-like protein fraction, and significantly less Cd in the cellular debris fraction than water-borne exposed chironomids, although these fractions accounted for only a small percent (~7%) of the total accumulated Cd. Despite this difference in prey subcellular fractionation, the subcellular storage of Cd in zebrafish fed on sediment-exposed chironomids and zebrafish fed on water-borne exposed chironomids was the same, with the highest accumulations in the organelles, enzymes, and metal rich granules fractions. Main areas of subcellular storage in zebrafish fed on a manufactured pellet food were identical to those ofzebrafish fed on chironomids. However, zebrafish fed on chironomids had significantly more Cd in the metallothionein-like protein fraction (5-10%) than did zebrafish fed on pellets :o%). Overall, TTE's were independent of concentration, but were dependent on route of prey exposure. Tissue-specific accumulations and tissue-specific distributions in zebrafish were independent of both concentration and route of prey exposure, and the gut consistently accounted for the highest proportion of overall body burden, and had the highest accumulation of all tissues; verifying its importance in preventing the internalization of Cd. Subcellular distributions were also independent of concentration, although zebrafish fed a biological food (chironomids) had more Cd in the metallothionein-like protein fraction than zebrafish fed a manufactured food. This has biological implications for Cd detoxification.