Physiological response to a metal-contaminated invertebrate diet in zebrafish: importance of metal speciation and regulation of metal transport pathways.

Dietary metal uptake in fish is determined by metal bioavailability in prey and the metal requirements of the fish. In this study zebrafish were fed the intertidal polychaete worm Nereis diversicolor (3% wet weight day(-1)) collected from Ag, Cd and Cu-impacted Restronguet Creek (RC) or a reference site, Blackwater estuary (BW), for 21 days. On days 0, 7, 14 and 21 fish were fed a single meal of RC or BW N. diversicolor labeled with (110m)Ag or (109)Cd for measurements of metal assimilation efficiency (AE). Zebrafish intestines were also taken for mRNA expression analysis of copper transporter 1 (ctr1), divalent metal transporter 1 (dmt1) and metallothionein 2 (mt2). No significant difference was observed in the AE of (109)Cd in metal naïve fish at day 0 between RC and BW worms, 11.8±2.1 and 15.3±2.8%, respectively. However, AE of (110m)Ag was significantly greater in fish fed worms from BW compared to RC, 5±1.2% and 1.6±0.5%, respectively at day 0. Fractionation analysis of radiolabeled metal partitioned in N. diversicolor from RC revealed a greater proportion of Ag (40±1.1%) in a fraction containing protein and organelle bound metal, associated with high trophic availability, compared to BW polychaetes (24±2.5%). Lower AE of (110m)Ag from RC polychaetes is therefore unlikely due to speciation of (110m)Ag in N. diversicolor from RC, but to the high concentration of Cu, a potential Ag antagonist. Exposure to RC polychaetes significantly increased the AE of (110m)Ag (6.2±1%), but not (109)Cd, from RC worms, after 21 days. AE of (110m)Ag and (109)Cd was unaffected by pre-exposure to BW. Elevated concentration of intestinal Cu and increased expression of ctr1, dmt1 and mt2 after 14 days exposure in fish fed worms from RC suggest altered Cu handling strategy of these fish which may increase AE of Ag via shared Ag and Cu transport pathways. These data suggest metal exposure history of invertebrates may affect metal bioavailability to fish, and fish may alter intestinal uptake physiology during chronic dietary exposure with implications for the assimilation and toxicity of dietary metals.