Copper and zinc detoxification in Gammarus pulex (L.)

To negate the toxicity of labile intracellular metals, some aquatic organisms partition metals into specific subcellular locations for detoxification, namely the soluble heat-stable cytosol and insoluble metal-rich granules. The aim of the present study was to characterise these subcellular storage sites in the freshwater crustacean Gammarus pulex (Linnaeus) following in situ exposures upstream (Drym, low metal) and downstream (Relubbus, elevated metal) of copper- and zinc-rich inflows into the River Hayle (Cornwall, UK). In the cytosol of gammarids exposed at Relubbus, copper and zinc associated to a 7.5-kDa metallothionein-like protein (MTLP) that was largely absent from gammarids prior to exposure. Exposure at Relubbus caused MTLP concentrations to increase 4- to 5-fold between days 2 and 4, indicating an induction response to increased labile intracellular metal. On day 16, spherical calcium-rich granules (0.5-2.5 μmol l(-1)) were visualised and analysed in the posterior caeca of gammarids exposed at both sites. Following exposure at Relubbus, granules contained trace amounts of copper, but zinc was absent. Granules in gammarids exposed at Drym contained no detectable copper or zinc. Granule formation appeared to be independent of exposure. Within the posterior caeca, granules have been associated with calcium storage during the crustacean molt, rather than in detoxification of trace metals. However, the granular copper burden appeared to follow environmental Cu availabilities. Thus, we describe Cu sequestration within molt-cycle calcium storage granules. As both MTLP concentrations and granule formation in crustaceans are affected upon by molting, we hypothesise that detoxification might impact upon this existing process.