Metal accumulation and toxicity: The critical accumulated concentration of metabolically available zinc in an oyster model

• Invertebrates detoxify accumulated trace metals.
• Toxicity is related to a metabolically available MA (not total) body concentration.
• This MA concentration is theoretical, and as yet undefined biochemically.
• A so-called ‘metal-sensitive fraction’ (MSF) is a physical candidate for MA metal.
• MSF may represent MA, but only in the absence of lysosomal metal detoxification.

Invertebrates typically carry out detoxification of accumulated metals. There is, therefore, no threshold total body concentration of accumulated metal initiating toxicity, the onset of toxic effects rather being related to a critical concentration of metabolically available (MA) accumulated metal. The challenge remains as to whether any particular combination of subcellular fractions of accumulated metal can be identified to represent this theoretical MA component. One candidate combined fraction is the so-termed metal sensitive fraction (MSF), consisting of metal bound to organelles and non-detoxificatory soluble proteins. In this study, we used laboratory zinc accumulation and toxicity data for four populations of the oyster Crassostrea hongkongensis with different histories of zinc exposure in the field to address the challenge. We conclude that in a ‘control’ population of the oyster, the MSF does approximate to the theoretical metabolically available zinc concentration. In populations with a history of field exposure to raised zinc bioavailabilities, however, the MSF would include more zinc detoxified in the lysosome component of organelle-bound metal, and the MSF in such populations would deviate more from the theoretical MA metal concentration.