Effects of elevated CO2 levels on subcellular distribution of trace metals (Cd and Cu) in marine bivalves

Hypercapnia (elevated CO2 levels) and pollution with trace metals such as Cu and Cd are common stressors in estuarine habitats that can negatively affect physiology and health of marine organisms. Hypercapnia can modulate toxicity of trace metals including Cu and Cd; however, the physiological and cellular mechanisms of the metal-CO2 interactions are not well understood. We investigated the effects of elevated PCO2 (∼800 and 2000 μatm) and metal exposure (50 μg l−1 of Cu or Cd) on subcellular distribution of metals in two common species of marine bivalves, Eastern oysters Crassostrea virginica and hard shell clams Mercenaria mercenaria. Oysters accumulated higher burdens of Cu and Cd in the gill tissues compared to clams. In both studied species, Cu was predominantly associated with the metabolically active cell compartments (mitochondria, lysosomes, microsomes and cytosolic enzymes), with a modest fraction sequestered by metallothioneins (∼30%) and the insoluble metal-containing granules (MCG) (∼15-20%). Unlike Cu, Cd was largely sequestered by metallothioneins (∼60-70%), with a relatively small fraction associated with the organelles and the cytosolic enzymes. Mitochondria were the main intracellular target for trace metals accumulating higher concentrations of Cd (and in the case of oysters - of Cu) than other organelles or cytosolic enzymes. Cu accumulation in the metabolically active cellular compartments was independent of the CO2 levels, while Cd content of the organelles and cytosolic enzymes increased at elevated PCO2 in both studied species indicating that hypercapnia may enhance cellular toxicity of Cd in bivalves. Hypercapnia suppressed the sequestration capacity of metallothioneins for Cu and Cd in oysters but increased Cu and Cd load in clam metallothioneins. Thus, metal-induced metabolic injury in oysters may be exaggerated by hypercapnia which enhances metal accumulation in the potentially sensitive intracellular fractions and suppresses the metal detoxification capacity. In contrast, clams appear to be more resistant to the combined effects of hypercapnia and metal exposure reflecting more efficient and robust detoxification mechanisms of this species.