Effects of environmental hypercapnia and metal (Cd and Cu) exposure on acid-base and metal homeostasis of marine bivalves

Environmental hypercapnia (elevated CO2 levels) is a common environmental stressor in estuaries which leads to changes in seawater pH and may affect bioavailability of trace metals to estuarine organisms. We studied the interactive effects of common metal pollutants (50 μg l- 1 Cd or Cu) and PCO2 (~ 395, 800 and 2000 μatm PCO2) on metal levels, intracellular pH, expression of metal binding proteins and cellular stress biomarkers in estuarine bivalves, the eastern oysters Crassostrea virginica and hard shell clams Mercenaria mercenaria. Cd (but not Cu or hypercapnia) exposure affected the acid-base balance of bivalve hemocytes resulting in an increase in the intracellular pH. Cd and Cu exposure led to the increase in the total metal burdens in bivalve tissues, and metal accumulation was reduced by elevated PCO2 in the mantle but not hemocytes. No change was found in the intracellular concentrations of free Cd2 +, Cu2 + or Fe2 + during Cu or Cd exposure indicating that these metals are bound to intracellular ligands. Free Zn2 + content in oyster hemocytes was suppressed by Cd and Cu exposure, and was below the detection limits in all treatments in clam hemocytes. The low levels of free intracellular metals went hand-in-hand with the elevated mRNA expression of metal binding proteins (metallothioneins and ferritin) in Cd- and Cu-exposed bivalves, which was enhanced by hypercapnia. Under most experimental conditions, the metal-binding and antioxidant mechanisms of oysters and clams were sufficient to effectively maintain intracellular redox status, even though metal exposure combined with moderate hypercapnia (~ 800 μtm PCO2) led to the elevated production of reactive oxygen species in hemocytes and elevated antioxidant capacities of the gill tissue indicating oxidative stress. Our study shows that hypercapnia modulates accumulation of Cd and Cu, the tissue metal-binding capacity and metal-induced oxidative stress in estuarine bivalves, but physiological effects of hypercapnia (~ 800-2000 μatm PCO2) are mild compared to the effects of other common stressors in estuaries.

Ivanina A. V., Hawkins C., Beniash E. & Sokolova I. M., in press. Effects of environmental hypercapnia and metal (Cd and Cu) exposure on acid-base and metal homeostasis of marine bivalves. Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology. Article (subscription required).

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