Ocean Acidification

Estuarine and coastal habitats experience large fluctuations of environmental factors such as temperature, salinity, partial pressure of CO₂ (P_CO2) and pH; they also serve as the natural sinks for trace metals. Benthic filter-feeding organisms such as bivalves are exposed to the elevated concentrations of metals in estuarine water and sediments that can strongly affect their physiology. The effects of metals on estuarine organisms may be exacerbated by other environmental factors. Thus, a decrease in pH caused by high P_CO2 (hypercapnia) can modulate the effects of trace metals by affecting metal bioavailability, accumulation or binding. To better understand the cellular mechanisms of interactions between P_CO2 and trace metals in marine bivalves, we exposed isolated mantle cells of the hard clams (Mercenaria mercenaria) to different levels of P_CO2 (0.05, 1.52 and 3.01 kPa) and two major trace metal pollutants – cadmium (Cd) and copper (Cu). Elevated P_CO2 resulted in a decrease in intracellular pH (pH_i) of the isolated mantle cells from 7.8 to 7.4. Elevated P_CO2 significantly but differently affected the trace metal accumulation by the cells. Cd uptake was suppressed at elevated P_CO2 levels while Cu accumulation has greatly accelerated under hypercapnic conditions. Interestingly, at higher extracellular Cd levels, labile intracellular Cd²⁺ concentration remained the same, while intracellular levels of free Zn²⁺ increased suggesting that Cd²⁺ substitutes bound Zn²⁺ in these cells. In contrast, Cu exposure did not affect intracellular Zn²⁺ but led to a profound increase in the intracellular levels of labile Cu²⁺ and Fe²⁺. An increase in the extracellular concentrations of Cd and Cu led to the elevated production of reactive oxygen species under the normocapnic conditions (0.05 kPa P_CO2); surprisingly, this effect was mitigated in hypercapnia (1.52 and 3.01 kPa). Overall, our data reveal complex and metal-specific interactions between the cellular effects of trace metals and P_CO2 in clams and indicate that variations in environmental P_CO2 may modulate the biological effects of trace metals in marine organisms.

Ivanina A. V., Beniash E., Etzkorn M., Meyers T. B., Ringwood A. H. & Sokolova I. M., in press. Short-term acute hypercapnia affects cellular responses to trace metals in the hard clams Mercenaria mercenaria. Aquatic Toxicology. Article (subscription required).