Effects of combined PS-Pb pollution on the calcification of Paphia undulata: Coexistence of antagonistic and synergistic effects

Similar to respiration, shellfish function as a carbon source because they release carbon dioxide during calcification which uses soluble inorganic carbon in seawater to form biological calcium deposits, such as shells, which is important for shellfish survival. Almost all filter feeding shellfish feed mainly on phytoplankton, which are the most dominant primary producers in marine ecosystems. Microplastics and Pb generally coexist in the habitat of Paphia undulata, and microplastics adsorb Pb. Both of these two matters negatively affect metabolism that affects calcification. Thus, it is speculated that the interaction between microplastics and Pb affects the calcification of P. undulata by affecting its metabolism. This hypothesis is related to the carbon cylce drived by filter feeding shellfish under combined pollution of microplastic and Pb. Nevertheless, related studies are extremely rare. To verify this hypothesis, P. undulata was exposed to PS-Pb combined pollution for 96 h, during which microplastics and Pb were represented by polystyrene (PS) with a particle size of 5.0 µm and Pb (NO3)2, respectively. Ammonia excretion was affected by PS and the interaction of PS and Pb. Not the oxygen consumption but the ratio of O/N was affected by PS, Pb and their interaction. If the concentrations of PS and Pb were 50 and 6 μg/L, respectively; the ratio of O/N of P. undulata was higher than that under the other treatments. The calcification was influenced by Pb and the interaction of PS and Pb, e.g., as the concentrations of PS and Pb were 25 and 6 μg/L, respectively, the calcification rate of P. undulata was the lowest. Moreover, the calcification rate was positively correlated with the ratio of O/N, i.e., there was a correlation between the calcification rate and metabolic substrates. PS and Pb at low concentrations had a synergistic effect on calcification, while they tended to produce antagonism at high concentrations. All these facts mean that the effect of compound pollution on calcification is not a simple addition of the effects of the two pollutants that influence calcification by affecting metabolism, and the effects of the PS-Pb combined pollution on the carbon cycle in offshore areas should be reassessed.