Influence of environmentally relevant concentrations of Zn, Cd and Ni and their binary mixtures on metal uptake, bioaccumulation and development in larvae of the purple sea urchin Strongylocentrotus purpuratus.

Metal accumulation, disturbance of Ca2+ homeostasis, and occurrence of abnormalities are well-established consequences of single metal exposure during early development stages of sea urchins. However, the effects caused by low concentrations of metals and metal mixtures need to be better understood in marine invertebrates. Therefore, the present study investigated the effects of environmentally relevant concentrations of Zn (9 µg/L), Cd (30 µg/L) and Ni (5 µg/L) in single and binary exposures (Zn + Cd, Cd + Ni and Ni + Zn) to the early life stages of the purple sea urchin Strongylocentrotus purpuratus. Endpoints checked in all treatments after 48-h exposure were unidirectional metal influx rates, bioaccumulation, and Ca2+ influx rates. Additionally, the presence of abnormal larvae and developmental delay was evaluated at 24 h, 48 h and 72 h of exposure. Unidirectional influx rates of all three metals were significantly higher than control background rates in all single exposures and binary mixtures, and were generally not different between them. Net accumulation (body burden) of both Zn and Cd increased significantly as a result of their respective single exposures, while Ni accumulation decreased considerably. When Zn or Cd were presented in binary exposures with other metals, the net accumulations of Zn or Cd were reduced relative to single exposures to these metals, whereas this did not occur for Ni accumulation. Thus, bioaccumulation proved to be a better metric than influx rate measurements to analyze metal competition at a whole organism level at these low metal concentrations. Short-term Ca2+ influx also did not appear to be a sensitive metric, as the measured rates did not vary among all single and binary exposures, with the exception of a lower rate in Ni + Zn binary exposure. A critical aspect observed was the relationship between bioaccumulation versus influx measurements, which proved positive for Cd, but negative for Zn and Ni, demonstrating possible capacities for both Zn and Ni regulation by sea urchin larvae. Increases in larval abnormalities relative to controls occurred only after binary mixtures, starting at 48 h exposure and maintained until 72 h. However, delay of the sea urchin development by the presence of gastrula stage at 72 h exposure occurred in Zn and Ni single exposures and all metal mixtures, with very high abnormal development when Ni was present.

Effects of sublethal Cd, Zn, and mixture exposures on antioxidant defense and oxidative stress parameters in early life stages of the purple sea urchin Strongylocentrotus purpuratus.

Oxidative stress parameters were evaluated during the first 72 h of embryonic development of purple sea urchin Strongylocentrotus purpuratus continuously exposed to control conditions, to cadmium alone (Cd, 30 µg/L), to zinc alone (Zn, 9 µg/L) or to a Cd (28 µg/L) plus Zn (9 µg/L) mixture. These sublethal concentrations represent ∼ 10% of the acute EC50. Bioaccumulation, antioxidant capacity against peroxyl radicals (ACAP), total glutathione (GSH) level, glutathione-S-transferase (GST), glucose-6-phosphate dehydrogenase (G6PDH) and superoxide dismutase (SOD) activity, and lipid peroxidation (LPO) were analyzed at 24 h (blastula), 48 h (gastrula), and 72 h (pluteus) stages of development. Zinc (an essential metal) was well-regulated, whereas Cd (non-essential) bioaccumulated and whole-body [Cd] increased from blastula to pluteus stage in sea urchin larvae. In controls, ACAP progressively declined from 24 h to 72 h, while LPO reciprocally increased, but other parameters did not change. Cd alone was more potent than Zn alone as a pro-oxidant, with the major effects being decreases in SOD activity and parallel increases in LPO throughout development; GST activity also increased at 24 h. Zn alone caused only biphasic disturbances of ACAP. In all cases, the simultaneous presence of the other metal prevented the effects, and there was no instance where the oxidative stress response in the presence of the Cd/Zn mixture was greater than in the presence of either Cd or Zn alone. Therefore the sublethal effects of joint exposures were always less than additive or even protective, in agreement with classical toxicity data. Furthermore, our results indicate that SOD and Zn can play important roles in protecting sea urchin embryos against Cd-induced lipid peroxidation.