Ni, Cu, and Zn isotopic compositions of bivalves and gastropods from the Korean coast

Nickel (Ni), copper (Cu), and zinc (Zn) are essential trace elements for marine organisms, and their deficiency can affect the growth and metabolism of these organisms. However, they are widely recognized as elements that cause toxic effects when they bioaccumulate in excessively high concentrations in organisms. Bivalves are sessile organisms that live attached to sediments or the water column and are considered important organisms for environmental pollution monitoring because they can accumulate pollutants, such as trace metals, in their bodies from the surrounding environment. Advances in isotope analysis technology have enabled the use of stable metal isotopes to determine whether metals bioaccumulate in bivalves. In the case of gastropods, they prey not only on seaweed but also on other shellfish, providing scientific information for the study of metal accumulation and isotope fractionation processes along the food chain. However, there is no research on the stable isotopes of metals. Therefore, this study presented the concentrations and stable isotope compositions of Ni, Cu, and Zn in Korean coastal bivalves and gastropods, and investigated bioaccumulation and isotope fractionation according to species, food chain, size, and habitat type.

Bivalve and gastropod samples were purchased from 16 fish markets along the Korean coast and completely digested on a hot plate using a closed digestion method with a mixture of acids. The concentrations of metals (Ni, Cu, Zn) in mollusks were analyzed using inductively coupled plasma mass spectrometry (ICP-MS; iCAP-Q). For high-precision, stable isotope analysis of Ni, Cu, and Zn, a three-step purification process was performed using AG 50W-X8, AG MP1, and Chelex-100. The isotope composition was then measured using a multi-collector ICP-MS at the Korea Institute of Ocean Science and Technology (KIOST). The accuracy of concentration and isotope measurements was verified using five certified reference materials (ERM-CE278k, SRM-1566b, IAEA-452, IAEA-461, BCR-668), and the results were consistent with reported values.

Oysters (Crassostrea gigas) had the highest Cu (113.48 mg/kg) and Zn (582.19 mg/kg) concentrations among bivalves, while their average Ni concentration (0.38 mg/kg) was the lowest. The concentrations of Cu and Zn in bivalves were within similar ranges except for oysters, but there were significant differences in isotopic composition. The average Cu isotope values in bivalves ranged from -0.18 to +1.43‰, and the Zn isotope showed a difference of 1.06‰ in the Mactra quadrangularis (+0.17‰) and the Lamarcka avellana (+1.23‰). The average concentrations of Ni, Cu, and Zn in marine gastropods were 2.41, 86.5, and 270.1 mg/kg, respectively, with a large concentration difference of 13 to 86 times depending on the species. Although the average Ni concentration in gastropods was lower than in bivalves, the concentrations of Cu and Zn were 6.1 and 1.9 times higher, respectively, in gastropods than in bivalves. The Cu isotope fractionation of Batillus cornutus was mainly negative, with a maximum difference of 1.21‰ from Rapana venosa, which had the heaviest isotope data. Both minimum (+0.18‰) and maximum (+0.81‰) values ​​of Zn isotopes appear in Rapana venosa, showing that isotopic composition varies depending on habitat and size, even within the same species.