Abstract
Cadmium (Cd), one of the most toxic metals found in inshore sediments of China, is a persistent environmental contaminant capable of exerting irreversible toxic effect on aquatic organisms and their associated ecosystems. Although Cd is known to be toxic to marine animals, the underlying mechanism of this toxicity is not clear. In this study, Meretrix meretrix, a commercially and ecologically important species of clam, was exposed to different concentrations of cadmium chloride (0, 1.5, 3, 6, 12 mg L-1) for 5 days, and the levels of Cd accumulation, antioxidant enzyme activity and expression of metallothionein (MT) in the hepatopancreas, gill, foot and mantle were evaluated. The results revealed a sharp increase in Cd accumulation in the tissues in response to increased Cd2+ concentrations in the water, and significant differences in Cd accumulation were observed among the different tissues. Increased Cd2+ level in the tissues also led to a significant increase in malonaldehyde (MDA) content, caused by increased lipid peroxidation. The activities of superoxide dismutase (SOD) and catalase (CAT) also increased, peaking at different Cd2+ concentrations, depending on the tissue. Glutathione peroxidases (GPx) activity in the gill and mantle initially increased, but then decreased with increasing external Cd2+ concentration. In the hepatopancreas and foot, GPx activity was inhibited by Cd2+, even at low concentrations. Furthermore, Cd2+ also stimulated the expression of MT in all four tissues. However, the levels of reduced glutathione (GSH) andoxidized glutathione (GSSG) in the gill and mantle, as well as the GSH/GSSG ratios in all four tissues decreased with increasing external Cd2+ concentrations. Taken together, the results suggested that in M. meretrix, response to the toxic effect of Cd2+ might occur through a combination of mechanisms, which involve both enhanced antioxidant enzyme activities and the ability to bind and sequester Cd2+ via cysteine-rich molecules such as GSH and MT, both of which would eventually lead to the reduction of heavy metal-induced oxidative stress.
Highlights
Toxic metal pollution, which can seriously threaten the biological sustainability of coastal ecosystems, has become a major problem for the aquatic environment (Liu et al, 2018)
As for all Cd2+-treated groups, the order was gill > mantle > foot > hepatopancreas, with significant differences among the different tissues from the same group, except for the hepatopancreas and foot in the group exposed to 12 mg L−1 Cd2+
Our results showed that lower Cd2+ levels in the tissues of M. meretrix could activate the antioxidant protective mechanisms to cope with the potential oxidative stress inflicted by the increasing levels of O2−· and H2O2, observations that have been reported by studies conducted with other mollusks (Chandurvelan et al, 2015; Liu and Wang, 2016; Yao et al, 2020)
Summary
Toxic metal pollution, which can seriously threaten the biological sustainability of coastal ecosystems, has become a major problem for the aquatic environment (Liu et al, 2018). Cadmium (Cd) is considered to be one of the most dangerous, and it is usually found in marine environments (Liu et al, 2015; Gu et al, 2019). The ROS in turn can cause oxidative damage, which is often linked to lipid peroxidation, protein denaturation, and inactivation of enzymes as well as DNA replication errors (Chandurvelan et al, 2015; Xia et al, 2016; Lin et al, 2017). It can be used to determine the extent of lipid peroxidation and the accumulation of ROS in an organism (Liu and Wang, 2016; Lin et al, 2017; Haque et al, 2018)
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