Seasonal and spatial variations of biomarker responses of rock oysters in a coastal environment influenced by large estuary input

Transcriptome Analysis and Identification of Cadmium-Induced Oxidative Stress Response Genes in Different Meretrix meretrix Developmental Stages

Cadmium (Cd) is one of the major pollutants in the aquatic environment, and it can easily accumulate in aquatic animals and result in toxic effects by changing the metabolism of the body, causing a serious impact on the immune system, reproductive system, and the development of offspring. The clam Meretrix meretrix is one of the commercially important species that is cultivated in large-scale aquaculture in China. To elucidate the underlying molecular mechanisms of Cd2+ in the developmental processes, fertilized eggs and larvae of M. meretrix at different developmental stages were exposed to Cd2+ (27.2 mg L-1 in natural seawater) or just natural seawater without Cd2+ (control), and high-throughput transcriptome sequencing and immunohistochemistry techniques were used to analyze the toxic effects of Cd on larvae at different early developmental stages. The results revealed 31,914 genes were differentially expressed in the different stages of M. meretrix development upon treatment with Cd2+. Ten of these genes were differentially expressed in all stages of development examined, but they comprised only six unigenes (CCO, Ndh, HPX, A2M, STF, and pro-C3), all of which were related to the oxidative stress response. Under Cd exposure, the expression levels of CCO and Ndh were significantly upregulated in D-shaped and pediveliger larvae, while pro-C3 expression was significantly upregulated in the fertilized egg, D-shaped larva, and pediveliger. Moreover, HPX, A2M, and STF expression levels in the fertilized egg and pediveliger larvae were also significantly upregulated. In contrast, CCO, Ndh, HPX, A2M, STF, and pro-C3 expression levels in the postlarva were all downregulated under Cd exposure. Besides the genes with changes in expression identified by the transcriptome, the expression of two other oxidative stress-related genes (MT and Nfr2) was also found to change significantly in the different developmental stages of M. meretrix upon Cd exposure, confirming their roles in combating oxidative stress. Overall, the findings of this study indicated that Cd would interfere with cellular respiration, ion transport, and immune response through inducing oxidative stress, and changes in the expression of oxidative stress-related genes might be an important step for M. meretrix to deal with the adverse effects of Cd at different stages of its development.

Dynamics of copper regulation in a marine clam Sinonovacula constricta at the organ level: Insight from a physiologically based pharmacokinetic model

Copper (Cu) is a common pollutant in estuaries and has received considerable attention worldwide. To gain an insight into the physiological mechanisms of waterborne Cu absorption, tissue distribution, storage, metabolism, and excretion in an estuarine razor clam Sinonovacula constricta, we developed a physiologically-based pharmacokinetic model based on prolonged Cu exposure with two exposure treatments. The tissues of S. constricta were divided into four parts: blood, digestive gland, gill, and other tissues. Our results showed that the waterborne Cu entered and exchanged with the gills and digestive gland, whereas digestive gland and other tissues were the main storage sites for Cu. Gills of S. constricta were able to maintain their Cu concentrations under both exposure treatments. Additionally, the gills exhibited a remarkable ability to remove Cu from water, with a transfer rate constant of 1.73 d-1 from the gills to water, while restricting its transfer from the blood with a transfer rate constant of 0.0131 d-1 from blood to gills. These results highlighted the crucial role of gills in regulating Cu levels in S. constricta as well as the detoxification and maintenance of metal homeostasis. Cu uptake rate constant in gill from waterborne was similar to that of digestive gland (0.294 vs. 0.364 L g-1 d-1), thus water entering the digestive tract was considered as another route of waterborne Cu absorption in bivalves. A significant amount of Cu in the blood was transferred to the digestive glands. These two factors explained the relatively higher Cu accumulation in the digestive glands than in other tissues in clams. The findings of this study enhanced our understanding of the homeostatic regulation and transportation mechanisms in marine bivalves.

Assessment of toxic metal pollution in Yueqing Bay and the extent of metal-induced oxidative stress in Tegillarca granosa raised in this water

Yueqing Bay is an important economic shellfish culture zone in Zhejiang Province, China. However, increased pollution in the water caused by toxic metals has led to the bioaccumulation of toxic metals in cockles such as Tegillarca granosa, and the consequence of toxic metal-associated toxicity in these animals. This study aimed to assess the concentration of toxic metals in the water and sediment in four different sites (Baisha, Qingjiang, Nanyue, and Wengyang) within Yueqing Bay and to evaluate the extent of metal bioaccumulation in T. granosa raised in the aquaculture farms located within the four sites, as well as the changes in biomarkers in T. granosa in response to the metals. The assessment was carried out at two different times of the year, January and July. The water and sediment samples taken from the aquaculture farms in Baisha (S1), Qingjiang (S2) and Nanyue (S3) were found to have a comprehensive toxic metal pollution index (Pc) <1, indicating that these farms were not polluted. However, the water and sediment samples taken from the aquaculture farm in Wengyang (S4) had a Pc between 1 and 2, indicating mild toxic metal pollution. The edible risk assessments (HQ) of T. granosa in all four farms were <1, and therefore, these cockles could be considered safe for human consumption. The toxic metal enrichment in T. granosa exhibited a strong correlation with the toxic metal content in the sediment. In all four farms, CAT and SOD activity levels in the visceral mass of T. granosa were higher than those found in the foot, and a significantly higher level of CAT activity was detected in July compared with January. Similarly, MDA and H2O2 contents in the visceral mass were also higher in July than in January. Tegillarca granosa individuals taken from S4 and S3 farms exhibited significantly higher levels of metallothionein (MT) mRNA and MDA compared with individuals from S1 and S2 farms. Furthermore, the levels of MDA and MT mRNA showed significant positive correlations with Cd, Cr, Hg, and Cu. Elevation of lipid peroxidation in these cockles coincided with increasing levels of endogenous antioxidants. The visceral mass of T. granosa and its MDA level could be used as a tissue indicator and a biochemical marker, respectively, for detecting toxic metal pollution. MT mRNA might also be used as a molecular marker of toxic metal pollution. The integrated biomarker response version 2 (IBRv2) values of the four aquaculture farms in Yueqing Bay showed the order S4 > S3 > S2 > S1, indicating that S4 had the most serious metal-induced stress. Furthermore, the IBRv2 values correlated with the Nemerow composite index (Pc) for all the cockles examined. Thus, as far as the contamination of aquaculture farms in Yueqing Bay by toxic metals is concerned, the aquaculture farm in Wengyang (S4) was mildly contaminated by toxic metals. However, the contamination was relatively low, presenting a low risk for the local population of T. granosa.

Cadmium-induced oxidative stress in Meretrix meretrix gills leads to mitochondria-mediated apoptosis

Cadmium (Cd) is one of the most important marine environmental pollutants that can cause oxidative damage and apoptosis in living organisms, and mitochondria are the key cell organelles affected by Cd toxicity. In this study, we investigated the effect of Cd on the mitochondria in the gill cells of the clam Meretrix meretrix and the underlying mechanism of mitochondria-mediated apoptosis following exposure to the metal. Exposure of the clams to artificial seawater containing 1.5, 3, 6 and 12 mg L^-1 Cd²⁺ led to swollen mitochondria compared with the untreated clams. The mitochondria also became vacuolated at the higher Cd²⁺ concentrations. Biochemical assays showed that monoamine oxidase (MAO) activity and mitochondrial membrane potential (Δψm) increased at 1.5 mg L^-1 Cd²⁺, but decreased at higher Cd²⁺ concentrations, while the activities of malate dehydrogenase (MDH) and cytochrome oxidase (CCO) and the scavenging capacities of anti-superoxide anion (ASA) and anti-hydroxy radical (AHR) all decreased with increasing Cd²⁺ concentrations. Significant increases in the levels of malondialdehyde (MDA) and H₂O₂ as well as in the activity levels of caspase-3, -8, and -9 were also observed in the Cd²⁺-treated clams. The results implied that Cd might induce apoptosis in M. meretrix via the mitochondrial caspase-dependent pathway.

Protein molecular responses of field-collected oysters Crassostrea hongkongensis with greatly varying Cu and Zn body burdens

The oyster Crassostrea hongkongensis is an ideal biomonitor due to its widespread distribution along the coast of Southern China and the ability to hyperaccumulate metals including Cu and Zn. In this study, we conducted the first investigation of the molecular responses to metal hyperaccumulation based on quantitative shotgun proteomics technique and genome information. Gill tissue of oysters collected from the uncontaminated environment (Site 1, 59.6 μg/g and 670 μg/g dry weight for Cu and Zn) displayed significant protein profile differentiation compared to those from a moderately contaminated (Site 2, 1,465 μg/g and 10,170 μg/g for Cu and Zn) and a severely contaminated environment (Site 3, 3,899 μg/g and 39,170 μg/g for Cu and Zn). There were 626 proteins identified to be differentially expressed at Site 3 but only 247 proteins at Site 2. Oysters from a moderately contaminated estuary (Site 2) displayed fewer effects as compared to oysters under severe contamination, with fluctuated small molecule metabolism and enhanced translation process. At Site 3, the induction of reactive oxygen species (ROS) was the main toxicity under the extremely high level of metal stress, which resulted in protein damage. Additionally, the impaired structure of cytoskeleton and modified membrane tracking process at Site 3 oysters led to the blockage or less efficient protein or macromolecule distribution within cells. Nonetheless, proteomic analysis in this study revealed that oysters could partly alleviate the adverse metal effects by boosting the translation process, enhancing the ability to recycle the misfolded proteins, and enhancing the potential to eliminate the excess ROS. Our study demonstrated an adaptive potential of oysters at the protein level to survive under conditions of metal hyper-accumulation.

Spatial-temporal variations and trends predication of trace metals in oysters from the Pearl River Estuary of China during 2011-2018

Estuaries are often considered to be the filters of pollutants from the land-derived outflows of freshwater to open seawater. Oysters are efficient bioaccumulators of metals in the estuarine environment, however, little information is available on the long-term tissue variability of metals in a large dynamic estuary under complex urbanized and anthropogenic impacts. Thus, an eight-year biomonitoring study of metals (Ag, Cd, Cr, Cu, Ni, Pb, and Zn) in the oysters from 10 sites were carried out to reveal the highly spatial-temporal variations in the Pearl River Estuary (PRE) of China during 2011-2018. Cd, Cu, and Zn in oysters were significantly correlated with the dissolved metals in seawater. Geographically, Ag, Cd, and Cr were higher in the western sites, and Cu, Ni, and Zn were higher in the eastern sites. High seasonal variations of Ag, Cu, and Zn were found in the wet season. The calculated annual change rates (v_c) of Cd, Cu, Zn, Ag, Pb, Ni, and Cr in the oysters were -1.1, -45, -48, 0.338, -0.216, -2.2, and -2.8 μg/g/y, respectively. If such decreasing rates of v_c (or natural logarithm rates v) were maintained, Cd, Cu, Zn, Pb, and Ni in oysters from PRE would be expected to recover to the national 50% concentrations in years 2022 (2024), 2045 (2079), 2073 (2110), 2021 (2023), and 2019 (2020), respectively. Long-term series observations of metals in organisms reflected the real bioavailability of metals, pollution status, and trends for environmental management and control in a large dynamic and contaminated estuary.

Heavy metals bioconcentration in Crassostrea rhizophorae: A site-to-site transplant experiment at the Potengi estuary, Rio Grande do Norte, Brazil

In this study, we analyzed the bioconcentration of Cd, Cr, Cu, Pb, Ni, and Zn in the soft tissue of transplanted oysters in two sites in the Potengi estuary for six months. Native oysters collected before and after the transplantation experiment provided the background for statistical analyses. Cd, Cr, and Ni showed a strong inverse correlation with oyster weight in both sites. Transplantation upstream of the estuary presented increasing concentrations of Zn, Cu, and Pb and condition index (CI) and decreasing trends for Cd and Ni, whereas Cr oscillated significantly. In the downstream transplantation, Cu, Pb, and Zn and the CI tended to decrease, whereas for Ni, Cd, and Cr, the concentrations increased. Spatiotemporal principal component analysis correlated these results mainly with proximity to the polluting source, seasonality, and previous exposure to heavy metals. These results helped interpret the responses provided by these biomonitors to environmental changes, whether they are natural or anthropogenic.

Comparative Proteomics on Deep-Sea Amphipods after in Situ Copper Exposure

The interest in deep-sea mining increased along with the environmental concerns of these activities to the deep-sea fauna. The discovery of optimal biomarkers of deep-sea mining activities in deep-sea species is a crucial step toward the supply of important ecological information for environmental impact assessment. In this study, an in situ copper exposure experiment was performed on deep-sea scavenging amphipods. Abyssorchomene distinctus individuals were selected among all the exposed amphipods for molecular characterization. Copper concentration within the gut was assessed, followed by a tandem mass tag-based coupled with two-dimensional liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) applied to identify and quantify the protein expression changes after 48 h of exposure. 2937 proteins were identified and annotated, and 1918 proteins among all identified proteins were assigned by at least two nonambiguous peptides. The screening process was performed based on the differences in protein abundance and the specific correlation between the proteins and copper in previous studies. These differentially produced proteins include Na⁺/K⁺ ATPase, cuticle, chitinase, and proteins with unknown function. Their abundances showed correlation with copper and had high sensitivity to indicate the copper level, being here proposed as biomarker candidates for deep-sea mining activities in the future. This is a key step in the development of environmental impact assessment of deep-sea mining activities integrating ecotoxicological data.

Biomarker responses in oysters Crassostrea hongkongensis in relation to metal contamination patterns in the Pearl River Estuary, southern China

The Pearl River Estuary (PRE) is the third largest estuary in China, where estuarine organisms are under metal stress at various biological levels. Based on the metal concentrations measured in oyster Crassostrea hongkongensis, we documented a change in dominance of metal contamination from Cd, Cr, Cu, Ni and Zn to Ag, Cd, Cu and Zn. In general, metal concentrations were higher in upstream stations and displayed a clear up-downstream gradient. Compared to the historical values, we noted the reductions in Cd, Cr and Ni concentrations, and the changing inputs due to evolving industrial activities were responsible for shaping the metal contamination profile in the PRE region. Along with metal concentrations, a suite of biomarkers was analyzed. Among the metals measured in the oyster tissues, Ag, Cd, Cu, Ni and Zn showed the strongest associations with pro-oxidant and oxidative stress responses (superoxide dismutase, lipid peroxidation and lysosomal membrane destabilization) and detoxification responses (glutathione and metallothionein), suggesting that the present metal contamination still exerts significant amount of stress in biota in the PRE. Metal contamination in estuaries in China is still severe compared to other countries, therefore continuous efforts should be taken to monitor the changing metal profiles with necessary control and remediation measures.

Seasonal variations of multi-biomarker responses to metals and pesticides pollution in M. galloprovincialis and T. decussatus from Homa Lagoon, Eastern Aegean Sea

Biomarkers (CAT, SOD, GPx, and AChE) were investigated in hepatopancreas and soft tissues of mussels (Mytilus galloprovincialis) and clams (Tapes decussatus) in response to metal and pesticide pollutions in Homa Lagoon. Mussel and clam samples were seasonally collected in January, May, August and November 2015. According to the results, higher metal concentrations determined in the soft tissues of both species in January. Hepatopancreas was found to accumulate higher levels of metals than soft tissue. Metal deposition potential in clams were higher than mussels. Pesticide levels were below detection limits in clams. In mussels, only 4,4'-DDE was detected in soft tissues and hepatopancreas in November. Highest AChE, CAT, SOD and GPx activities in mussels were determined in January. On the other hand, maximum AChE and CAT activities in clams were found in November. Seasonal variations of metals and biomarker levels might be related to species-specific physiology and environmental conditions.

Cadmium bioaccumulation and elimination in tissues of the freshwater mussel Anodonta woodiana

Experiments were carried out to investigate the bioaccumulation and elimination of cadmium (Cd) in tissues (kidney, gills, digestive gland, mantle, visceral mass, foot, adductor muscle and hemolymph) from the freshwater mussel, Anodonta woodiana. The mussels were exposed to subchronic Cd at concentrations of 0.168 and 0.675 mg L^-1 for 28 d of bioaccumulation and 28 d of elimination. During the bioaccumulation phase, Cd bioaccumulations increased in all tissues. The highest bioaccumulation of Cd was found in the kidney. The second-highest and third-highest bioaccumulations of Cd were found in the digestive gland and gills, respectively. The Cd bioaccumulations in the tissues of A. woodiana increased with exposure time and concentration, except for hemolymph, which reached the highest value on d 14. The bioaccumulation factors (BCFs) increased with exposure time, but an inverse relationship was observed between BCFs and exposure concentration. During the elimination phase, the visceral mass showed the highest Cd elimination rate. In the kidney, digestive gland and gills, the elimination rates almost reached 40%, but their concentrations were still higher than in other tissues. Thus, we concluded that the kidney, gills and digestive gland of A. woodiana are target tissues for subchronic Cd toxicity.

Spatial and temporal variations of bulk and colloidal dissolved organic matter in a large anthropogenically perturbed estuary

In the present study, we characterized the sources of dissolved organic matter (DOM) and the processes controlling the evolution of DOM in a large anthropogenically perturbed estuary in Southern China. Three-dimensional excitation emission matrices (3D EEMs) revealed that DOM in the estuary was primarily from autochthonous origin although terrestrial sources also contributed to the DOM production. Parallel factor analysis (PARAFAC) of the EEMs dataset indicated that DOM in the estuary was a legacy from the upstream wastewater discharging activities, and estuarine mixing was the primary mechanism responsible for the decreasing concentrations of DOM components. However, non-conservative behavior of DOM components was also observed along the center transect as a result of additional removal process and spatial heterogeneity in the distribution of DOM. Seasonal influence on the concentrations and compositions of colloidal DOM in the estuary was also found. Asymmetric flow field-flow fractionation (AF4) analysis showed distinct distribution patterns of the colloidal DOM during the dry and wet seasons. Behavior of colloidal DOM was size-independent and the distribution of colloidal DOM was controlled by multiple depletion and replenishment cycles in the dry season, in contrast to those observed in the wet season. By combining both EEMs-PARAFAC modeling and AF4 technique, this study provided important information on the sources and fate of bulk and colloidal DOM in a large estuary with intensive anthropogenic influences.