The role of copper and zinc accumulation in defense against bacterial pathogen in the fujian oyster (Crassostrea angulata)

Metal bioaccumulation in marine invertebrates and risk assessment in sediments from South African coastal harbours and natural rocky shores

Industrial and urban activities are major contributors to metal contamination in coastal systems, often impacting the physiology, distribution and diversity of marine invertebrates. This study assessed metal contaminations in sediments, seawater, algae and invertebrates across four armoured systems (harbours) and two natural sites along the south coast of South Africa. Bioaccumulation factors such as Biosediment (BSAF), Biowater (BWAF), Bioaccumulation (BAF) and bioremediation of metals by invertebrate bioindicators were also determined. Spatial variation in metal concentrations were observed, however, bioaccumulation of metals was site and species-specific. Invertebrates bioaccumulated higher metal concentrations in armoured than natural sites, with filter feeders exhibiting higher concentrations than grazers. Among filter feeders, Octomeris angulosa and Crassostrea gigas bioaccumulated elevated aluminium (Al), arsenic (As), chromium (Cr), zinc (Zn) and copper (Cu), while, Perna perna accumulated elevated nickel (Ni), cadmium (Cd) and lead (Pb). Among grazers, Siphonaria serrata and Scutellastra longicosta bioaccumulated elevated Al, Cr, Cd, cobalt (Co), Cu, Ni and Zn. Bioaccumulation factors indicated that (As, Ni, Zn) were bioaccumulated by algae, and invertebrates from sediment (BSAF>1) and from seawater (BWAF>1). Additionally, invertebrates bioaccumulated metals from their prey item, algae as indicated by (BAF>1). Arsenic Cd and Pb in invertebrates were above the maximum limit set for human consumption by various regulatory bodies. Our findings underscore the significant role of coastal invertebrates in bioaccumulating and bioremediating metals, suggesting a natural mechanism for water quality enhancement, especially in urbanised coastal areas.

The c.503A>G polymorphism in ZIP1-II of Pacific oyster Crassostrea gigas associated with zinc content

The Pacific oyster Crassostrea gigas is renowned for its high zinc content, but the significant variation among individuals diminishes its value as a reliable source of zinc supplementation. The Zrt/Irt-like protein 1 (ZIP1), a pivotal zinc transporter that facilitates zinc uptake in various organisms, plays crucial roles in regulating zinc content. In the present study, polymorphisms of a ZIP1 gene in C. gigas (CgZIP1-II) were investigated, and their association with zinc content was evaluated through preliminary association analysis in 41 oysters and verification analysis in another 200 oysters. A total of 17 single nucleotide polymorphisms (SNPs) were identified in the exonic region of CgZIP1-II gene, with c.503A>G significantly associated with zinc content. Protein sequence and structure prediction showed that c.503A>G caused a p.Met110Val nonsynonymous mutation located in the metal-binding region of CgZIP1-II, which could influence its affinity for zinc ions, thereby modulating its zinc transport functionality. These results indicate the potential influence of CgZIP1-II polymorphisms on zinc content and provide candidate markers for selecting C. gigas with high zinc content.

Effects of Dietary Zinc on Growth Performance, Digestive Enzyme Activities, Antioxidant Status, and Immune Responses of Sea Cucumber Apostichopus japonicus

Zinc is an essential micronutrient for organisms involved in regulating various biological processes. This study evaluated the effects of dietary zinc on growth performance, digestive enzyme activities, antioxidant status, and immune responses of sea cucumber Apostichopus japonicus. Five experimental diets were formulated with graded levels of zinc (0, 20, 40, 60, and 80 mg/kg, respectively), and the actual dietary zinc values were 31.4, 51.0, 68.2, 91.9, and 110.8 mg/kg diet, respectively. Sea cucumbers were fed with diets for 2 months. The results showed the growth performance, amylase, and trypsin activities of sea cucumber increased significantly with zinc supplementation, and the best growth performance and enzyme activities were observed at 40 mg/kg zinc diet. Zinc supplementation significantly increased activities of superoxide dismutase, catalase, anti-superoxide anion, and inhibiting hydroxyl radical, while significantly reduced the malondialdehyde content. Furthermore, the higher zinc supplementation levels resulted in significantly upregulated immune-related genes of hsp90, p105, rel, and lsz, suggesting that excessive zinc caused oxidative stress. The broken-line regression analysis of specific growth rate indicated dietary zinc requirement in juvenile sea cucumber was ~ 66.3 mg/kg diet. Overall, dietary zinc contributes to the growth and immune resistance of juvenile sea cucumber, and our study will provide insights into the rational use of dietary zinc in aquaculture.

Integrating pollutant levels and biochemical biomarkers in oysters (Crassostrea rhizophorae and Crassostrea gasar) indicates anthropic impacts on marine environments along the coast of Santa Catarina state, Brazil

This study aimed to carry out a general diagnosis of the contamination of the coastal marine environment of the Santa Catarina state (SC, Brazil) by different classes of environmental pollutants, as well as to evaluate possible adverse effects of the contaminants on biochemical biomarkers of oysters, Crassostrea gasar and Crassostrea rhizophorae. 107 chemicals were evaluated in water, sediment and oyster samples from nine sites along the coastline of SC. We also examined various biochemical biomarkers in the oysters' gills and digestive glands to assess potential effects of contaminants. In general, the northern and central regions of the littoral of SC presented higher occurrences and magnitudes of contaminants than the southern region, which is probably related to higher urbanization of center and northern areas of the littoral. The biomarker analysis in the oysters reflected these contamination patterns, with more significant alterations observed in regions with higher levels of pollutants. Our results may serve as a first baseline for future and more extensive monitoring actions and follow-up of the degree of contamination in the state, allowing for inspection actions and management of areas most affected by marine pollutants.

Lysosomal Cu(I)/Cu(II) Dependence of Antimicrobial Ability of Oyster Hemocytes and Regulation of Phagolysosomal System

Copper (Cu) is hyperaccumulated in oyster hemocytes and is an essential trace metal indispensable for diverse innate immune functions. However, the roles of Cu in oyster immune defense are still unclear. In this study, Cu exposure enhanced the phagocytosis of zymosan by increasing the number and length of filopodia, as well as mitochondrial ROS (mitoROS) production mainly in granulocytes, followed by semigranulocytes and agranulocytes. The intracellular calcium level increased to promote the phagosome-lysosome fusion after Cu exposure. The enhancement of phagosomal acidification and mitochondrion-phagosome juxtaposition were also found in granulocytes after Cu exposure. These results indicated that Cu could regulate the phagolysosomal system to enhance the antimicrobial ability of oyster hemocytes with the assistance of mitoROS. Furthermore, Cu(I) and Cu(II) were predominately located in lysosomes, and degranulation may provide a mechanism for exposing Cu to bacteria to prevent their survival and proliferation. Specifically, we showed that the newly formed Cu(I) arising from lysosomal Cu(II) moved to lysosomes and mitochondria in activated hemocytes to induce strong immune responses. The ability of the transformation of Cu(I) from Cu(II) followed granulocytes > semigranlocytes > agranulocytes, indicating that granulocytes played important roles in immune functions of oysters. Our results provided new insights into the understanding of antimicrobial effects of Cu in oyster hemocytes.

Hemocytes of bivalve mollusks as cellular models in toxicological studies of metals and metal-based nanomaterials

Understanding the impacts of environmental pollutants on immune systems is indispensable in ecological and health risk assessments due to the significance of normal immunological functions in all living organisms. Bivalves as sentinel organisms with vital ecological importance are widely distributed in aquatic environments and their innate immune systems are the sensitive targets of environmental pollutants. As the central component of innate immunity, bivalve hemocytes are endowed with specialized endolysosomal systems for particle internalization and metal detoxification. These intrinsic biological features make them a unique cellular model for metal- and nano-immunotoxicology research. In this review, we firstly provided a general overview of bivalve's innate immunity and the classification and immune functions of hemocytes. We then summarized the recent progress on the interactions of metals and nanoparticles with bivalve hemocytes, with emphasis on the involvement of hemocytes in metal regulation and detoxification, the interactions of hemocytes and nanoparticles at eco/bio-nano interface and hemocyte-mediated immune responses to the exposure of metals and nanoparticles. Finally, we proposed the key knowledge gaps and future research priorities in deciphering the fundamental biological processes of the interactions of environmental pollutants with the innate immune system of bivalves as well as in developing bivalve hemocytes into a promising cellular model for nano-immuno-safety assessment.

Roles of hemocyte subpopulations in silver nanoparticle transformation and toxicity in the oysters Crassostrea hongkongensis

Hemocytes are the main immune cells in bivalve mollusks and one of the sensitive targets for nanoparticle toxicity. Bivalve hemocytes consist of multiple functional heterogeneous cell types, but their different roles in immune system against foreign particles remain largely unknown. In order to clarify the different immune responses of hemocyte subpopulations to silver nanoparticles (AgNPs) and Ag ions, in this study, the Hong Kong oyster (Crassostrea hongkongensis) hemocytes were employed and separated into three subpopulations based on their cell size and granularity, including agranulocytes (R1), semigranulocytes (R2), and granulocytes (R3). We first demonstrated that AgNPs could rapidly enter into the oyster hemocytes within 3 h by phagocytosis process and resulted in different immune responses in hemocyte subpopulations. The most affected cell subtype by AgNPs was the granulocytes, followed by semigranulocytes, whereas agranulocytes were not affected following exposure to AgNPs. Interestingly, AgNPs induced the granule formation in semigranulocytes and further increased the proportion of granulocytes, whereas their ionic counterparts had no such effects on hemocyte composition, indicating the different detoxification mechanisms for nanoparticulate and ionic form. Following AgNP exposure, the dissolved Ag ions were accumulated in lysosomes and caused lysosomal dysfunction, indicating that lysosomes were the main targets for AgNP toxicity and the dissolved Ag ions were the main contributor of AgNP toxicity. Furthermore, AgNP exposure induced reactive oxygen production and impeded the lysosome function and phagocytosis in granulocytes, with impaired immunity system in oysters. Our study identified the different immune responses of oyster hemocyte subpopulations to AgNPs based on the in vitro short-term exposure assays, which may be applied to rapidly evaluate the ecotoxicological risks of different nanoparticles in aquatic systems.

Zinc alters behavioral phenotypes, neurotransmitter signatures, and immune homeostasis in male zebrafish (Danio rerio)

Anthropogenic activities discharge zinc into aquatic ecosystems, and the effects of long-term and low-concentration zinc exposure on fish behavior are unclear. We evaluated the behavior and physiology of male zebrafish (Danio rerio) after a 6-week exposure to 1.0 or 1.5 ppm (mg/L) zinc chloride. The exposure caused anxiety-like behaviors and altered the social preferences in both exposure groups. Analysis of transcriptional changes suggested that in the brain, zinc exerted heterogenetic effects on immune and neurotransmitter functions. Exposure to 1.0 ppm zinc chloride resulted in constitutive immune dyshomeostasis, while exposure to 1.5 ppm zinc chloride impaired the neurotransmitter glutamate. In the intestine, zinc dysregulated self-renewal of intestinal cells, a potential loss of defense function. Moreover, exposure to 1.5 ppm zinc chloride suppressed intestinal immune functions and dysregulated tyrosine metabolism. These behavioral alterations suggested that the underlying mechanisms were distinct and concentration-specific. Overall, environmental levels of zinc can alter male zebrafish behaviors by dysregulating neurotransmitter and immunomodulation signatures.

Does roundup affect zinc functions in a bivalve mollusk in ex vivo exposure?

Roundup (Rn), a glyphosate-based formulation, is one of the most commonly used herbicides in the world. It affects non-targeted organisms in several ways, including adhesive activity towards metal ions. Zinc (Zn) plays a crucial role in a number of biochemical processes. In this study, we aim to elucidate the direct impact of Rn on Zn accumulation and Zn-dependent activities in the ex vivo system. To this end, we exposed the samples of the digestive gland of a bivalve mollusk Unio tumidus to 3 µM of Rn (calculated as 3 µM of glyphosate), Zn, Zn chelator (N,N,N',N'-Tetrakis(2-pyridylmethyl)ethylenediamine) (TPEN, Tp), and their combinations ZnTp and ZnRn for 17 h. We determined the levels of Zn in the tissue (Zn t) and metallothioneins (Zn-MT), metallothioneins (MTSH), and glutathione (GSH & GSSG), total antioxidant capacity (TAC), lysosomal membrane integrity, and caspase-3 activity. Our study demonstrated that Rn and Tp had different effects on the accumulation and functionality of Zn. Rn did not affect the accumulation of Zn (Zn t, Zn-MT) in the Zn- and ZnRn-groups. On the contrary, Tp produced effects antagonistic to Zn on caspase-3 activity, lysosomal stability, and MTSH concentration. Rn caused particular pro-oxidative effect that decreased GSH level (Rn- and ZnRn-groups) and lysosomal stability (Rn-group). The shared affected index was the GSH/GSSG ratio, which decreased by 2-8 times in each exposure. As the first experience with the application of Tp to indicate Zn activity in mollusks, the study concluded that the ex vivo approach could be useful in the study of numeral aquatic pollutants.

Immune responses of oyster hemocyte subpopulations to in vitro and in vivo zinc exposure

Oysters are an excellent biomonitor of coastal pollution and the hyper-accumulator of toxic metals such as copper and zinc (Zn). One unique feature of molluscs is their hemocytes which are mainly involved in immune defenses. Different subpopulations of hemocytes have been identified, but their functions in metal transport and detoxification are not clear. In this study, we examined the immune responses of different subpopulations of oyster Crassostrea hongkongensis hemocytes under different periods of Zn exposure by using flow cytometer and confocal microscopy. In vitro exposure to Zn resulted in acute immune responses by increasing the reactive oxygen species (ROS) production and phagocytosis and decreased number of granulocytes and mitochondrial membrane potential (MMP) within 3 h. Granulocyte mortality and lysosomal pH increased whereas glutathione (GSH) decreased within 1 h of in vitro exposure, indicating the immune stimulation of granulocytes. Within the first 7 days of in vivo exposure, immunocompetence of granulocytes was inhibited with increasing granulocyte mortality but decreasing ROS production and phagocytosis. However, with a further extension of Zn exposure to 14 days, both phagocytosis and lysosomal content increased with an increasing number of granulocytes, indicating the increase of hemocyte-mediated immunity. Our study demonstrated that granulocytes played important roles in oyster immune defenses while other subpopulations may also participate in immune functions. The degranulation and granulation due to transition between semigranulocytes and granulocytes after Zn exposure were important in metal detoxification. The study contributed to our understanding of the immune phenomena and the adaptive capability of oysters in metal contaminated environments.

A nationwide survey of trace metals and Zn isotopic signatures in mussels (Mytilus edulis) and oysters (Crassostrea gigas) from the coast of South Korea

Thirteen trace metals and Zn isotopic signatures were investigated in mussels and oysters collected from the coast of South Korea to evaluate their bioavailability in bivalve mollusks. The average Cu, Zn, and Cd concentrations were 2.6-17.7 times higher in oysters than mussels, and high biota sediment accumulation factors (>30) were observed for these metals in oysters. Except for Pb in mussels, most metals had no correlation with total sediment concentrations. In oysters, Fe, V, Cu, Zn, Sn, and Pb concentrations were significantly correlated with sediments. The average values of δ⁶⁶Zn_IRMM3702 in mussels, oyster, and sediments were +0.09‰, +0.12‰, and -0.06‰, respectively. Soft tissues of mussels and oysters with high Zn concentrations tended to contain lighter Zn isotopes. The results indicate that oysters are a better biomonitoring organism for metal contamination than mussels and can be used in the monitoring and management of coastal environments and ecosystems.

Copper promoting oyster larval growth and settlement: Molecular insights from RNA-seq

As a cofactor of key enzymes, Cu is required in living organisms, although Cu levels in the natural environment are typically low. In this study, the promotion of growth and settlement on the larvae of oyster Crassostrea angulata was observed at an environmentally relevant concentration (10 μg/L Cu). Interestingly, Cu accumulation in the soft tissue of oyster larvae increased during the larval development and exhibited a sharp increase at the late pelagic stage. With the help of RNA-seq, we constructed a high-quality transcriptional database of the oyster C. angulata larvae (24,257 genes with an average length of 1594 bp) via de novo assembly, which provided the basic molecular changes during the larval development. Network analysis of five early developmental stages and differential expression under Cu exposure were integrated to examine the roles of Cu in oyster larvae. Our molecular analysis demonstrated that both ion channels and organic transporters contributed to Cu internalization from the external environment, including zinc transporters and amino acid transporters. The followed distribution of Cu across cells was achieved by ATP7A, the circulatory system, and the Cu transporters (CTRs). Cu exposure enhanced the ribosome and the calcium binding proteins with a higher rate of translation and shell formation, giving rise to faster growth of oyster larvae. Furthermore, Cu facilitated the settling process by upregulating the chitin binding genes and then promoting the formation of the proteinaceous matrix between larvae and substrate. Our study presents the molecular basis for Cu promotion (i.e., hormesis) effects on oyster larval growth and settlement.

Accumulation of different metals in oyster Crassostrea gigas: Significance and specificity of SLC39A (ZIP) and SLC30A (ZnT) gene families and polymorphism variation

The Zrt/Irt-like proteins (ZIP, SLC39A) and zinc transporters (ZnT, SLC30A) are the two major gene families responsible for the import/export of Zn and other metals. In this study, the mRNA expression levels and genetic variations of eight ZnTs and 14 ZIPs were identified in Crassostrea gigas after exposure to Zn, Cd, Cu, Hg, and Pb. Metal exposure induced reactive oxygen species (ROS) and malondialdehyde (MDA) accumulation and antioxidant enzyme expression. The expanded gene numbers of superoxide dismutase (SOD) in the oysters exhibited diverse expression under exposure to the five metals, and the contrasting expressions of both ZnTs and ZIPs under different metal exposures were observed, revealing their ion-specific responses. Zn and Cu have similar transporters and induce high expression levels of ZnT1, 2, 7, and 9 and ZIP1, 3, 6, 9, 10, 11, and 14. Pb induced high expression levels of ZIP7, and 13 and ZnT5, 6, and 7, which are mainly expressed in the endoplasmic reticulum (ER). Cd induced high expression levels of ZnT1, 2, and 7 and ZIP1, 6, 9, 10, 11, and 13. Hg exposure was found to have little effect on the ZIP and ZnT expression levels. Based on 3784 single nucleotide polymorphisms (SNPs) within the ZnTs and ZIPs, genetic association analysis for Zn accumulation was conducted on 427 oyster samples. The 38 SNPs, which were located within 12 genes, were identified to be associated with Zn content (p < 0.01), explaining the phenotypic variation from 1.61% to 3.37%. One nonsynonymous mutation and related haplotypes were identified within ZIP1, explaining 1.69% of the variation in Zn. Its high expression under Zn exposure revealed its important role in Zn transportation. To the best of our knowledge, this study is the first comprehensive investigation of the transportation mechanisms of ZIPs and ZnTs under different metal exposures and the genetic effect of Zn accumulation in oysters, and provides valuable biomarkers and genetic resources to evaluate environmental metal pollution.

Vibrio parahaemolyticus Infection Influenced Trace Element Homeostasis, Impaired Antioxidant Function, and Induced Inflammation Response in Litopenaeus vannamei

Vibrio parahaemolyticus (V. parahaemolyticus) caused huge diseases and economic losses in shrimp aquaculture. Understanding the infection mechanism might help develop new strategies for controlling pathogen outbreak. Redistribution of trace element homeostasis, accompanied by impairment of antioxidant status and immune response, was observed during various infections. Accordingly, we hypothesized that V. parahaemolyticus infection might influence trace element homeostasis, impair antioxidant function, and induce inflammation response in shrimp. In the present study, the aim of this study was to investigate the influence of V. parahaemolyticus infection on trace element homeostasis, antioxidant status, and inflammation response in Litopenaeus vannamei (L. vannamei). The results showed that compared with the control group, V. parahaemolyticus infection significantly increased (P < 0.05) intestinal V. parahaemolyticus number, serum copper (Cu) concentration at 24, 48, and 72 h and significantly increased (P < 0.05) serum zinc (Zn), iron (Fe), and manganese (Mn) concentrations at 24 h but decreased (P < 0.05) at 72 h. The intestinal gene expressions of metal transporters ZIP13, CTR1, and MT1 were significantly decreased at 24, 48, and 72 h, and DMT1 was significantly decreased at 48 h and 72 h in the infection group. The activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx) were suppressed at 48 h and 72 h, and the malondialdehyde (MDA) content was increased at 24, 48, and 72 h in the infection group; the pro-inflammatory genes including necrosis factor-α (TNF-α), lipopolysaccharide-induced TNF-α factor (LITAF), and Ras-related protein Rab6A (RAB6A) were significantly upregulated at 48 and 72 h in the infection group. These results suggest that V. parahaemolyticus infection influenced trace element homeostasis, impaired antioxidant function, and induced inflammation response in L. vannamei, which might help understand the infection mechanism. The results provide a better understanding of the L. vannamei and V. parahaemolyticus interactions and may deliver the basis for further research in preventing the bacterial diseases.

Vibrio-bivalve interactions in health and disease

In the marine environment, bivalve mollusks constitute habitats for bacteria of the Vibrionaceae family. Vibrios belong to the microbiota of healthy oysters and mussels, which have the ability to concentrate bacteria in their tissues and body fluids, including the hemolymph. Remarkably, these important aquaculture species respond differently to infectious diseases. While oysters are the subject of recurrent mass mortalities at different life stages, mussels appear rather resistant to infections. Thus, Vibrio species are associated with the main diseases affecting the worldwide oyster production. Here, we review the current knowledge on Vibrio-bivalve interaction in oysters (Crassostrea sp.) and mussels (Mytilus sp.). We discuss the transient versus stable associations of vibrios with their bivalve hosts as well as technical issues limiting the monitoring of these bacteria in bivalve health and disease. Based on the current knowledge of oyster/mussel immunity and their interactions with Vibrio species pathogenic for oyster, we discuss how differences in immune effectors could contribute to the higher resistance of mussels to infections. Finally, we review the multiple strategies evolved by pathogenic vibrios to circumvent the potent immune defences of bivalves and how key virulence mechanisms could have been positively or negatively selected in the marine environment through interactions with predators.