Using zebrafish as a model to assess the individual and combined effects of sub-lethal waterborne and dietary zinc exposure during development

Arsenic bioaccumulation and biotransformation in the marine copepod Tigriopus japonicus under chronic dietborne and waterborne exposure

Arsenic (As) can be transferred along the food chain, while little is known about the toxic effects of dietborne As on marine copepods. In this study, we investigated the short-term and long-term effects of waterborne and dietborne As exposure on the bioaccumulation and biotransformation, as well as developmental toxicity of Tigriopus japonicus. Under acute As exposure, As bioaccumulation increased and reached a plateau with increasing exposure concentration. Moreover, As accumulation at dietborne exposure was 4.3 and 5.7 times greater than that at control group for AsIII and AsV, respectively. At chronic As exposure, As accumulation increased continuously with exposure time, with a 2.8-day extension of development time and a 45% reduction in 10-d fecundity under dietborne exposure compared to control, whereas 2.3-day extension of development time and a 20% reduction in 10-d fecundity were observed under waterborne exposure. Among As species, inorganic As had the highest concentrations, but the proportion of inorganic As decreased from 89% to 63% during 4 to 21 d of exposure, suggesting the conversion of inorganic As to organic As. The organic As was dominated by arsenobetaine (AsB, 13-25%), followed by monomethylarsenic (MMA, 8-25%). These results suggest that dietborne exposure has more pronounced toxic effects on T. japonicus, but the toxicity of As could be reduced through biotransformation under chronic exposure. Therefore, the arsenic species should be considered when assessing As toxicity.

Trace metals in the teleost fish gill: biological roles, uptake regulation, and detoxification mechanisms

In fish, the gill plays a vital role in regulating the absorption of trace metals and is also highly susceptible to metal toxicity. Trace metals such as iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn) are involved in various catalytic activities and molecular binding within the gill, thereby supporting a range of physiological processes in this organ. While beneficial at normal levels, these metals can become toxic when present in excess. Conversely, nonessential metals like cadmium (Cd) and lead (Pb) can gain entry into gill cells through similar metal transport pathways, potentially interfering with various cellular processes. The transepithelial transport of these metals across the gill epithelium is governed by a variety of metal transport and metal binding proteins. These include the Cu transporter 1 (CTR1), divalent metal transporter 1 (DMT1), and members of the Zrt-/Irt-like protein (ZIP) and zinc transport (ZnT) families. Additionally, some of these metals can compete with major ions (e.g., calcium, sodium) for absorption sites in the gill. This complex crosstalk suggests an interdependent mechanism that balances metal uptake to meet physiological needs while preventing excessive accumulation. In this article, I review the roles of trace metals in proteins/enzymes that support the different functions in the gill of teleost fish. I also discuss current understanding of the pathways involved in regulating the branchial uptake of metals and their influence on ionic regulation, and the potential detoxification mechanisms in the gill. Finally, I summarize knowledge gaps and potential areas for further investigation.

Diet supplementation of organic zinc positively affects growth, antioxidant capacity, immune response and lipid metabolism in juvenile largemouth bass, Micropterus salmoides

Zn is an important trace element involved in various biochemical processes in aquatic species. An 8-week rearing trial was thus conducted to investigate the effects of Zn on juvenile largemouth bass (Micropterus salmoides) by feeding seven diets, respectively, supplemented with no Zn (Con), 60 and 120 mg/kg inorganic Zn (Sul60 and Sul120), and 30, 60, 90 and 120 mg/kg organic Zn (Bio30, Bio60, Bio90 and Bio120). Sul120 and Bio120 groups showed significantly higher weight gain and specific growth rate than Con group, with Bio60 group obtaining the lowest viscerosomatic index and hepatosomatic index. 60 or 90 mg/kg organic Zn significantly facilitated whole body Zn retention. Up-regulation of hepatic superoxide dismutase, glutathione peroxidase and catalase activities and decline of malondialdehyde contents indicated augmented antioxidant capacities by organic Zn. Zn treatment also lowered plasma aminotransferase levels while promoting acid phosphatase activity and hepatic transcription levels of alp1, acp1 and lyz-c than deprivation of Zn. The alterations in whole body and liver crude lipid and plasma TAG contents illustrated the regulatory effect of Zn on lipid metabolism, which could be possibly attributed to the changes in hepatic expressions of acc1, pparγ, atgl and cpt1. These findings demonstrated the capabilities of Zn in potentiating growth and morphological performance, antioxidant capacity, immunity as well as regulating lipid metabolism in M. salmoides. Organic Zn could perform comparable effects at same or lower supplementation levels than inorganic Zn, suggesting its higher efficiency. 60 mg/kg supplementation of organic Zn could effectively cover the requirements of M. salmoides.

Comparative transcriptome and antioxidant biomarker response reveal molecular mechanisms to cope with zinc ion exposure in the unicellular eukaryote Paramecium

The development of industry has resulted in excessive environmental zinc exposure which has caused various health problems in a wide range of organisms including humans. The mechanisms by which aquatic microorganisms respond to environmental zinc stress are still poorly understood. Paramecium, a well-known ciliated protozoan and a popular cell model in heavy metal stress response studies, was chosen as the test unicellular eukaryotic organism in the present research. In this work, Paramecium cf. multimicronucleatum cells were exposed in different levels of zinc ion (0.1 and 1.0 mg/L) for different periods of exposure (1 and 4 days), and then analyzed population growth, transcriptomic profiles and physiological changes in antioxidant enzymes to explore the toxicity and detoxification mechanisms during the zinc stress response. Results demonstrated that long-term zinc exposure could have restrained population growth in ciliates, however, the response mechanism to zinc exposure in ciliates is likely to show a dosage-dependent and time-dependent manner. The differentially expressed genes (DEGs) were identified the characters by high-throughput sequencing, which remarkably enriched in the phagosome, indicating that the phagosome pathway might mediate the uptake of zinc, while the pathways of ABC transporters and Na⁺/K⁺-transporting ATPase contributed to the efflux transport of excessive zinc ions and the maintenance of osmotic balance, respectively. The accumulation of zinc ions triggered a series of adverse effects, including damage to DNA and proteins, disturbance of mitochondrial function, and oxidative stress. In addition, we found that gene expression changed significantly for metal ion binding, energy metabolism, and oxidation-reduction processes. RT-qPCR of ten genes involved in important biological functions further validated the results of the transcriptome analysis. We also continuously monitored changes in activity of four antioxidant enzymes (SOD, CAT, POD and GSH-PX), all of which peaked on day 4 in cells subjected to zinc stress. Collectively, our results indicate that excessive environmental zinc exposure initially causes damage to cellular structure and function and then initiates detoxification mechanisms to maintain homeostasis in P. cf. multimicronucleatum cells.

Meeting Zn Needs during Medaka Eye Development: Nanoscale Visualization of Retina by Expansion Microscopy

Fish eyes require high Zn levels to support their early development. Although numerous studies have been conducted on the nutritional and toxic effects of Zn on the eye, the Zn requirement for retinal cell development is still debatable. Moreover, due to the complexity of the retinal structure, it is difficult to clearly visualize each retinal layer and accurately separate cell morphology in vivo by conventional methods. In the present study, we for the first time have achieved nanoscale imaging of retinal anatomy affected by dietary and waterborne Zn exposure by novel expansion microscopy. We demonstrated that the fish retina showed different developmental strategies in response to dietary and aqueous Zn exposures. Excess dietary Zn produced toxicity to retinal photoreceptor cells, resulting in a reduction in cell number and cell area, and this toxicity became severe with biological development. In contrast, waterborne Zn in the natural environment probably failed to meet the Zn requirements of retinal development. Overall, our results indicated that during early development, the Zn requirement of the fish eyes was sensitive, and oversupplementation led to impaired photoreceptor cell development. Our study has provided new perspectives using the powerful and novel expansion microscopy technique in toxicity assessment, enabling ultra-clear visualization of small but complex organ development.

Transgenerational effects of zinc in zebrafish following early life stage exposure

Although toxic effects of zinc (Zn) have been well established in the different developmental stages in fish, long-lasting effects of Zn exposure during embryonic development have not been explored. Exposure to an environmentally relevant Zn concentration of 10 μM (650 μg/L) during the first five days after fertilization did not affect survival, body weight, malformations or overall hatching success of F0 and F1 larvae. Zn exposure did, however, result in delayed hatching in both the F0 and F1 generations and caused significant changes in homeostasis of Zn and selenium (Se) in F0 and F1 fish. This was especially pronounced when F1 embryos from Zn-exposed parents were treated with 30 μM (2000 μg/L) Zn. In the F0 generation, skewed sex ratio towards males and changes in homeostasis of Zn, Se and manganese (Mn) in the brain, gill, liver and gonad of adult fish were also observed. These changes were associated with altered expression of Zn- and Mn-regulatory genes and sex differentiation genes in F0 and F1 fish. The present study suggests that fish may carry memory from embryo-larval Zn exposure into adulthood and further to the next generation. The present study shows that ecotoxicological risk of an exposure to Zn during embryo-larval development may persist long after recovery and may also manifest in the F1 generation.

Toxicological assessment of cadmium-containing quantum dots in developing zebrafish: Physiological performance and neurobehavioral responses

The present research investigated the effects of exposure to sublethal concentrations of cadmium selenide/zinc sulfide (CdSe_core/ZnS_shell)-containing quantum dots (QDs; 0 - 100 µg/L QDs) on the neurophysiological performance of developing zebrafish (Danio rerio). The results suggested that exposure to CdSe QDs for 5 days increased the whole-body content of Cd without affecting the general physiological conditions of larvae. Interestingly, CdSe QD exposure reduced swimming distance but increased swimming velocity of larvae, suggesting that the exposure may lead to burst/episodic swimming. The findings also suggested that CdSe QD exposure reduced the wall-hugging behavior of larvae during a sudden light-to-dark transition test, and that the exposure significantly decreased the locomotor activity of fish during the dark period. On the other hand, control larvae displayed a dark avoidance behavior, whereas CdSe QD-exposed larvae exhibited an increase in the time spent in the dark zone, providing further support that CdSe QDs inhibited anxiety-related responses in larvae. Additional analysis with droplet digital PCR revealed that CdSe QD exposure altered the mRNA levels of genes that are associated with dopamine signaling and oxidative stress response. Collectively, our findings suggested that CdSe QD exposure may induce neurobehavioural toxicity and alters the mRNA abundance of dopamine- and oxidative stress-related genes in developing animals.