The effect of sublethal concentrations of zinc on reproduction in the zebrafish, Brachydanio rerio Hamilton-Buchanan

Zinc pre-exposure improves Zn resistance by demethylation of metallothionein 2 and transcription regulation of zinc-regulatory genes in zebrafish ZF4 cells

Mild zinc (Zn) pre-exposure can promote Zn resistance of organism, but the underlying molecular mechanisms are largely unknown. Two experiments were performed using zebrafish ZF4 cells, including short-term and long-term Zn pre-exposure experiments. In the short-term test, the cells were pre-exposed to 100 µM Zn for 24 h, transferred into fresh medium with 4.4 µM Zn for 24 h, and then re-exposed to 250 µM Zn. In the long-term test, the cells were pre-exposed to 100 µM Zn intermittently for 10 passages (3 days per passage), transferred into fresh medium with 4.4 µM Zn for 5 passages, and then re-exposed to 250 µM Zn. Both pretreatments resulted in higher resistance to 250 µM Zn. Exposure to 250 µM Zn caused a more than 2-fold increase in Zn content without Zn pretreatment but did not affect Zn content in the Zn pretreated cells. The Zn pretreated cells had low methylation levels of the metal-response element (MRE) at locus -87 in the promoter of mt2 (metallothionein 2). The up-regulated mRNA expression of Zn-regulatory genes (mtf-1, mt2, slc30a1a, slc30a4, slc30a5, slc30a6 and slc30a7) in the long-term Zn pretreated cells and mt2, slc30a4, slc30a6 and slc30a7 in the short-term Zn pretreated cells were observed. Exposure to 250 µM Zn in combination with the Zn pretreatments up-regulated mRNA expression of these genes and reduced methylation levels of the MRE compared with 250 µM Zn alone and the control. Taken together, the data suggested that demethylation of MRE in the promoter of mt2 and transcriptional induction of mt2 and Zn exporter genes offered Zn resistance in fish ZF4 cells. The traditional toxicological evaluation based on continuous exposure may overestimate the risk of fluctuating concentrations of Zn in the environment.

Regulation of metal homeostasis and zinc transporters in early-life stage zebrafish following sublethal waterborne zinc exposure

In the present research, the effects of exposure to a sublethal concentration of zinc (Zn) on metal and ion homeostasis, and the regulation and the localization of various Zn transporters (i.e., the Zrt-Irt Like Protein (ZIP) family of Zn transporters), were investigated in zebrafish (Danio rerio) during early development. Exposure to an elevated level of Zn [4 μM (high) vs. 0.25 μM (control)] from 0 day post-fertilization (dpf) resulted in a significant increase in the whole body content of Zn at 5 dpf. A transient decrease in the whole body calcium (Ca) level was observed in 3 dpf larvae exposed to high Zn. Similarly, whole body nickel (Ni) and copper (Cu) contents were also reduced in 3 dpf larvae exposed to high Zn. Importantly, the magnitude of reduction in whole body Ni and Cu contents following Zn exposure was markedly higher than that in Ca content, suggesting that internal Ni and Cu balance were likely more sensitive to Zn exposure in developing zebrafish. Exposure to high Zn altered the mRNA expression levels of specific zip transporters, with an increase in zip1 (at 3 dpf) and zip8 (at 5 dpf), and a decrease in zip4 (at 5 dpf). The expression levels of most zip transporters tended to decrease from 3 dpf to 5 dpf with the exception of zip4 and zip8. Results from in situ hybridization revealed that several zip transporters exhibited distinct spatial distribution (e.g., zip8 in the intestinal tract, zip14 in the pronephric tubules). Overall, our findings suggested that exposure to sublethal concentrations of Zn disrupts the homeostasis of essential metals during early development and that different ZIP transporters may play unique roles in regulating Zn homeostasis in various organs in developing zebrafish.

Arabian killifish (Aphanius dispar) embryos: A model organism for the risk assessment of the Arabian Gulf coastal waters

Fish embryos are excellent models for studies aimed at understanding toxic mechanisms and indications of possible acute and chronic effects. For the past 3 yr, an Arabian killifish (Aphanius dispar) fish embryo test has been developed in the authors' laboratory as a routine ecotoxicological test that can be used to support risk assessment of potential contaminants in Arabian Gulf coastal waters. Tests were conducted with 3 reference toxicants (3,4-dichloroaniline [DCA], sodium dodecyl sulfate, and zinc sulfate [Zn]) and chlorine, a disinfectant used widely in industrial cooling systems around the Arabian Gulf region. The 50% effect concentration (EC50) for DCA was 0.47 mg/L and 1.89 mg/L for embryos exposed before 6 hpf and after 168 hpf, respectively. Sublethal effects were mainly observed at concentrations above 2.5 mg/L, the effects included severe pericardial edema and tail shortage. The sodium dodecyl sulfate ionic surfactant caused mortality at both early and late stages of embryo development; it caused coagulation, severe deformity, and hemolysis. Both the EC50 and the 50% lethal concentration (LC50) for sodium dodecyl sulfate were 9.37 mg/L. Salinity influenced the toxicity of Zn to killifish embryos: at 40 psu Zn was found not to be toxic, whereas at 20 psu toxicity had increased significantly (p < 0.05). Values of EC50 and LC50 were 2.5 mg/L and 4 mg/L, respectively. Concentrations above 15 mg/L in embryos were often accompanied by upper abdominal edema and inhibition of growth, especially evident in the tail. Chlorine caused mortality at a lower concentration; for example, at 0.05 mg/L 33% of embryos were found dead at the end of the experiment. The LC50 for chlorine was determined to be 0.08 mg/L. Examination of the existing literature showed similar results to the present study's findings. The results suggest a more comparable sensitivity of killifish embryos to that of other fish embryo test recommended species. The present study's findings support the ability of killifish to be an indicator organism for environmental risk assessments of Arabian Gulf waters. Benefits include sensitivity to a wide range of substances and conditions, animal alternative, ease of fish breeding, and clarity of the embryos.

Toxic effects of zinc on the development, growth, and survival of red sea bream Pagrus major embryos and larvae

This study investigated the zinc toxicity to red sea bream Pagrus major embryos and larvae at 18 +/- 1 degrees C (33 +/- 1 per thousand in salinity) under laboratory conditions. The acute toxicity tests indicated that zinc 48-h LC50 to embryos and 96-h LC50 to larvae were 4.3 (3.3-6.3; 95% confidence limits) and 10.1 (9.0-11.4) mg l(-1), respectively, suggesting that embryos were more sensitive than larvae to zinc exposure. The subchronic toxicity test, in which embryos and larvae were continuously exposed to 0, 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, 2.0, and 2.5 mg Zn2+ l(-1) solutions for 10 days, demonstrated that waterborne zinc had distinctly toxic effects on the development, growth, and survival of red sea bream embryos and larvae. Zinc exposure at concentrations > or = 0.5 mg l(-1) would lead to a low hatching rate (19-78%, vs. 98% in controls), high mortality (29-91%, vs. 10% in controls), and morphological abnormality (12-77%, vs. 0.3% in controls) in embryos and larvae, while it caused delay in time-to-hatch in embryos at concentrations > or = 1.0 mg l(-1). These four biological parameters were zinc concentration dependent and could be effective bioindicators for evaluating the toxicity of zinc to the early life stage of this fish. Heartbeats of embryos (9-13 beats 10 s(-1)) were relatively low and were not significantly influenced by zinc concentration, although they rose remarkably with elevated zinc concentration in larvae at the end of the test, particularly when it was > or = 1.0 mg l(-1) (36-38, vs. 31 beats 10 s(-1) in controls). The total length (LT) of the larvae at the end of the test was reduced by 12.2% and 15.6% in the 1.0 and 2.0 mg l(-1) solutions but did not vary significantly in other solutions in comparison with the controls. Heartbeat and LT were less sensitive to zinc exposure and might not be good biological parameters for determining the toxicity of zinc to the early life stage of red sea bream.

The effects of heavy metals on embryonic development of fish (a review)

Early developmental stages of fish are particularly sensitive to water pollution. Heavy metals may affect various developmental processes during the embryonic period, which results in a reduction of offspring quantity and quality. Waterborne metals may accumulate in the gonads of spawners and adversely affect gamete production and viability, or exert direct toxic influence upon developing embryos. The egg shell does not fully protect the embryo against metal penetration, particularly during the swelling phase; thus, metals may accumulate in the egg. The results depend on metal concentration and range from developmental disturbances to death of the embryo. Metals disturb various processes of fish embryonic development and affect the development rate. Early stages just after fertilization are particularly sensitive to metal intoxication, when most disturbances and the highest embryonic mortality occur. Waterborne metals also promote developmental anomalies during organogenesis, including body malformations. Heavy metals often induce a delay in the hatching process, premature hatching, deformations and death of newly hatched larvae. All these disturbances result in reduced numbers and poor quality of the larvae, which show small body size, high frequency of malformations and reduced viability.

Multiple stressors in the Sacramento River watershed

Aquatic biota in the Sacramento River watershed are stressed by diversion of river flows, by historical mining resulting in cadmium, copper, zinc, and mercury, and, more recently, contamination by agricultural and urban chemical runoff. In addition, the proposed redirection of drainage of saline waters--containing selenium--from the western slope of the San Joaquin River into the Delta formed by the confluence of the Sacramento and San Joaquin Rivers could add to the stress on resident organisms. These combined stressors have led to deterioration in surface water quality and the aquatic habitat. The potential interaction of these stressors, coupled with invasions of foreign species and the export of juvenile fish into aqueducts, has driven several species of fish to near extinction in the system. Effects of historical contamination by heavy metals are potentially exacerbated by presence of organophosphate pesticides, at concentrations exceeding National Academy of Sciences recommendations, throughout the lower watershed and the San Francisco Bay. The Asian clam, Potamocorbula amurensis, an introduced non-indigenous species has apparently become a preferred food item of the sturgeon, Accipenser transmontanus, an important sport and aquaculture species. Since this introduction, sturgeon body burdens for selenium have increased dramatically and analytical chemistry of P. amurensis indicates that these organisms are effective bioaccumulators of selenium. This review examines potential ecotoxicity associated with multiple stressors in the watershed. Data from field monitoring, laboratory toxicity assays with ambient water, and ecotoxicologic investigations are reviewed. Potential designs for multiple stressor investigations are discussed. The information presented on this watershed illustrates the challenge to investigators seeking to evaluate multiple stressor effects on riverine and estuarine organisms.

The impact of sewage sludge exposure on the reproduction of the sand goby, Pomatoschistus minutus

Adult sand gobies were exposed to 0.1% sewage sludge for 19 weeks prior to the end of spawning. Exposure to sewage sludge had a significant effect on male mortality rates but no significant effects on the gonadosomatic index of males or females. There were no major effects of sludge exposure on testes androgen content or on testes release of androgens after in vitro gonadotrophin stimulation. Fecundity and the number of larvae produced were not significantly affected by the sludge exposure. There was a tendency for eggs and larvae from sludge-exposed females to have a higher mortality rate. At a population level, however, there was a major reduction in the number of eggs and larvae produced in the sludge-exposed population which reflected a failure of some females to spawn. Of the larvae produced, 60-70% from sludge-exposed parents were lighter and had a larger yolk-sac volume compared to larvae from non-exposed parents which may have indicated impaired yolk utilisation.

Comparative effects of the sublethal poisoning of zinc, copper and lead on the gonads of the teleost Puntius conchonius ham

The histopathological impact of the sublethal concentrations of zinc, copper and lead on the gonads of Puntius conchonius was evaluated. Copper (Cu) interfered with spermatogenesis temporarily while zinc (Zn) and lead (Pb) produced dilation in the testicular blood capillaries with necrosis and disintegration of the seminiferous tubules. All three metals induced significant atresia in the ovary. The damaged mainly the younger oocytes, whereas Cu and Pb were more effective on relatively older oocytes. The studies suggest a direct action of heavy metals on the gonads.

Impaired spermatogenesis in arsenic treated freshwater fish, Colisa fasciatus (Bl. and Sch.)

The testicular architecture of Colisa fasciatus was studied after 15 and 30 days of exposure to 14.0 or 2.0 mg/l arsenic(III) oxide. Marked alterations occurred at 14.0 mg/l after 30 days of treatment, while 2.0 mg/l (the concentration in Lake Chilwa) produced little change. In addition to the degenerative changes in the lobules, the interstitial Leydig cells (steroid-secreting cells) underwent significant reduction (P less than 0.01) in diameter. Varying degrees of necrosis and pyknosis suggested reduced secretory levels. Such changes may be correlated with the corresponding altered spermatogenesis.

Impaired ovarian functions in arsenic-treated freshwater fish, Colisa fasciatus (bl. and sch.)

Arsenic(III) oxide (2.0 mg/l) after 15 and 30 days exposure, and 14.0 mg/l concentration after 15 days exposure, produced no marked histological alteration in the ovary of Colisa fasciatus during its mature phase, whereas 14.0 mg/l arsenic(III) oxide concentration after 30 days exposure decreased the development of oocyte (II and III stage), reduced the number and diameter of nucleoli and increased the number of atretic follicles. The possible mechanism for these alterations is discussed.