Martin Tsz Ki Tsui

Biokinetics and tolerance development of toxic metals in Daphnia magna

Daphnia magna is widespread in many freshwater systems of temperate regions and frequently is used to test metal toxicity. Recently, studies have been performed to determine metal biokinetics and development of tolerance in this important zooplankton species. In the present paper, we review the recent progress in these areas and suggest possible directions for future studies. Substantial differences exist in aqueous uptake, dietary assimilation, and elimination of several metals (Cd, Se, Zn, Ag, Hg, and MeHg) by D. magna. The routes of uptake are metal-specific, with Se and MeHg being accumulated predominantly through diet. All metals except Ag can be biomagnified from algae to D. magna, providing that metal concentrations in algae and algal food density are relatively low. Methylmercury is biomagnified in all situations. As a route for metal elimination in D. magna, maternal transfer is especially important for Se, Zn, and MeHg. On the other hand, the effect of single-generation exposure to metals on D. magna is very different from multigeneration exposure, which often results in a significantly higher metal tolerance. Moreover, D. magna easily loses metal tolerance developed through long-term exposure. Recovery from metal stress can temporarily increase the sensitivity of D. magna to metal toxicity. Finally, metallothionein-like protein is responsible for minimizing metal toxicity in D. magna. The results inferred from these studies can be extrapolated to other aquatic invertebrates as well as to other pollutants in the aquatic environment.

Influences of maternal exposure on the tolerance and physiological performance of Daphnia magna under mercury stress

We examined the tolerance development to mercury (Hg) by a population of freshwater zooplankton (Daphnia magna) with different pre-exposure histories to Hg. The growth and reproductive performance of the F1 offspring as affected by the maternal (F0) and offspring (F1) exposures was quantified. The F0 daphnids exposed to 2.5 and 25 nM of Hg for 4 d and followed by 4 d of depuration had elevated levels of Hg and metallothionein-like proteins (MTLPs), as well as higher tolerance to Hg toxicity than the control daphnids. The higher Hg tolerance may be attributed to the higher proportion of Hg partitioned to the MTLPs. Moreover, significant enhancement of Hg tolerance also was found in the F1 offspring originating from the F0 mothers exposed to 25 nM of Hg, but there was no significant induction of MTLPs in these F1 offspring when compared to the offspring from the control mothers. The Hg tissue concentrations in the F1 neonates were approximately 25% of those in the F0 adults. However, there was similar Hg tolerance in the F2 offspring originating from both the control and Hg-exposed F0 mothers, indicating that the Hg tolerance in the daphnids disappeared two generations after Hg contamination. Further exposure of the F1 offspring to different Hg concentrations (1.5 and 15 nM for 28 d) indicated that maternal exposure history did not affect their growth and reproductive performance, which solely were influenced by the offspring exposure. Unexpectedly, the F1 offspring exposed to Hg had significantly higher final wet weights and reproductive rates than the control groups, suggesting the possibility of Hg hormesis. Furthermore, the maternal exposure had no effect on the Hg accumulation and the MTLP concentrations in the F1 offspring. Therefore, we concluded that the Hg tolerance might disappear quickly once the Hg contamination was removed and the maternal exposure history was not important in determining the physiological performance and Hg accumulation of the subsequent generations.

Maternal transfer efficiency and transgenerational toxicity of methylmercury in Daphnia magna

We examined maternal transfer efficiency, retention by subsequent generations, and transgenerational toxicity of methylmercury (CH3Hg or MeHg) in a population of freshwater zooplankton (Daphnia magna). The effect of dietary MeHg residence time in the daphnids on the efflux system also was quantified. After ingesting a relatively high dosage of MeHg, D. magna exhibited a reduction of live neonates and an increase of undeveloped eggs (or embryos), which reflected the sublethal toxicity of MeHg. The daily maternal transfer efficiency of MeHg to both reproductive outputs ranged from 0.42 to 4.9% over different ages of the parental daphnids, which was dependent on the daily reproductive output. During the lifetime of D. magna, reproduction contributed to 10.8% +/- 1.74% (n = 3) SD of total MeHg loss from the parental daphnids. The percentage of MeHg retention by the second generation (F1) of D. magna (40-60%) was generally higher than that by the parental generation (F0; approximately 25%) after 20 d of depuration. Methylmercury imposed sublethal toxicity to the F0 and F1 generations, but a smaller effect was observed on the F2 generation. Because of the very low MeHg body burden in the subsequent generations, we hypothesized that factors other than MeHg, such as nutritional deficiency in the offspring contributed to the transgenerational toxicity. Different MeHg residence times did not significantly affect the efflux rate of MeHg but did significantly affect the relative importance of reproduction as the elimination pathway for MeHg. Based on the MeHg body burden of neonates, we estimated that MeHg took 2.5 to 3.0 d to be optimally transferred from assimilation (e.g., gut) to the site of egg development (e.g., brood chamber) in D. magna. Our study demonstrated that maternal transfer of MeHg in freshwater zooplankton is an important predictor of MeHg concentration in their offspring and is a time-dependent and highly dynamic process.

Multigenerational acclimation of Daphnia magna to mercury: Relationships between biokinetics and toxicity

We examined the effects of multigenerational exposure of mercury (Hg) on Hg toxicity and biokinetics in a population of Daphnia magna. After chronic Hg exposure at 3.8 microg Hg/L, the first generation (F0) adults had an elevated 24-h median lethal concentration (LC50) of Hg (76 microg/L) when compared to the control adults (56 microg/L). The dissolved influx rate of Hg was depressed significantly in the Hg-treated adults, which was accompanied by a reduced ingestion rate and enhanced induction of metallothionein-like proteins (MTLP). The second-generation (F1) juveniles originating from the control and exposed lines had no major differences in these parameters (except the dietary assimilation efficiency). Recovery from Hg stress enhanced the vulnerability of F1 adults to Hg toxicity, with a reduced 48-h LC50 (44 microg/L) and a decreased concentration of MTLP (80% of control). Nevertheless, Hg-treated F1 adults had similar tolerance (in terms of LC50s) as the control line, indicating that D. magna acclimated to Hg stress after the first generation of exposure. No major difference occurred in the Hg biokinetics and toxicity among different groups of F2 daphnids. However, the F2 neonates produced by the Hg-treated F1 adults had much higher 48-h LC50 (149 microg/L) and MTLP concentration (148% of control) when there was continuous Hg exposure after birth. We concluded that acclimation to Hg stress occurred quickly in D. magna, though animals recovering from Hg stress were more vulnerable to Hg toxicity. Both ingestion rate and MTLP may not be good biomarkers of Hg stress in the field, because acclimation can be achieved through multigenerational exposure to elevated Hg concentrations.

Understanding sources of methylmercury in songbirds with stable mercury isotopes: Challenges and future directions

Mercury (Hg) stable isotope analysis is an emerging technique that has contributed to a better understanding of many aspects of the biogeochemical cycling of Hg in the environment. However, no study has yet evaluated its usefulness in elucidating the sources of methylmercury (MeHg) in songbird species, a common organism for biomonitoring of Hg in forested ecosystems. In the present pilot study, we examined stable mercury isotope ratios in blood of 4 species of songbirds and the invertebrates they are likely foraging on in multiple habitats in a small watershed of mixed forest and wetlands in Acadia National Park in Maine (USA). We found distinct isotopic signatures of MeHg in invertebrates (both mass-dependent fractionation [as δ202 Hg] and mass-independent fractionation [as Δ199 Hg]) among 3 interconnected aquatic habitats. It appears that the Hg isotopic compositions in bird blood cannot be fully accounted for by the isotopic compositions of MeHg in lower trophic levels in each of the habitats examined. Furthermore, the bird blood isotope results cannot be simply explained by an isotopic offset as a result of metabolic fractionation of δ202 Hg (e.g., internal demethylation). Our results suggest that many of the birds sampled obtain MeHg from sources outside the habitat they were captured in. Our findings also indicate that mass-independent fractionation is a more reliable and conservative tracer than mass-dependent fractionation for identifying sources of MeHg in bird blood. The results demonstrate the feasibility of Hg isotope studies of songbirds but suggest that larger numbers of samples and an expanded geographic area of study may be required for conclusive interpretation. Environ Toxicol Chem 2018;37:166-174.