Comparison of toxicity of silver nanomaterials and silver nitrate on developing zebrafish embryos: Bioavailability, osmoregulatory and oxidative stress.

The mechanisms of toxicity of engineered nanomaterials (ENMs) to the early life stages of freshwater fish, and the relative hazard compared to dissolved metals, is only partially understood. In the present study, zebrafish (Danio rerio) embryos were exposed to lethal concentrations of silver nitrate (AgNO3) or silver (Ag) ENMs (primary size 42.5 ± 10.2 nm). The 96 h-LC50 for AgNO3 was 32.8 ± 0.72 µg Ag L-1 (mean ± 95% CI) compared to 6.5 ± 0.4 mg L-1 of the whole material for Ag ENMs; with the ENMs being orders of magnitude less toxic than the metal salt. The EC50 for hatching success was 30.5 ± 1.4 µg Ag L-1 and 6.04 ± 0.4 mg L-1 for AgNO3 and Ag ENMs, respectively. Further sub-lethal exposures were performed with the estimated LC10 concentrations for both AgNO3 or Ag ENMs over 96 h, where about 3.7% of the total Ag as AgNO3 was internalised, as measured by Ag accumulation in the dechorionated embryos. However, for the ENM exposures, nearly all (99.8%) of the total Ag was associated with chorion; indicating the chorion as an effective barrier to protect the embryo in the short term. Calcium (Ca2+) and sodium (Na+) depletion was induced in embryos by both forms of Ag, but hyponatremia was more pronounced in the nano form. Total glutathione (tGSH) levels declined in embryos exposed to both Ag forms, but a superior depletion occurred with the nano form. Nevertheless, oxidative stress was mild as superoxide dismutase (SOD) activity stayed uniform and the sodium pump (Na+/K+-ATPase) activity had no appreciable inhibition compared to the control. In conclusion, AgNO3 was more toxic to the early life stage zebrafish than the Ag ENMs, still differences were found in the exposure and toxic mechanisms of both Ag forms.

Differences in toxicity and accumulation of metal from copper oxide nanomaterials compared to copper sulphate in zebrafish embryos: Delayed hatching, the chorion barrier and physiological effects.

The mechanisms of toxicity of engineered nanomaterials (ENMs) to the early life stages of freshwater fish, and the relative hazard compared to dissolved metals, is only partially understood. In the present study, zebrafish embryos were exposed to lethal concentrations of copper sulphate (CuSO4) or copper oxide (CuO) ENMs (primary size ∼15 nm), and then the sub-lethal effects investigated at the LC10 concentrations over 96 h. The 96 h-LC50 (mean ± 95% CI) for CuSO4 was 303 ± 14 µg Cu L-1 compared to 53 ± 9.9 mg L-1 of the whole material for CuO ENMs; with the ENMs being orders of magnitude less toxic than the metal salt. The EC50 for hatching success was 76 ± 11 µg Cu L-1 and 0.34 ± 0.78 mg L-1 for CuSO4 and CuO ENMs respectively. Failure to hatch was associated with bubbles and foam-looking perivitelline fluid (CuSO4), or particulate material smothering the chorion (CuO ENMs). In the sub-lethal exposures, about 42% of the total Cu as CuSO4 was internalised, as measured by Cu accumulation in the de-chorionated embryos, but for the ENMs exposures, nearly all (94%) of the total Cu was associated with chorion; indicating the chorion as an effective barrier to protect the embryo from the ENMs in the short term. Both forms of Cu exposure caused sodium (Na+) and calcium (Ca2+), but not magnesium (Mg2+), depletion from the embryos; and CuSO4 caused some inhibition of the sodium pump (Na+/K+-ATPase) activity. Both forms of Cu exposure caused some loss of total glutathione (tGSH) in the embryos, but without induction of superoxide dismutase (SOD) activity. In conclusion, CuSO4 was much more toxic than CuO ENMs to early life stage zebrafish, but there are subtle differences in the exposure and toxic mechanisms for each substance.

Phytotoxicity of silver nanoparticles to Lemna minor: Surface coating and exposure period-related effects.

Silver nanoparticles (Ag NPs) exponential production raises concern about their environmental impact. The effects of Ag NPs to aquatic plants remain scarcely studied, especially in extended exposures. This paper aims to evaluate Ag NPs effects in Lemna minor at individual and sub-individual levels, focusing on three variables: Ag form (NPs versus ions - Ag+), NPs surface coating (citrate vs polyvinylpyrrolidone - PVP) and exposure period (7 vs 14days). Endpoints were assessed at individual level (specific growth rate, chlorosis incidence and number of fronds per colony) and sub-individual level (enzymatic activities of catalase (CAT), guaiacol peroxidase (GPx) and glutathione-S-transferase (GST)). Generally, plants exposed to all Ag forms underwent decays on growth rate and fronds per colony, and increases on chlorosis, GPX and GST, but no effects on CAT. The most sensitive endpoints were specific growth rate and GPx activity, showing significant effects down to 0.05mg/L for Ag NPs and 3µg/L for Ag+, after 14days. Ag+ showed higher toxicity with a 14d-EC50 of 0.0037mg Ag/L. Concerning surface coating, PVP-Ag NPs were more deleterious on growth rate and fronds per colony, whereas citrate-Ag NPs affected more the chlorosis incidence and GPx and GST activities. The exposure period significantly affected chlorosis: 14days triggered a chlorosis increase in Ag+-exposed plants and a decrease in Ag NPs-exposed plants when compared to 7days. Ag NPs induced an oxidative stress status in cells, thus ensuing upregulated enzymatic activity as a self-defense mechanism. Since Ag NPs dissolution might occur on a steady and continuous mode along time, and the average longevity of fronds, we propose longer exposures periods than the recommended by the OECD guideline. This approach would provide more relevant and holistic evidences on the overall response of freshwater plants to Ag NPs in an ecological relevant scenario.

From sub cellular to community level: toxicity of glutaraldehyde to several aquatic organisms.

The biocide glutaraldehyde (GA) is widely used as a disinfectant and sterilizing agent against bacteria and virus in hospital and veterinary facilities. GA or its metabolites may reach aquatic ecosystems due to incomplete or inadequate treatment of wastewaters. Data about GA effects at lethal and a sub lethal level to non-target organisms is needed so that a risk assessment to aquatic ecosystems can be done. Thus, in this work a battery of toxicity tests with primary producers, primary consumers and secondary consumers were performed and a species sensitive distribution (SSD) for GA was built. Moreover, effects on biomarkers (catalase, lactate dehydrogenase, glutathione-S-transferase, and cholinesterase) were measured in Danio rerio embryos and adults. Primary consumers (Thamnocephalus platyurus 24h - EC50=3.6 mg/l; Daphnia magna 48 h - EC50=6.6 mg/l) and D. rerio adults (96 h - LC50=5.5mg/l) were slightly more sensitive to GA than D. rerio embryos (96 h - LC50=22.2mg/l) and primary producers (Lemna minor 168 h - EC50=73.8 mg/l; Pseudokirchneriella subcapitata 72h - EC50=12.3mg/l; Chlamydomonas reinhardtii 72 h - EC50=14.6 mg/l; Chlorella vulgaris 72 h - EC50=31.3mg/l). However, no significant differences between the trophic levels were found and general HC5 and HC50 values of 0.6 and 11.4 mg/l were respectively estimated. Despite the low GA toxicity to D. rerio embryos, hatching delay and malformations were found (96 h - EC50=11.9 mg/l). For biomarkers, an inhibition of lactate dehydrogenase activity was observed in embryos whereas an inhibition in catalase, lactate dehydrogenase and glutathione-S-transferase activities was observed in adults. Thus, GA is moderately toxic (doses>1mg/l) to aquatic organisms, independently of the trophic level. However, considering the varied range of effects depending on the life stage and organism tested and relatively low HC5 value of 0.6 mg/l, mesocosm and chronic toxicity tests seem to be the next step in direction of more realistic scenarios of GA risk assessment in aquatic ecosystems.