Thermal stress-stimulated ZnO toxicity inhibits reproduction of freshwater crustacean Daphnia magna.

Elevated temperatures due to climate change pose a variety of environmental risks to the freshwater ecosystem. At the same time, zinc oxide (ZnO) has become widely used and has entered the freshwater environment. As thermal stress may potentially impact the physicochemical properties of ZnO, its toxicity to freshwater organisms in the face of global warming is poorly understood. The potential effects on reproductive performances, including oogenesis, are of particular concern. In this study, we investigate the reproductive performances and related mRNA abundance of the zooplankton Daphnia magna under conditions of ZnO exposure and heat stress. The results revealed that ZnO and elevated temperature delayed maturity and juvenile production of D. magna. Histological observations indicated that oogenesis was inhibited, and the number and size of oocytes were reduced in the condition of ZnO exposure under heat stress. Eventual offspring in the same treatment exhibited decreased numbers, size, and quality. Congenital juvenile anomalies were increased, such as deformed eye, and impaired antenna and tail spine. Moreover, both ZnO and elevated temperature treatments inhibited expression levels of reproduction-related genes (vtg, EcR and VMO1) and induced the dmrt93b gene involved in the production of male offspring. Furthermore, we found that D. magna tried to cope with ZnO and thermal stress by upregulating hsp90, HIF-1α and HIF-1ß. ZnO and heat stress inhibited the reproductive capacity of D. magna, produced deleterious effects on reproduction-associated physiological pathways, and damaged reproductive outcomes.

Combined effects of nanoplastics and elevated temperature in the freshwater water flea Daphnia magna.

Global warming and nanoplastics (NPs) are critical global issues. Among NPs, one of the most hazardous types of plastics, polystyrene (PS), poses ecotoxicological threats to several freshwater organisms. The degree of toxicity of PS-NPs is strongly influenced by various environmental factors. This study illustrates the combined effects of temperature and PS-NPs on the water flea Daphnia magna. The sensitivity of D. magna to PS-NPs was tested under control (23 °C) and elevated temperatures (28 °C). As a result, increased temperatures influenced the uptake and accumulation of PS-NPs. Co-exposure to both higher temperatures and PS-NPs resulted in a drastic decrease in reproductive performance. The level of oxidative stress was found to have increased in a temperature-dependent manner. Oxidative stress was stimulated by both stressors, leading to increased levels of reactive oxygen species and antioxidant enzyme activity supported by upregulation of antioxidant enzyme-related genes under combined PS-NPs exposure and elevated temperature. In the imbalanced status of intracellular redox, activation of the p38 mitogen-activated protein kinase signaling pathway was induced by exposure to PS-NPs at high temperatures, which supported the decline of the reproductive capacity of D. magna. Therefore, our results suggest that PS-NPs exposure along with an increase in temperature significantly affects physiological processes triggered by damage from oxidative stress, leading to severely inhibited reproduction of D. magna.

Ecotoxicological, ecophysiological, and mechanistic studies on zinc oxide (ZnO) toxicity in freshwater environment.

The world has faced climate change that affects hydrology and thermal systems in the aquatic environment resulting in temperature changes, which directly affect the aquatic ecosystem. Elevated water temperature influences the physico-chemical properties of chemicals in freshwater ecosystems leading to disturbing living organisms. Owing to the industrial revolution, the mass production of zinc oxide (ZnO) has been led to contaminated environments, and therefore, the toxicological effects of ZnO become more concerning under climate change scenarios. A comprehensive understanding of its toxicity influenced by main factors driven by climate change is indispensable. This review summarized the detrimental effects of ZnO with a single ZnO exposure and combined it with key climate change-associated factors in many aspects (i.e., oxidative stress, energy reserves, behavior and life history traits). Moreover, this review tried to point out ZnO kinetic behavior and corresponding mechanisms which pose a problem of observed detrimental effects correlated with the alteration of elevated temperature.

Disturbances in growth, oxidative stress, energy reserves and the expressions of related genes in Daphnia magna after exposure to ZnO under thermal stress.

The toxicological effects of metal contamination are influenced by the ambient temperature. Therefore, global warming affects the toxicity of metal contamination in aquatic ecosystems. ZnO is widely used as a catalyst in many industries, and causes contamination in aquatic ecosystems. Here, we investigated the effects of ZnO concentration under elevated temperature by observing growth, oxidative stress, energy reserves and related gene expression in exposed Daphnia magna. Body length and growth rate increased in neonates exposed to ZnO for 2 days but decreased at 9 and 21 days under elevated temperature. ZnO concentration and elevated temperature induced oxidative stress in mature D. magna by reducing superoxide dismutase (SOD) activity and increasing malondialdehyde (MDA) levels. In contrast, juveniles were unaffected. Carbohydrate, protein and caloric contents were reduced throughout development in D. magna treated with ZnO and elevated temperature in all exposure periods (2, 9 and 21 days). However, lipid content also decreased in mature D. magna treated with ZnO cultured under elevated temperature, while that of juveniles showed an increase in lipid content. Therefore, energy was perhaps allocated to physiological processes for detoxification and homeostasis. Moreover, expression patterns of genes related to physiological processes changed under elevated temperature and ZnO exposure. Taken together, our results highlight that the combination of temperature and ZnO concentration induced toxicity in D. magna. This conclusion was confirmed by the Integrated Biological Response (IBR) index. This study shows that changes in biological levels of organization could be used to monitor environmental change using D. magna as a bioindicator.

Temperature and concentration of ZnO particles affect life history traits and oxidative stress in Daphnia magna.

Temperature affects physiological processes in organisms and the toxicity of chemicals. The widespread industrial use of ZnO causes contamination in aquatic ecosystems. This study aimed to investigate the chronic toxicity of ZnO at different temperatures using Daphnia magna as a model organism. The chronic toxicity of five different concentrations of ZnO was assessed at 23 °C and 28 °C. The results showed that higher concentrations of ZnO inhibited growth, production of first clutch eggs and juvenile accumulation at both 23 °C and 28 °C. Growth rate, numbers of first clutch eggs and juvenile accumulation were lower at 28 °C than at 23 °C. We also observed the levels of malondialdehyde (MDA) and superoxide dismutase (SOD) activity. At higher concentrations of ZnO, oxidative stress was induced leading to increase MDA level and decrease SOD activity at 28 °C. These findings indicated that high temperature and high concentration of ZnO inhibited the activity of enzymatic proteins. Nonetheless, among all treatments, the accumulation of zinc in D. magna was not significantly different. Our results suggested that both ZnO and higher temperature induced oxidative stress in D. magna. As a result, MDA concentration increased, SOD activity changed and the growth and reproduction of D. magna was adversely affected.