Document Type

Dissertation

Date of Award

2021

School/College

College of Science, Engineering, and Technology (COSET)

Degree Name

Ph.D. in Environmental Toxicology

Committee Chairperson

Ayodotun Sodipe Shishir Shishodia, Sonya Good, Alamelu Sundaresan, Mofolorunso Enigbokan

Committee Member 1

Shishir Shishodia

Committee Member 2

Sonya Good

Committee Member 3

Alamelu Sundaresan

Committee Member 4

Mofolorunso Enigbokan

Keywords

Cadmium, HSP-70, Hypsibius Exemplaris, Reactive Oxygen Species, Tardigrades

Abstract

Heavy metals such as Cadmium are known stressors in aquatic environments due to being persistent in nature and the ability to bioaccumulate in organisms. Cadmium ranks 7th on the Agency for Toxic Substance and Disease Registry’s 2019 Substance Priority List. It is also listed as a substance of very high concern by the European Chemicals Agency. Thus, it is considered one of the most important environmental and occupational pollutants existing. This study aimed at investigating the cellular responses to the heavy metal Cadmium (Cd) and observe the processes that are involved in its detoxification. Tardigrade specimens (Hypsibius exemplaris) were exposed to varying concentrations of cadmium ranging from 1mg/l to 15mg/l at 6, 12 and 24-hours intervals. The expressions of reactive oxygen species (ROS) and antioxidant enzymes (superoxide dismutase, catalase and glutathione-s-transferase) were measured after cadmium exposure. Tardigrade viability was also assessed based on observed locomotive movements of the tardigrades. The results of this study indicate that antioxidant defense components in the tardigrade Hypsibius exemplaris are significantly affected by cadmium toxicity. The viability analysis showed that the in vivo acute exposures decreased starting from the 5 mg/L concentration over a 24-hours period. The results show that cadmium can induce the excessive production of reactive oxygen species (ROS) resulting in oxidative stress as ROS was statistically significantly higher than control after 24 hours of cadmium exposure (p<0.05). Cadmium triggers a protective cellular antioxidant response involving glutathione-s-transferase (GST). After 12 hours, GST in exposed specimens was statistically significantly lower (p<0.01) than in control specimens, further decreasing after 24 hours. Glutathione (GSH), highly abundant in cells, is a primary target for free cadmium ions. Therefore, the observed cadmium-induced depletion of the GSH pool indicates a disturbance of the redox balance leading to an oxidative environment. This research contributes to the existing knowledge on how cadmium alters the cellular redox state, leading to cadmium-induced pathologies. Overall, the results support the notion that the mode of cadmium toxicity in tardigrades is evident through both decreases in antioxidants and general cellular oxidative stress from accumulated ROS. This study also investigated the impact of cadmium exposure on the differential gene expression of the stress-induced protein HSP70. The results reveal a time-dependent effect of cadmium exposure on HSP70 expression. Exposure to 7.5mg/l of cadmium over 6hrs, 12hrs and 24hrs induced an increase in HSP70 mRNA expression. This supports the literature on the important role of heat shock proteins in conferring protection and tolerance against deleterious concentrations of heavy metals such as cadmium. The results also show that Hypsibius exemplaris has a lower tolerance towards cadmium when compared to the higher LC50 values reported with the well-studied bioindicator species Caenorhabditis elegans. However, H. Exemplaris has a higher tolerance than reported with the other well-investigated species Daphnia magna.

Included in

Biology Commons

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