Antioxidative Effect of Selenium in Cadmium-Exposed Tardigrade (H. exemplaris)

Abstract

On a daily basis, aquatic ecosystem is exposed to heavy metals, drugs, and chemicals; the continuous exposure results in a toxic effect, depending on the dose and duration of exposure. Studies have confirmed tardigrade to be highly tolerant to very high temperature, pressure, gravity, etc. This study aimed at confirming various toxic damages caused by exposure to cadmium solution and the antioxidative effect of selenium in the amelioration of the effects. To model the environmental route of exposure, toxicants were added to tardigrade (H. exemplaris) culture done in deionized water to remove non-bioavailability of cadmium caused by culturing tardigrades in agar medium. As was observed in pure tardigrades, with no exposure to cadmium, there was no alteration in the metabolism of tardigrades. However, when solutions of cadmium were added to the tardigrade culture (at concentrations of 0.5 ppb, 1.0 ppb), increased sensitivity to oxidative stress as well as significantly reduced metabolism was observed. More specifically, after 6 h of exposure, the metabolism dropped relative to the control organism. The produced reactive oxygen species was confirmed with fluorescence microscopy. Finally, treating the exposed tardigrades with deionized water did not ameliorate the toxic effects of the cadmium to the tardigrades; instead, the effects were reduced, and this is due to water only diluting the toxicants. However, with 0.5 ppm of selenium, using sodium selenite counteracted the cadmium-induced reactive oxygen species produced over a 6-h period from the point of exposure through the exposure period. The antioxidative enzymes (catalase (CAT), superoxide dismutase (SOD), and acetylcholine (Ach)) produced were quantified using various assay methods. In conclusion, from an environmental perspective, selenium is a relevant antioxidant using the appropriate dosage.