Selenium (Se) is a necessary micro-element for humanbeing. It is also a component of selenosysteine (SeCys). In bony fishes, Se is necessary for more than 40 Seleneproteines including GPx, seleneprotein P (SelP). GPx has significant detoxifying effect in the oxidative stresses caused by hydrogen peroxide and organic peroxides. SelP has some detoxifying  effect to heavymetal toxicity. Low concentration Se is beneficial to fish growth, e.g. it increases the body weight. Fish show best GPx activity when Se concentration in diet was 0.15-0.7 mg/kg. The diet containing 3-4 mg/kg Se is toxic to fish.

The Se concentration is as high 140-1400 mg/ kg in the water inlet of sewage disposal work.  Such concentration is highly toxic, may cause death, deformity, hinder in growth and reproduction, tissue damage, GSH metabolism disorder of organisms and even distinction of sensitive species. The Se content in water and soil environment of China varies in  large extent. Too high or too low environment of Se concentration both are harmful.

Prof XIE Lingtian, Associate Prof WU Xing and coworkers in the research group of Environmental Toxicology in IAE tested the toxic effect of diet Se on Heterandria formosa and effect of Se predisposition on the toxicity of Cd exposure to fish. In the experiment, the fish were pre-disposed to safe concentration of SE (2 µg/g dw feed) 10 days and then exposed to 0.5 mg/L for 5days. The accumulation of both Se(IV) and Organic Se(Se-Met) very fast, the lipid peroxidation level decreased and the CAT level increased in the fish body, but Na⁺/K⁺-ATPase level in gill decreased after predisposition to Se. Cd exposure induced increase in lipid peroxidation and decrease in CAT activity. This is contradictory to Se exposure. However, Cd exposure, similarly to Se, reduced the Na⁺/K⁼-ATPAse level, implicated that both Cd and Se have certain toxicity at low concentrations. The Se predisposition reduced the Cd absorption and accumulation in fish body and reduced the Cd toxicity to some extent.

The results indicate that Se enrichment in fish body decreased Cd toxicity. The beneficial and harmful effect of Se may show together. This phenomenon increases the difficulty in assessment of the diet Se effect.

The work was supported by NNSFC (31270549、41303087、21267015、21567019), Inner Mongolia Science Foundation (2015MS0804、2015e2016) and CAS One Hundred Talent Program. The results were published in Chemosphere (2016: 161, 358-364. doi: 10.1016/j.chemosphere.2016.07.035), entitled as A low level of dietary selenium has both beneficial and toxic effects and is protective against Cd-toxicity in the least killifish Heterandria formosa”.