Selenium (Se) is commonly recognized as a protective element with an antagonistic effect against mercury (Hg) toxicity.However,the mechanisms of this Hg-Se antagonism are complex and remain controversial.To gain insight into the Hg-Se antagonism,a type of unicellular eukaryotic protozoa (Tetrahymena malaccensis,T.malaccensis) was selected and individually or jointly exposed to two Hg and three Se species.We found that Se species showed different toxic effects on the prohferation of T.malaccensis with the toxicity following the order:selenite (Se(Ⅳ))＞ selenomethionine (SeMeth)＞ selenate (Se(Ⅵ)).The Hg-Se antagonism in Tetrahymena was observed because the joint toxicity significantly decreased under co-exposure to highly toxic dosages of Hg and Se versus individual toxicity.Unlike Se(Ⅳ) and Se(Ⅵ),non-toxic dosage of SeMeth significantly decreased the Hg toxicity,revealing the influence of the Se species and dosages on the Hg-Se antagonism.Unexpectedly,inorganic divalent Hg (Hg2+) and monomethylmercury (MeHg) also displayed detoxification towards extremely highly toxic dosages of Se,although their detoxifying efficiency was discrepant.These results suggested mutual Hg-Se detoxification in T.malaccensis,which was highly dependent on the dosages and species of both elements.As compared to other species,SeMeth and MeHg promoted the Hg-Se joint effects to a higher degree.Additionally,the Hg contents decreased for all the Hg-Se co-exposed groups,revealing a sequestering effect of Se towards Hg in T.malaccensis.