Arsenic is a pollutant that can be detected in different chemical forms in soil. However, the toxicological
effects of different arsenic species on organisms have received little attention. In this study, we exposed
earthworms Eisenia fetida to artificial soils contaminated by arsenite [As(III)], arsenate [As(V)], monomethylarsonate
(MMA) and dimethylarsinate (DMA) for 28 and 56 days. Three biomarkers including lipid
peroxidation (LPO), metallothioneins (MTs) and lysosomal membrane stability (LMS) were analyzed in
the organisms. In addition, the contents of total arsenic and arsenic species in earthworms were also
determined to investigate the effects of bioaccumulation and biotransformation of arsenic on biomarkers
and to evaluate the dose-response relationships. The results showed that the relationship between the
three biomarkers and the two inorganic arsenic species were dose dependent, and the correlation levels
between the biomarkers and As(III) were higher than that between the biomarkers and As(V). Trivalent
arsenic species shows more toxicity than pentavalent arsenic on the earthworms at molecular and
subcellular level, including oxidative damage, MTs induction and lysosomal membrane damage. The
toxicity of MMA and DMA was lower than inorganic arsenic species. However, the occurrence of
demethylation of organic arsenics could lead to the generation of highly toxic inorganic arsenics and
induce adverse effects on organisms. The biotransformation of highly toxic inorganic arsenics to the less
toxic organic species in the earthworms was also validated in this study. The biomarker responses of the
earthworm to different arsenic species found in this study could be helpful in future environment
monitoring programs.