Arsenic-contaminated water is a serious hazard for human health.Plankton plays a critical role in the fate and toxicity of arsenic in water by accumulation and biotransformation.Spirulina platensis (S.platensis),a typical plankton,is often used as a supplement or feed for pharmacy and aquiculture,and may introduce arsenic into the food chain,resulting in a risk to human health.However,there are few studies about how S.platensis biotransforms arsenic.In this study,we investigated arsenic biotransfoation by S.platensis.When exposed to arsenite (As(Ⅲ)),S.platensis accumulated arsenic up to 4.1 mg/kg dryu weight.After exposure to As(Ⅲ),arsenate (As(Ⅴ)) was the predominant species making up 64％ to 86％ of the total arsenic.Monomethylarsenate (MMA(Ⅴ)) and dimethylarsenate (DMA(Ⅴ)) were also detected.An arsenite S-adenosylmethionine methyltransferase from S.platensis (SpArsM) was identified and characterized.SpArsM showed low identity with other reported ArsM enzymes.The Escherichia coli AW3110 bearing SparsM gene resulted in As(Ⅲ) methylation and conferring resistance to As(Ⅲ).The in vitro assay showed that SpArsM exhibited As(Ⅲ) methylation activity.DMA(Ⅴ) and a small amount of MMA(Ⅴ) were detected in the reaction system within 0.S hr.A truncated SpArsM derivative lacking the last 34 residues still had the ability to methylate As(Ⅲ).The three single mutants of SpArsM (C595,C1865,and C2385) abolished the capability of As(Ⅲ) methylation,suggesting the three cysteine residues are involved in catalysis.We propose that SpArsM is responsible for As methylation and detoxification of As(Ⅲ) and may contribute to As biogeochemistry.