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A freshwater microalga,Chlorella vulgaris,was grown in the presence of varying phosphate concentrations (＜10-500 μg/L P) and environmentally realistic concentrations of arsenate (As(Ⅴ)) (5-50 μg/L As).Arsenic speciation in the culture medium and total cellular arsenic were measured using AEC-ICP-MS and ICP-DRC-MS,respectively,to determine arsenic biotransformation and uptake in the various phosphorus scenarios.At high phosphate concentration in the culture medium,＞ 100 μg/L P,the uptake and biotransformation of As(Ⅴ) was minimal and dimethylarsonate (DMAs(Ⅴ)) was the dominant metabolite excreted by C.vulgaris,albeit at relatively low concentrations.At common environmental P concentrations,0-50 μg/L P,the uptake and biotransformation of As(Ⅴ) increased.At these higher As-uptake levels,arsenite (As(Ⅲ)) was the predominant metabolite excreted from the cell.The concentrations of As(Ⅲ) in these low P conditions were much higher than the concentrations of methylated arsenicals observed at the various P concentrations studied.The switchover threshold between the (small) methylation and (large) reduction of As(Ⅴ) occurred around a cellular As concentration of 1 fg/cell.The observed nearly quantitative conversion of As(Ⅴ) to As(Ⅲ) under low phosphate conditions indicates the importance of As(Ⅴ) bio-reduction at common freshwater P concentrations.These findings on the influence of phosphorus on arsenic uptake,accumulation and excretion are discussed in relation to previously published research.The impact that the two scenarios of As(Ⅴ) metabolism,As(Ⅲ) excretion at high As(Ⅴ)-uptake and methylarsenical excretion at low As(Ⅴ)-uptake,have on freshwater arsenic speciation is discussed.