Toxicity of arsenic species to three freshwater organisms and biotransformation of inorganic arsenic by freshwater phytoplankton (Chlorella sp. CE-35).

In the environment, arsenic (As) exists in a number of chemical species, and arsenite (As(III)) and arsenate (As(V)) dominate in freshwater systems. Toxicity of As species to aquatic organisms is complicated by their interaction with chemicals in water such as phosphate that can influence the bioavailability and uptake of As(V). In the present study, the toxicities of As(III), As(V) and dimethylarsinic acid (DMA) to three freshwater organisms representing three phylogenetic groups: a phytoplankton (Chlorella sp. strain CE-35), a floating macrophyte (Lemna disperma) and a cladoceran grazer (Ceriodaphnia cf. dubia), were determined using acute and growth inhibition bioassays (EC₅₀) at a range of total phosphate (TP) concentrations in OECD medium. The EC₅₀ values of As(III), As(V) and DMA were 27 ± 10, 1.15 ± 0.04 and 19 ± 3 mg L(-1) for Chlorella sp. CE-35; 0.57 ± 0.16, 2.3 ± 0.2 and 56 ± 15 mg L(-1) for L. disperma, and 1.58 ± 0.05, 1.72 ± 0.01 and 5.9 ± 0.1 mg L(-1) for C. cf. dubia, respectively. The results showed that As(III) was more toxic than As(V) to L. disperma; however, As(V) was more toxic than As(III) to Chlorella sp. CE-35. The toxicities of As(III) and As(V) to C. cf. dubia were statistically similar (p>0.05). DMA was less toxic than iAs species to L. disperma and C. cf. dubia, but more toxic than As(III) to Chlorella sp. CE-35. The toxicity of As(V) to Chlorella sp. CE-35 and L. disperma decreased with increasing TP concentrations in the growth medium. Phosphate concentrations did not influence the toxicity of As(III) to either organism. Chlorella sp. CE-35 showed the ability to reduce As(V) to As(III), indicating a substantial influence of phytoplankton on As biogeochemistry in freshwater aquatic systems.