Arsenic distribution and speciation in Daphnia pulex

Abstract

Rat Lake, Yellowknife, Northwest Territories, is situated on arsenic-rich tailings from a historical gold mine. The abundant zooplankton species Daphnia pulex in this lake was used to study the impact of arsenic at the base of the freshwater food web; the speciation and distribution of arsenic in D. pulex and its food sources; and the origin of formation of organoarsenicals in freshwater systems. The arsenic concentration in lake water was measured as 0.25 mg L(-1), while the zooplankton organisms contained up to 35 mg kg(-1) d.w. arsenic. Plankton samples were analyzed for arsenic speciation, by using X-ray Absorption Near Edge Structure (XANES) on the whole, dried samples and High Performance Liquid Chromatography coupled to Inductively Coupled Plasma Mass Spectrometry (HPLC-ICP-MS) on water extracts. XANES data suggest that D. pulex mainly contain inorganic arsenicals with 56% of arsenic with +5 oxidation state and 10% of arsenic with +3 oxidation state, but also 34% of organoarsenic compounds that were identified with HPLC-ICP-MS as monomethylarsonate (MMA), dimethylarsinate (DMA), and arsenosugars. The most abundant of the organoarsenicals was the glycerol sugar (Sugar 1). X-ray Fluorescence (XRF) mapping of D. pulex for arsenic distribution showed that arsenic was mainly distributed in the gut of the animal, where its concentration was ten times higher than in the surrounding tissues. Moreover, the analysis of residues from extractions targeting water-soluble and lipid-soluble arsenicals suggested that part of the measured arsenic signal comes from ingested sediments, phytoplankton, or other food sources. These food sources contain inorganic arsenic only, with As(V)-O in phytoplankton and As(III)-S in sediments, suggesting the possibility that the organoarsenicals compounds detected in the tissues of the organism are created by the Daphnia.