Arsenic cycling in freshwater phytoplankton and zooplankton cultures

Freshwater phytoplankton (Chlamydomonas) and zooplankton (Daphnia pulex) were exposed to arsenic to trace the arsenic transformations and the formation of organoarsenic compounds at the base of the freshwater food chain. Plankton were cultured in artificial lake water, and exposed to arsenic through several pathways, hypothesised to be the main exposure sources: through water, food and contaminated sediments. High performance liquid chromatography-inductively coupled plasma-mass spectrometry and X-ray absorption spectroscopy were used to determine arsenic speciation in the studied organisms, and X-ray fluorescence mapping was used to locate the arsenic in a single Daphnia specimen. The results show that the formation of methylated arsenic compounds and arsenosugars by the zooplankton organisms was independent of the exposure route, but instead dependent on arsenic concentration in the environment. Specifically, organoarsenic compounds were dominant in extracts of Daphnia organisms exposed to low arsenic concentrations through water at 10 mu gL(-1) (67%), and through contaminated food (75%), but inorganic arsenic was dominant in Daphnia exposed to high arsenic concentrations, including contaminated sediments. Phytoplankton cultures contained variable amounts of arsenosugars, but on average the dominant compound in phytoplankton was inorganic arsenic. The main implications of the present study for understanding arsenic cycling in the freshwater plankton community are that arsenosugars are formed at possibly both the phytoplankton and zooplankton trophic levels; and that higher arsenic loads in plankton correspond to higher inorganic arsenic concentrations, which could indicate a saturation of the arsenic methylation process by plankton organisms.