Accumulation and elimination of silver in Daphnia magna and the effect of natural organic matter

Body burden is often used as an indicator of the toxic impact of metals such as silver. Natural organic matter (NOM) is reported to reduce silver toxicity to the highly sensitive freshwater crustacean Daphnia magna. However, the effect of NOM on silver burden in these organisms has not been investigated, and literature reports from other aquatic animals suggest that NOM can actually promote silver accumulation. In 24 h accumulation trials NOM exhibited a general trend of reducing whole body silver accumulation. Differences in accumulation profiles between NOM samples were attributed to chloride content stimulating uptake by the formation of diffusible silver chloride complexes. Silver accumulation assayed over 1 h exhibited considerable heterogeneity. Subsequent experiments conducted with varying light conditions during exposure and utilising gut dissection, suggested that these differences were in part due to variable gut silver accumulation. In addition to a general reduction in silver accumulation, NOM also facilitated enhanced elimination of silver from the animals. Rapid elimination of silver from Daphnia, coupled with speciation-, body compartment- and time of day-dependent accumulation suggests that silver body burden may be a poor indicator of silver toxicity in natural environments.