Abstract
Experiments examining mercury (Hg) toxicity in Daphnia are usually conducted in highly standardized conditions that prevent the formation of biofilm. Although such standardization has many advantages, extrapolation of results to natural conditions and inference of ecological effects is challenging. This is especially true since biofilms can accumulate metals/metalloids and play a key role in their transfer to higher trophic level organisms. In this study, we experimentally tested the effects of spontaneously appearing biofilm in Daphnia cultures on accumulation of Hg and its natural antagonist selenium (Se) in Daphnia magna. We added Hg (in the form of mercury (II) chloride) at two concentrations (0.2 µg/L and 2 µg/L) to experimental microcosms and measured the uptake of Hg and Se by D. magna in the presence and absence of biofilm. To test for consistent and replicable results, we ran two identical experimental sets one week apart. Biofilm presence significantly reduced the accumulation of Hg, while increasing the tissue Se content in D. magna, and these findings were reproducible across experimental sets. These findings indicate that highly standardized tests may not be adequate to predict the bioaccumulation and potential toxicity of metals/metalloids under natural conditions.
Highlights
Lab-based aquatic toxicity tests are usually conducted under highly standardized conditions (OECD 1992, 2004), which allows for comparisons of toxicity of different compounds from experiments run in different laboratories and during different times
We examined whether biofilm could affect Hg and Se accumulation in Daphnia magna through aqueous and dietary uptake pathways
The Hg exposure concentrations used were lower than the acute LC50 of 2.2 μg/L Hg in cladocerans (Nichols et al 1997), exceeding mean total concentrations of 0.006 μg/L typically found in the aquatic environment (Chen et al 2000)
Summary
Lab-based aquatic toxicity tests are usually conducted under highly standardized conditions (OECD 1992, 2004), which allows for comparisons of toxicity of different compounds from experiments run in different laboratories and during different times. The inclusion of biotic interactions between Daphnia and biofilm in toxicity tests could provide a more ecologically realistic approach. Biofilms are aggregates of microorganisms (algae, cyanobacteria, bacteria, fungi, and protozoa) growing on surfaces and embedded in a matrix of extracellular polymeric substances (EPS; Decho 2000). Biofilms are involved in organic matter cycling, primary production and respiration (Wetzel 1993; Kühl et al 1996; Decho 2000). They serve as food to higher trophic levels through grazing (Huws et al 2005; Siehoff et al 2009), and their algal components exude organic carbon to be taken up by bacteria (Søndergaard et al 1995; Goto et al 2001)
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