Biofilms grown in aquatic microcosms affect mercury and selenium accumulation in Daphnia

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.

Matrix in River Water, Sediments, and Biofilms Mitigates Mercury Toxicity to Medaka (Oryzias Latipes)

“…de Carvalho et al also observed that the Hg present in contaminated sediments was scarcely available to the Danio rerio and Oreochromis niloticus, for there was no difference in Hg concentrations in these two fishes before and after the sediment exposure experiment . Moreover, Issa et al showed that the presence of a biofilm could significantly reduce 60–70% accumulation of inorganic Hg in Daphnia by decreasing bioavailable Hg in the aqueous phase . Such conclusion was based on the fact that Daphnia accumulated Hg(II) mainly through Hg absorption from the aqueous phase rather than from ingesting Hg-containing solid phases, e.g., biofilms and sediments …”

“…16 Such conclusion was based on the fact that Daphnia accumulated Hg(II) mainly through Hg absorption from the aqueous phase rather than from ingesting Hg-containing solid phases, e.g., biofilms and sediments. 16 It was also found that the accumulation pattern of Hg in medaka differed remarkably among matrix-containing and -free treatments (Table 2). Namely, there were strong and significant correlations between THg (r = 0.960, p < 0.01) and MeHg concentrations (r = 0.930, p < 0.01) in medaka and THg amounts in matrix-free treatments (Figure S4), revealing that Hg accumulation in medaka in matrix-free treatments was dose-dependent.…”

“…14 Moreover, Issa et al showed that the presence of a biofilm could significantly reduce 60−70% accumulation of inorganic Hg in Daphnia by decreasing bioavailable Hg in the aqueous phase. 16 Such conclusion was based on the fact that Daphnia accumulated Hg(II) mainly through Hg absorption from the aqueous phase rather than from ingesting Hg-containing solid phases, e.g., biofilms and sediments. 16 It was also found that the accumulation pattern of Hg in medaka differed remarkably among matrix-containing and -free treatments (Table 2).…”

“…Biofilms are usually the first microbial media in nature to interact with dissolved metal­(loid)­s in aquatic systems and can alter the bioavailability and toxicity of metal­(loid)­s by influencing mineral solubility, adsorption, and transformations between oxidized and reduced species as well as metabolism of aquatic biota. , The Hg levels in water, biofilms, and sediments have been investigated widely to assess the acute, short-term, and long-term pollution status of the aquatic environment, respectively. , However, Hg levels in environmental media cannot be taken directly to assess the toxic effects of organisms exposed to Hg-contaminated environments. Namely, there are several investigations showing that the presence of sediments or biofilms alters Hg availability to aquatic organisms. − The past studies have focused exclusively on exploring the effect of a single matrix of biofilms or sediments on the accumulation of Hg in organisms. Little is known about the combined effects of single and multiple environmental media on Hg bioaccumulation, as well as its toxicity to aquatic organisms.…”