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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.

SUBMITTER: Issa S

PROVIDER: S-EPMC7182615 | biostudies-literature | 2020 May

REPOSITORIES: biostudies-literature

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

Issa Semona S Ciesielski Tomasz Maciej TM Mikkelsen Øyvind Ø Einum Sigurd S Jaspers Veerle L B VLB

Ecotoxicology (London, England) 20200415 4

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 ...[more]

PMID: 32297060

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Project description:Eutrophication of the planet's aquatic systems is increasing at an unprecedented rate. In freshwater systems, nitrate-one of the nutrients responsible for eutrophication-is linked to biodiversity losses and ecosystem degradation. One of the main sources of freshwater nitrate pollution in New Zealand is agriculture. New Zealand's pastoral farming system relies heavily on the application of chemical fertilisers. These fertilisers in combination with animal urine, also high in nitrogen, result in high rates of nitrogen leaching into adjacent aquatic systems. In addition to nitrogen, livestock waste commonly carries human and animal enteropathogenic bacteria, many of which can survive in freshwater environments. Two strains of enteropathogenic bacteria found in New Zealand cattle, are K99 and Shiga-toxin producing Escherichia coli (STEC). To better understand the effects of ambient nitrate concentrations in the water column on environmental enteropathogenic bacteria survival, a microcosm experiment with three nitrate-nitrogen concentrations (0, 1, and 3 mg NO3-N /L), two enteropathogenic bacterial strains (STEC O26-human, and K99-animal), and two water types (sterile and containing natural microbiota) was run. Both STEC O26 and K99 reached 500 CFU/10 ml in both water types at all three nitrate concentrations within 24 hours and remained at those levels for the full 91 days of the experiment. Although enteropathogenic strains showed no response to water column nitrate concentrations, the survival of background Escherichia coli, imported as part of the in-stream microbiota did, surviving longer in 1 and 3 mg NO3-N/Lconcentrations (P < 0.001). While further work is needed to fully understand how nitrate enrichment and in-stream microbiota may affect the viability of human and animal pathogens in freshwater systems, it is clear that these two New Zealand strains of STEC O26 and K99 can persist in river water for extended periods alongside some natural microbiota.

Project description:Geobacter species are common in iron-rich environments and can contribute to formation of methylmercury (MeHg), a neurotoxic compound with high bioaccumulation potential formed as a result of bacterial and archaeal physiological activity. Geobacter sulfurreducens can utilize various electron acceptors for growth including iron hydroxides or fumarate. However, it remains poorly understood how the growth on these compounds affects physiological properties of bacterial cells in biofilms, including the capacity to produce MeHg. The purpose of this study was to determine changes in the biochemical composition of G. sulfurreducens during biofilm cultivation in media containing iron hydroxide or fumarate, and to quantify mercury (Hg) methylation capacity of the formed biofilms. Biofilms were characterized by Fourier-transform infrared spectroscopy in the attenuated total reflection mode (ATR-FTIR), Resonance Raman spectroscopy and confocal laser scanning microscopy. MeHg formation was quantified by mass spectrometry after incubation of biofilms with 100 nM Hg. The results of ATR-FTIR experiments showed that in presence of fumarate, G. sulfurreducens biofilm formation was accompanied by variation in content of the energy-reserve polymer glycogen over time, which could be cancelled by the addition of supplementary nutrients (yeast extract). In contrast, biofilms cultivated on Fe(III) hydroxide did not accumulate glycogen. The ATR-FTIR results further suggested that Fe(III) hydroxide surfaces bind cells via phosphate and carboxylate groups of bacteria that form complexes with iron. Furthermore, biofilms grown on Fe(III) hydroxide had higher fraction of oxidized cytochromes and produced two to three times less biomass compared to conditions with fumarate. Normalized to biofilm volume, the content of MeHg was similar in assays with biofilms grown on Fe(III) hydroxide and on fumarate (with yeast extract and without). These results suggest that G. sulfurreducens biofilms produce MeHg irrespectively from glycogen content and cytochrome redox state in the cells, and warrant further investigation of the mechanisms controlling this process.

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

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

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