A physiological role for HgII during phototrophic growth

Grégoire, Daniel S.; Poulain, Alexandre J.

doi:10.1038/ngeo2629

Cited by 49 publications

(53 citation statements)

References 30 publications

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“…Nutrient cycling and cometabolic Hg transformations stand to be closely linked in Hg methylation hotspots such as periphytic microbial mats (Olsen, Brandt, & Brooks, ), aquatic sediments (Correia & Guimarães, ; Fleming, Mack, Green, & Nelson, ) and waterlogged soils (Eklof et al, ). While our study makes a case for sulphur cycling as a coupling point for competing cometabolic Hg transformations, other nutrients such as iron (Bravo et al, ; Fleming et al, ; Sugio et al, ; Wiatrowski et al, ) and organic carbon (Christensen et al, ; Grégoire et al, ; Grégoire & Poulain, ) that are known to control Hg methylation and reduction have the potential to fulfil a similar role. Curiously, the coupling between other macronutrients under strong microbial control, such as nitrogen, and Hg cycling remains unexplored despite the importance of nitrogen cycling in aquatic and terrestrial ecosystems.…”

Section: Resultsmentioning

confidence: 91%

“…All conditions tested in the bioreactor are summarized in Table S3. The bioreactor subsampling methodology is identical to that used in previous work (Grégoire et al, ; Grégoire & Poulain, ). All bioreactor experiments were carried out at 37°C at a light intensity of 20 µmol photon m ‐2 s ‐1 to limit abiotic photoreduction of Hg (Grégoire & Poulain, ).…”

Section: Methodsmentioning

confidence: 99%

“…The bioreactor subsampling methodology is identical to that used in previous work (Grégoire et al, ; Grégoire & Poulain, ). All bioreactor experiments were carried out at 37°C at a light intensity of 20 µmol photon m ‐2 s ‐1 to limit abiotic photoreduction of Hg (Grégoire & Poulain, ). All live cell bioreactor treatments were performed in triplicate with initial H. mobilis cell inocula of 10% (v/v) from cultures grown until mid‐ to late‐exponential phase, and abiotic controls were performed in duplicate.…”

Section: Methodsmentioning

confidence: 99%

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Reduced sulphur sources favour Hg^II reduction during anoxygenic photosynthesis by Heliobacteria

et al. 2019

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The consumption of rice has become a global food safety issue because rice paddies support the production of high levels of the potent neurotoxin, methylmercury. The production of methylmercury is carried out by chemotrophic anaerobes that rely on a diversity of terminal electron acceptors, namely sulphate. Sulphur can be a limiting nutrient in rice paddies, and sulphate amendments are often used to stimulate crop production, which can increase methylmercury production. Mercury (Hg) redox cycling can affect Hg methylation by controlling the delivery of inorganic Hg substrates to methylators in anoxic habitats. Whereas sulphur is recognized as a key substrate controlling methylmercury production, the controls sulphur exerts on other microbe‐mediated Hg transformations remain poorly understood. To explore the potential coupling between sulphur assimilation and anaerobic HgII reduction to Hg0, we studied Heliobacillus mobilis, a mesophilic anoxygenic phototroph representative from the Heliobacteriacea family originally isolated from a rice paddy. Here, we tested whether the redox state of the sulphur sources available to H. mobilis would affect its ability to reduce HgII. By comparing Hg0 production over a redox gradient of sulphur sources, we demonstrate that phototrophic HgII reduction is favoured in the presence of reduced sulphur sources such as thiosulphate and cysteine. We also show that cysteine exerts dynamic control on Hg cycling by affecting not only Hg's bioavailability but also its abiotic photoreduction under low light conditions. Specifically, in the absence of cells we show that organic matter (as yeast extract) and cysteine are both required for photoreduction to occur. This study offers insights into how one of the most primitive forms of photosynthesis affects Hg redox transformations and frames Heliobacteria as key players in Hg cycling within paddy soils, forming a basis for management strategies to mitigate Hg accumulation in rice.

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Section: Resultsmentioning

confidence: 91%

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Reduced sulphur sources favour Hg^II reduction during anoxygenic photosynthesis by Heliobacteria

et al. 2019

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“…The donor side is much more resistant to Hg than the acceptor side. Although these data support that Hg(II) has no physiological function, it has been shown recently that Hg(II) might act in the presence of intracellular redox imbalance as sink of electrons to maintain redox homeostasis in purple bacteria (Gregoire and Poulain 2016).…”

Section: Introductionmentioning

confidence: 71%

Stoichiometry and kinetics of mercury uptake by photosynthetic bacteria

2017

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Mercury adsorption on the cell surface and intracellular uptake by bacteria represent the key first step in the production and accumulation of highly toxic mercury in living organisms. In this work, the biophysical characteristics of the mercury bioaccumulation are studied in intact cells of photosynthetic bacteria by use of analytical (dithizone) assay and physiological photosynthetic markers (pigment content, fluorescence induction and membrane potential) to determine the amount of mercury ions bound to the cell surface and taken up by the cell. It is shown that the Hg(II) uptake mechanism 1) has two kinetically distinguishable components, 2) includes co-opted influx through heavy metal transporters since the slow component is inhibited by Ca 2+ channel blockers, 3) shows complex pH-dependence demonstrating the competition of ligand binding of Hg(II) ions with H + ions (low pH) and high tendency of complex formation of Hg(II) with hydroxyl ions (high pH) and 4) is not a passive but an energy-dependent process as evidenced by light-activation and inhibition by protonophore. Photosynthetic bacteria can accumulate Hg(II) in amounts much (about 10 5 ) greater than their own masses by well defined strong and weak binding sites with equilibrium binding constants in the range of 1 (μM) -1 and 1 (mM) -1 , respectively. The strong binding sites are attributed to sulfhydryl groups as the uptake is blocked by use of sulfhydryl modifying agents and their number is much (two orders of magnitude) smaller than the number of weak binding sites. Biofilms developed by some bacteria (e.g. Rvx. gelatinosus) increase the mercury binding capacity further by a factor of about five. Photosynthetic bacteria in the light act as sponge of Hg(II) and can be potentially used for biomonitoring and bioremediation of mercury contaminated aqueous cultures.2

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“…The Δ 199 Hg values of surface (25 m) zooplankton display a clear diurnal pattern, where Δ 199 Hg values are greater during the day than at night, and their isotopic composition is significantly higher than deeper samples (125–1,250 m, Wilcoxon test, W = 15, p < 0.01). A diurnal cycle of Δ 199 Hg values in zooplankton is expected given the recent reports that marine phytoplankton and bacterioplankton can photochemically degrade Hg (Grégoire & Poulain, ; Kritee et al, ; Lee & Fisher, ). We suggest the Δ 199 Hg value in surface zooplankton represents the isotopic composition of photodegraded MMHg in phytoplankton or particle‐associated MMHg that has been photodemethylated before entering the food web.…”

Section: Discussionmentioning

confidence: 96%

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

Motta

Blum

Johnson

et al. 2019

Global Biogeochemical Cycles

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The oceans are an important global reservoir for mercury (Hg), and marine fish consumption is the dominant human exposure pathway for its toxic methylated form. A more thorough understanding of the global biogeochemical cycle of Hg requires additional information on the mechanisms that control Hg cycling in pelagic marine waters. In this study, Hg isotope ratios and total Hg concentrations are used to explore Hg biogeochemistry in oligotrophic marine environments north of Hawaii. We present the first measurements of the vertical water column distribution of Hg concentrations and the Hg isotopic composition in precipitation, marine particles, and zooplankton near Station ALOHA (22°45′N, 158°W). Our results reveal production and demethylation of methylmercury in both the euphotic (0–175 m) and mesopelagic zones (200–1,000 m). We document a strong relationship between Hg isotopic composition and depth in particles, zooplankton, and fish in the water column and diurnal variations in Δ199Hg values in zooplankton sampled near the surface (25 m). Based on these observations and stable Hg isotope relationships in the marine food web, we suggest that the Hg found in large pelagic fish at Station ALOHA was originally deposited largely by precipitation, transformed into methyl‐Hg, and bioaccumulated in situ in the water column. Our results highlight how Hg isotopic compositions reflect abiotic and biotic production and degradation of methyl‐Hg throughout the water column and the importance of particles and zooplankton in the vertical transport of Hg.

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A physiological role for HgII during phototrophic growth

Cited by 49 publications

References 30 publications

Reduced sulphur sources favour Hg^II reduction during anoxygenic photosynthesis by Heliobacteria

Reduced sulphur sources favour Hg^II reduction during anoxygenic photosynthesis by Heliobacteria

Stoichiometry and kinetics of mercury uptake by photosynthetic bacteria

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

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A physiological role for HgII during phototrophic growth

Cited by 49 publications

References 30 publications

Reduced sulphur sources favour HgII reduction during anoxygenic photosynthesis by Heliobacteria

Reduced sulphur sources favour HgII reduction during anoxygenic photosynthesis by Heliobacteria

Stoichiometry and kinetics of mercury uptake by photosynthetic bacteria

Mercury Cycling in the North Pacific Subtropical Gyre as Revealed by Mercury Stable Isotope Ratios

Contact Info

Product

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Reduced sulphur sources favour Hg^II reduction during anoxygenic photosynthesis by Heliobacteria

Reduced sulphur sources favour Hg^II reduction during anoxygenic photosynthesis by Heliobacteria