About

Professor Jeffra Schaefer's research centers on the microbial communities and their activities in peatlands, particularly focusing on boreal and arctic wetlands that play a significant role in global soil carbon sequestration. Her work investigates how microbial populations in these environments break down organic matter into methane and carbon dioxide, contributing to greenhouse gas emissions. She studies the effects of climate change, such as warming trends and permafrost melting, on the transition of wetlands from nutrient-poor bogs to richer fens, and how these changes influence microbial metabolic pathways and community dynamics. This research is crucial for predicting future environmental trends related to carbon cycling in high latitude ecosystems. Another major focus of Professor Schaefer's research is the microbial production of methylmercury in northern wetlands. Methylmercury is a potent neurotoxin that bioaccumulates in food webs, posing health risks to humans and wildlife. Her work explores the biogeochemical and microbial controls that regulate mercury methylation in wetland environments, which are known hotspots for this process due to their anoxic conditions and organic matter abundance. Collaborating with researchers from institutions such as Rutgers University, University of Massachusetts Amherst, and the US Geological Survey, she examines the links between wetland type, microbial populations, and methylmercury accumulation to better understand and predict mercury cycling in these ecosystems. Additionally, Professor Schaefer investigates the mechanisms of mercury uptake and methylation at the microbial level, including the role of thiol compounds and metal interactions in mercury bioavailability. Her research extends to copper homeostasis in strict anaerobic microorganisms, particularly the iron-reducing bacterium Geobacter sulfurreducens, which is relevant to environments contaminated with heavy metals. She also studies metal contamination in the Raritan River, New Jersey, assessing the distribution of toxic metals in water, sediment, and plankton in collaboration with marine sciences colleagues and students. Overall, Professor Schaefer's work integrates microbial ecology, biogeochemistry, and environmental chemistry to address critical issues related to greenhouse gas emissions and toxic metal cycling in wetland ecosystems.

Research topics

• Biology
• Ecology
• Environmental chemistry
• Chemistry
• Organic chemistry
• Inorganic chemistry

Selected publications

• Metabolic turnover of cysteine-related thiol compounds at environmentally relevant concentrations by Geobacter sulfurreducens

  Frontiers in Microbiology · 2023-01-11 · 7 citations

  articleOpen access

  Low-molecular-mass (LMM) thiol compounds are known to be important for many biological processes in various organisms but LMM thiols are understudied in anaerobic bacteria. In this work, we examined the production and turnover of nanomolar concentrations of LMM thiols with a chemical structure related to cysteine by the model iron-reducing bacterium Geobacter sulfurreducens . Our results show that G. sulfurreducens tightly controls the production, excretion and intracellular concentration of thiols depending on cellular growth state and external conditions. The production and cellular export of endogenous cysteine was coupled to the extracellular supply of Fe(II), suggesting that cysteine excretion may play a role in cellular trafficking to iron proteins. Addition of excess exogenous cysteine resulted in a rapid and extensive conversion of cysteine to penicillamine by the cells. Experiments with added isotopically labeled cysteine confirmed that penicillamine was formed by a dimethylation of the C-3 atom of cysteine and not via indirect metabolic responses to cysteine exposure. This is the first report of de novo metabolic synthesis of this compound. Penicillamine formation increased with external exposure to cysteine but the compound did not accumulate intracellularly, which may suggest that it is part of G. sulfurreducens ’ metabolic strategy to maintain cysteine homeostasis. Our findings highlight and expand on processes mediating homeostasis of cysteine-like LMM thiols in strict anaerobic bacteria. The formation of penicillamine is particularly noteworthy and this compound warrants more attention in microbial metabolism studies.

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• The Combined Effect of Hg(II) Speciation, Thiol Metabolism, and Cell Physiology on Methylmercury Formation by <i>Geobacter sulfurreducens</i>

  Environmental Science & Technology · 2023-04-25 · 15 citations

  articleOpen access

  The chemical and biological factors controlling microbial formation of methylmercury (MeHg) are widely studied separately, but the combined effects of these factors are largely unknown. We examined how the chemical speciation of divalent, inorganic mercury (Hg(II)), as controlled by low-molecular-mass thiols, and cell physiology govern MeHg formation by Geobacter sulfurreducens. We compared MeHg formation with and without addition of exogenous cysteine (Cys) to experimental assays with varying nutrient and bacterial metabolite concentrations. Cysteine additions initially (0–2 h) enhanced MeHg formation by two mechanisms: (i) altering the Hg(II) partitioning from the cellular to the dissolved phase and/or (ii) shifting the chemical speciation of dissolved Hg(II) in favor of the Hg(Cys)2 complex. Nutrient additions increased MeHg formation by enhancing cell metabolism. These two effects were, however, not additive since cysteine was largely metabolized to penicillamine (PEN) over time at a rate that increased with nutrient addition. These processes shifted the speciation of dissolved Hg(II) from complexes with relatively high availability, Hg(Cys)2, to complexes with lower availability, Hg(PEN)2, for methylation. This thiol conversion by the cells thereby contributed to stalled MeHg formation after 2–6 h Hg(II) exposure. Overall, our results showed a complex influence of thiol metabolism on microbial MeHg formation and suggest that the conversion of cysteine to penicillamine may partly suppress MeHg formation in cysteine-rich environments like natural biofilms.

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• The role of oxygen in stimulating methane production in wetlands

  Global Change Biology · 2021 · 77 citations

  • Environmental chemistry
  • Chemistry
  • Ecology

  emissions; and guiding wetland C management strategies.

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• Nutrient Inputs Stimulate Mercury Methylation by Syntrophs in a Subarctic Peatland

  Frontiers in Microbiology · 2021-10-04 · 24 citations

  articleOpen accessCorresponding

  Climate change dramatically impacts Arctic and subarctic regions, inducing shifts in wetland nutrient regimes as a consequence of thawing permafrost. Altered hydrological regimes may drive changes in the dynamics of microbial mercury (Hg) methylation and bioavailability. Important knowledge gaps remain on the contribution of specific microbial groups to methylmercury (MeHg) production in wetlands of various trophic status. Here, we measured aqueous chemistry, potential methylation rates (k meth ), volatile fatty acid (VFA) dynamics in peat-soil incubations, and genetic potential for Hg methylation across a groundwater-driven nutrient gradient in an interior Alaskan fen. We tested the hypotheses that (1) nutrient inputs will result in increased methylation potentials, and (2) syntrophic interactions contribute to methylation in subarctic wetlands. We observed that concentrations of nutrients, total Hg, and MeHg, abundance of hgcA genes, and rates of methylation in peat incubations (k meth ) were highest near the groundwater input and declined downgradient. hgcA sequences near the input were closely related to those from sulfate-reducing bacteria (SRB), methanogens, and syntrophs. Hg methylation in peat incubations collected near the input source (FPF2) were impacted by the addition of sulfate and some metabolic inhibitors while those down-gradient (FPF5) were not. Sulfate amendment to FPF2 incubations had higher k meth relative to unamended controls despite no effect on k meth from addition of the sulfate reduction inhibitor molybdate. The addition of the methanogenic inhibitor BES (25 mM) led to the accumulation of VFAs, but unlike molybdate, it did not affect Hg methylation rates. Rather, the concurrent additions of BES and molybdate significantly decreased k meth , suggesting a role for interactions between SRB and methanogens in Hg methylation. The reduction in k meth with combined addition of BES and molybdate, and accumulation of VFA in peat incubations containing BES, and a high abundance of syntroph-related hgcA sequences in peat metagenomes provide evidence for MeHg production by microorganisms growing in syntrophy. Collectively the results suggest that wetland nutrient regimes influence the activity of Hg methylating microorganisms and, consequently, Hg methylation rates. Our results provide key information about microbial Hg methylation and methylating communities under nutrient conditions that are expected to become more common as permafrost soils thaw.

  PublisherDOI

• Extracellular sulfite is protective against reactive oxygen species and antibiotic stress in <i>Shewanella oneidensis</i> MR‐1

  Environmental Microbiology Reports · 2021-04-18 · 2 citations

  article

  In this study, we investigated the extracellular reactive sulfur species produced by Shewanella oneidensis MR-1 during growth. The results showed that sulfite is the major extracellular sulfur metabolite released to the growth medium under both aerobic and anaerobic growth conditions. Exogenous sulfite at physiological concentrations protected S. oneidensis MR-1 from hydrogen peroxide toxicity and enhanced tolerance to the beta-lactam antibiotics cefazolin, meropenem, doripenem and ertapenem. These findings suggest that the release of extracellular sulfite is a bacterial defence mechanism that plays a role in the mitigation of environmental stress.

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• Production of methylmercury by methanogens in mercury contaminated estuarine sediments

  FEMS Microbiology Letters · 2020 · 25 citations

  • Environmental chemistry
  • Chemistry
  • Ecology

  Anaerobic bacteria are known to produce neurotoxic methylmercury [MeHg] when elemental mercury [Hg(0)] is provided as the sole mercury source. In this study, we examined the formation of MeHg in anaerobic incubations of sediment collected from the San Jacinto River estuary (Texas, USA) amended with aqueous Hg(0) to investigate the microbial communities involved in the conversion of Hg(0) to MeHg. The results show that the addition of the methanogen inhibitor 2-bromoethanesulfonate (BES) significantly decreased MeHg production. The mercury methylation gene, hgcA, was detected in these sediments using archaeal specific primers, and 16S rRNA sequencing showed that a member of the Methanosarcinaceae family of methanogens was active. These results suggest that methanogenic archaea play an underappreciated role in the production of MeHg in estuarine sediments contaminated with Hg(0).

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• Anaerobic guilds responsible for mercury methylation in boreal wetlands of varied trophic status serving as either a methylmercury source or sink

  Environmental Microbiology · 2020 · 38 citations

  1st authorCorresponding
  • Biology
  • Ecology

  Wetlands are common sites of active Hg methylation by anaerobic microbes; however, the amount of methylmercury produced varies greatly, as Hg methylation is dependent upon both the availability of Hg and the composition and activity of the microbial community involved. In this study, we identified the major microbial guilds responsible for Hg methylation along a trophic gradient composed of two sites and three different types of wetlands: a bog-fen peatland gradient and a black alder swamp, serving as net sources and a sink for methylmercury respectively. Iron-reducing bacteria in the Geobacteraceae were important Hg methylators across all wetlands and seasons examined, as evidenced by abundant 16S rRNA and hgcA transcripts clustering with this family. Molybdate inhibited Hg methylation more efficiently in the peatlands than in the swamp, suggesting an increasing role of sulfate-reducing bacteria and/or related syntrophs in the methylation of Hg with decreasing trophic status. Sulfate addition failed to increase Hg methylation rates in the peatlands, suggesting that SRBs/syntrophs were instead likely metabolizing alternative substrates such as syntrophic fermentation of organic compounds with methanogens. These results highlight the interconnectivity of anaerobic metabolism and importance of community dynamics on the methylation of Hg in wetlands with different trophic status.

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• Microbial formation of thiols control the chemical speciation and methylation of Hg(II)

  2020-03-09

  articleOpen access

  &amp;lt;p&amp;gt;The formation of the neurotoxin methylmercury (MeHg) is a biotic process where anaerobic bacteria methylate inorganic divalent Hg (Hg(II)) intracellularly. The cellular uptake mechanisms are still not identified, but low molecular mass (LMM) thiols play an important role together with thiol groups on the outer membrane in controlling the chemical speciation of Hg(II). For example, increased concentration of specific LMM thiols, especially cysteine, is known to enhance the formation of MeHg. A recent study showed that metabolically active anaerobic microorganisms produced LMM thiols in vivo and exported them to concentrations up to 100 nM in the assay medium. The concentration range was sufficient to significantly affect the chemical speciation, uptake and methylation of Hg(II) without any external addition of LMM thiols.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In this study we investigate the kinetics of microbial formation and cellular export of LMM thiols by the iron-reducing bacterium Geobacter sulfurreducens and the sulfate-reducing bacterium Desulfovibrio sp. ND132 in high time resolution and the impact on the chemical speciation and methylation of Hg(II).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;LMM thiols were separated by liquid chromatography and determined by electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Hg(LMM-RS)&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; complexes were determined by thermodynamic modeling and by direct measurements using LC-Inductively coupled plasma MS (LC-ICPMS).&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Results will be presented for the production of LMM thiol compounds, formation of Hg(LMM-RS)&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; complexes and how this change in Hg speciation impacts the Hg(II) methylation rate in short-term washed cell assays. Characterizing the time-dependent molecular composition of LMM thiols associated with methylating microbes are important to further understand their multiple roles on Hg(II) uptake and MeHg formation in bacteria assays and in the environment.&amp;amp;#160;&amp;lt;/p&amp;gt;

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• Tracing the Uptake of Hg(II) in an Iron-Reducing Bacterium Using Mercury Stable Isotopes

  Environmental Science & Technology Letters · 2020-06-24 · 21 citations

  article

  Anaerobic microorganisms play a key role in the biological mercury (Hg) cycle due to their ability to produce bioaccumulative neurotoxic methylmercury (MeHg). However, despite recent advances, how bacteria accumulate inorganic Hg [Hg(II)] prior to methylation is largely unknown. In this study, we applied Hg stable isotopes to measure changes in cellular compartments of Geobacter sulfurreducens and a nonmethylating mutant strain to investigate intracellular transport of Hg(II). Both strains accumulated intracellular Hg(II) that was lower in δ202Hg relative to dissolved extracellular Hg(II), demonstrating mass-dependent fractionation during uptake. Hg reduction by the mutant strain (50% Hg concentration loss in 24 h) resulted in higher δ202Hg values of cellular Hg than in wild-type cells. Further observations showed increasing δ202Hg values in dissolved extracellular MeHg and Hg(II) but decreasing δ202Hg values of intracellular Hg(II) in wild-type G. sulfurreducens suggesting that external Hg pools may be the proximate source of Hg for methylation in this bacterium. This investigation demonstrates that cellular uptake is comprised of multiple processes and transformations that influence Hg(II) prior to methylation, which can impart distinct isotopic signatures to Hg(II) and MeHg pools in the environment.

  PublisherDOI

• Mercury methylating microbial communities of boreal forest soils

  Scientific Reports · 2019-01-24 · 69 citations

  articleOpen access

  The formation of the potent neurotoxic methylmercury (MeHg) is a microbially mediated process that has raised much concern because MeHg poses threats to wildlife and human health. Since boreal forest soils can be a source of MeHg in aquatic networks, it is crucial to understand the biogeochemical processes involved in the formation of this pollutant. High-throughput sequencing of 16S rRNA and the mercury methyltransferase, hgcA, combined with geochemical characterisation of soils, were used to determine the microbial populations contributing to MeHg formation in forest soils across Sweden. The hgcA sequences obtained were distributed among diverse clades, including Proteobacteria, Firmicutes, and Methanomicrobia, with Deltaproteobacteria, particularly Geobacteraceae, dominating the libraries across all soils examined. Our results also suggest that MeHg formation is also linked to the composition of non-mercury methylating bacterial communities, likely providing growth substrate (e.g. acetate) for the hgcA-carrying microorganisms responsible for the actual methylation process. While previous research focused on mercury methylating microbial communities of wetlands, this study provides some first insights into the diversity of mercury methylating microorganisms in boreal forest soils.

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Frequent coauthors

• Tamar Barkay

  Rutgers, The State University of New Jersey

  15 shared

• Ulf Skyllberg

  Swedish University of Agricultural Sciences

  11 shared

• Erik Björn

  Umeå University

  11 shared

• Andrea G. Bravo

  Institut de Ciències del Mar

  11 shared

• François M. M. Morel

  Princeton University

  9 shared

• John R. Reinfelder

  Rutgers, The State University of New Jersey

  8 shared

• Moritz Buck

  Swedish University of Agricultural Sciences

  6 shared

• Stefan Bertilsson

  Swedish University of Agricultural Sciences

  6 shared

Education

• PhD

  Rutgers University

  2005

• BA

  University of California Berkeley

  1997