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Peterson BD, McDaniel EA, Schmidt AG, Lepak RF, Janssen SE, Tran PQ, Marick RA, Ogorek JM, DeWild JF, Krabbenhoft DP, McMahon KD. Mercury Methylation Genes Identified across Diverse Anaerobic Microbial Guilds in a Eutrophic Sulfate-Enriched Lake. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2020; 54:15840-15851. [PMID: 33228362 PMCID: PMC9741811 DOI: 10.1021/acs.est.0c05435] [Citation(s) in RCA: 28] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
Mercury (Hg) methylation is a microbially mediated process that converts inorganic Hg into bioaccumulative, neurotoxic methylmercury (MeHg). The metabolic activity of methylating organisms is highly dependent on biogeochemical conditions, which subsequently influences MeHg production. However, our understanding of the ecophysiology of methylators in natural ecosystems is still limited. Here, we identified potential locations of MeHg production in the anoxic, sulfidic hypolimnion of a freshwater lake. At these sites, we used shotgun metagenomics to characterize microorganisms with the Hg-methylation gene hgcA. Putative methylators were dominated by hgcA sequences divergent from those in well-studied, confirmed methylators. Using genome-resolved metagenomics, we identified organisms with hgcA (hgcA+) within the Bacteroidetes and the recently described Kiritimatiellaeota phyla. We identified hgcA+ genomes derived from sulfate-reducing bacteria, but these accounted for only 22% of hgcA+ genome coverage. The most abundant hgcA+ genomes were from fermenters, accounting for over half of the hgcA gene coverage. Many of these organisms also mediate hydrolysis of polysaccharides, likely from cyanobacterial blooms. This work highlights the distribution of the Hg-methylation genes across microbial metabolic guilds and indicate that primary degradation of polysaccharides and fermentation may play an important but unrecognized role in MeHg production in the anoxic hypolimnion of freshwater lakes.
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Affiliation(s)
- Benjamin D. Peterson
- Environmental Science & Technology Program, University of Wisconsin - Madison, 660 N. Park Street, Madison, WI 53706, USA
- Corresponding author:
| | - Elizabeth A. McDaniel
- Department of Bacteriology, University of Wisconsin - Madison, 1550 Linden Drive, Madison, WI 53706, USA
| | - Anna G. Schmidt
- Department of Bacteriology, University of Wisconsin - Madison, 1550 Linden Drive, Madison, WI 53706, USA
| | - Ryan F. Lepak
- Environmental Science & Technology Program, University of Wisconsin - Madison, 660 N. Park Street, Madison, WI 53706, USA
- U.S. Geological Survey, Upper Midwest Water Science Center, Mercury Research Laboratory, 8505 Research Way, Middleton, WI 53562, USA
- U.S. Environmental Protection Agency Office of Research and Development, Center for Computational Toxicology and Exposure, Great Lakes Toxicology and Ecology Division, 6201 Congdon Blvd, Duluth, MN 55804, USA
| | - Sarah E. Janssen
- U.S. Geological Survey, Upper Midwest Water Science Center, Mercury Research Laboratory, 8505 Research Way, Middleton, WI 53562, USA
| | - Patricia Q. Tran
- Department of Bacteriology, University of Wisconsin - Madison, 1550 Linden Drive, Madison, WI 53706, USA
- Department of Integrative Biology, University of Wisconsin - Madison, 250 N. Mills St.Madison, WI 53706, USA
| | - Robert A. Marick
- Department of Biochemistry, University of Wisconsin - Madison, 433 Babcock Drive, Madison, WI 53706, USA
| | - Jacob M. Ogorek
- U.S. Geological Survey, Upper Midwest Water Science Center, Mercury Research Laboratory, 8505 Research Way, Middleton, WI 53562, USA
| | - John F. DeWild
- U.S. Geological Survey, Upper Midwest Water Science Center, Mercury Research Laboratory, 8505 Research Way, Middleton, WI 53562, USA
| | - David P. Krabbenhoft
- U.S. Geological Survey, Upper Midwest Water Science Center, Mercury Research Laboratory, 8505 Research Way, Middleton, WI 53562, USA
| | - Katherine D. McMahon
- Department of Bacteriology, University of Wisconsin - Madison, 1550 Linden Drive, Madison, WI 53706, USA
- Department of Civil and Environmental Engineering, University of Wisconsin – Madison, 1415 Engineering Drive, Madison WI 53706, USA
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Gionfriddo CM, Wymore AM, Jones DS, Wilpiszeski RL, Lynes MM, Christensen GA, Soren A, Gilmour CC, Podar M, Elias DA. An Improved hgcAB Primer Set and Direct High-Throughput Sequencing Expand Hg-Methylator Diversity in Nature. Front Microbiol 2020; 11:541554. [PMID: 33123100 PMCID: PMC7573106 DOI: 10.3389/fmicb.2020.541554] [Citation(s) in RCA: 26] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2020] [Accepted: 08/25/2020] [Indexed: 01/27/2023] Open
Abstract
The gene pair hgcAB is essential for microbial mercury methylation. Our understanding of its abundance and diversity in nature is rapidly evolving. In this study we developed a new broad-range primer set for hgcAB, plus an expanded hgcAB reference library, and used these to characterize Hg-methylating communities from diverse environments. We applied this new Hg-methylator database to assign taxonomy to hgcA sequences from clone, amplicon, and metagenomic datasets. We evaluated potential biases introduced in primer design, sequence length, and classification, and suggest best practices for studying Hg-methylator diversity. Our study confirms the emerging picture of an expanded diversity of HgcAB-encoding microbes in many types of ecosystems, with abundant putative mercury methylators Nitrospirae and Chloroflexi in several new environments including salt marsh and peat soils. Other common microbes encoding HgcAB included Phycisphaerae, Aminicenantes, Spirochaetes, and Elusimicrobia. Combined with high-throughput amplicon specific sequencing, the new primer set also indentified novel hgcAB sequences similar to Lentisphaerae, Bacteroidetes, Atribacteria, and candidate phyla WOR-3 and KSB1 bacteria. Gene abundance data also corroborate the important role of two "classic" groups of methylators (Deltaproteobacteria and Methanomicrobia) in many environments, but generally show a scarcity of hgcAB+ Firmicutes. The new primer set was developed to specifically target hgcAB sequences found in nature, reducing degeneracy and providing increased sensitivity while maintaining broad diversity capture. We evaluated mock communities to confirm primer improvements, including culture spikes to environmental samples with variable DNA extraction and PCR amplification efficiencies. For select sites, this new workflow was combined with direct high-throughput hgcAB sequencing. The hgcAB diversity generated by direct amplicon sequencing confirmed the potential for novel Hg-methylators previously identified using metagenomic screens. A new phylogenetic analysis using sequences from freshwater, saline, and terrestrial environments showed Deltaproteobacteria HgcA sequences generally clustered among themselves, while metagenome-resolved HgcA sequences in other phyla tended to cluster by environment, suggesting horizontal gene transfer into many clades. HgcA from marine metagenomes often formed distinct subtrees from those sequenced from freshwater ecosystems. Overall the majority of HgcA sequences branch from a cluster of HgcAB fused proteins related to Thermococci, Atribacteria (candidate division OP9), Aminicenantes (OP8), and Chloroflexi. The improved primer set and library, combined with direct amplicon sequencing, provide a significantly improved assessment of the abundance and diversity of hgcAB+ microbes in nature.
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Affiliation(s)
- Caitlin M Gionfriddo
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, United States
| | - Ann M Wymore
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, United States
| | - Daniel S Jones
- BioTechnology Institute, University of Minnesota, St. Paul, MN, United States.,Department of Earth Sciences, Minneapolis, MN, United States
| | - Regina L Wilpiszeski
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, United States
| | - Mackenzie M Lynes
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, United States
| | - Geoff A Christensen
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, United States
| | - Ally Soren
- Smithsonian Environmental Research Center, Edgewater, MD, United States
| | | | - Mircea Podar
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, United States
| | - Dwayne A Elias
- Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, United States
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Capo E, Bravo AG, Soerensen AL, Bertilsson S, Pinhassi J, Feng C, Andersson AF, Buck M, Björn E. Deltaproteobacteria and Spirochaetes-Like Bacteria Are Abundant Putative Mercury Methylators in Oxygen-Deficient Water and Marine Particles in the Baltic Sea. Front Microbiol 2020; 11:574080. [PMID: 33072037 PMCID: PMC7536318 DOI: 10.3389/fmicb.2020.574080] [Citation(s) in RCA: 22] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2020] [Accepted: 08/25/2020] [Indexed: 11/13/2022] Open
Abstract
Methylmercury (MeHg), a neurotoxic compound biomagnifying in aquatic food webs, can be a threat to human health via fish consumption. However, the composition and distribution of the microbial communities mediating the methylation of mercury (Hg) to MeHg in marine systems remain largely unknown. In order to fill this knowledge gap, we used the Baltic Sea Reference Metagenome (BARM) dataset to study the abundance and distribution of the genes involved in Hg methylation (the hgcAB gene cluster). We determined the relative abundance of the hgcAB genes and their taxonomic identity in 81 brackish metagenomes that cover spatial, seasonal and redox variability in the Baltic Sea water column. The hgcAB genes were predominantly detected in anoxic water, but some hgcAB genes were also detected in hypoxic and normoxic waters. Phylogenetic analysis identified putative Hg methylators within Deltaproteobacteria, in oxygen-deficient water layers, but also Spirochaetes-like and Kiritimatiellaeota-like bacteria. Higher relative quantities of hgcAB genes were found in metagenomes from marine particles compared to free-living communities in anoxic water, suggesting that such particles are hotspot habitats for Hg methylators in oxygen-depleted seawater. Altogether, our work unveils the diversity of the microorganisms with the potential to mediate MeHg production in the Baltic Sea and pinpoint the important ecological niches for these microorganisms within the marine water column.
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Affiliation(s)
- Eric Capo
- Department of Chemistry, Umeå University, Umeå, Sweden.,Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden
| | - Andrea G Bravo
- Institut de Ciències del Mar (ICM-CSIC), Barcelona, Spain
| | - Anne L Soerensen
- Department of Environmental Research and Monitoring, Swedish Museum of Natural History, Stockholm, Sweden
| | - Stefan Bertilsson
- Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden
| | - Jarone Pinhassi
- Centre for Ecology and Evolution in Microbial Model Systems - EEMiS, Linnaeus University, Kalmar, Sweden
| | - Caiyan Feng
- Department of Chemistry, Umeå University, Umeå, Sweden
| | - Anders F Andersson
- Department of Gene Technology, Science for Life Laboratory, KTH Royal Institute of Technology, Solna, Sweden
| | - Moritz Buck
- Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden
| | - Erik Björn
- Department of Chemistry, Umeå University, Umeå, Sweden
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