Investigating the geomicrobiology of mercury methylation in northern peatlands

Biogeochemical Cycles

IAP2-20-032

Overview

Scotland’s ombrotrophic peatlands have accumulated atmospherically-deposited mercury (Hg) for over two millennia [Farmer et al. 2009], originally from mining and fossil fuel burning during Roman times, and later from 19th- and 20th -century industrialisation. A combination of the high organic matter content of peat with elevated sulphate levels in rainfall promotes environmental conditions conducive for native microorganisms to transform Hg into methylmercury (MeHg) [Blythe 2020], a neurotoxin that can bioaccumulate through ecosystem food webs [e.g., Liu et al. 2019]. However, little is known about the MeHg content, the potential for MeHg formation, or the putative microorganisms methylating Hg in peatlands. Based on climate change modelling [e.g., Chen et al. 2018], we hypothesize that peatlands could be potential MeHg sources under forecast environmental conditions. In contrast, we also hypothesize that changes in Hg association with organic matter over time could act to decrease the bioavailability of Hg for methylation [e.g., Moreau et al. 2015].

The aim of the proposed research is to apply field- and lab-based experiments, stable mercury isotope geochemistry, molecular microbiology, and bioinformatics to:

• test hypotheses regarding peatland MeHg generation
• determine potential microbial (phylogenetic) sources for MeHg production
• understand effects of varying environmental conditions on Hg methylation activity.

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Image Captions

Peat bog below the top of Doune Hill. Luss Hills, Scotland.
Creative Commons License; photo credit: Michal Klajban

Methodology

We will visit several Scottish peatland study areas to take peat cores for total Hg (HgT) and MeHg quantification, DNA/RNA extraction, and laboratory experiments. Measurements of HgT and MeHg will be performed at the University of Glasgow, using cold-vapour atomic fluorescence spectrometry (CVAFS). Microcosm experiments under anaerobic conditions, with varying temperatures and partial pressures of CO2 representative of current and plausible climate warming scenarios, will be conducted at Heriot-Watt University. Hg stable isotope tracer measurements will be made at the U.S. Geological Survey Water Lab in WI, USA. Whole microbial community DNA/RNA will be extracted and sequenced at the University of Glasgow, and bioinformatic analyses will be performed on the CLIMB HPC cluster to search for genes encoding for mercury methylation [e.g., Podar et al. 2015]) in both field and lab samples.

Integrating these results and their interpretations from the above workflows will allow us to construct a conceptual model for Hg methylation potential in peatlands. We will be able to test hypotheses about the potential for Scotland’s peat bogs to act as a net contributor to environmental MeHg levels, in the context of Hg interaction with organic matter over time, and to understand how this process may be affected by plausible climate change scenarios. Given recent discoveries of new and non-canonical pathways for microbial Hg methylation [e.g., Gionfriddo et al. 2016], we envision that novel Hg-methylating microorganisms will be discovered, which will help to elucidate yet unknown biochemical pathways for Hg methylation.

Project Timeline

Year 1

1a) Sampling and processing of peat cores
1b) Quantification of total and methylated Hg in peat cores by CVAFS
1c) Extraction of DNA/RNA from peat cores, metagenomic/metatranscriptomic sequencing
1d) Write-up first manuscript for publication

Year 2

2a) Metagenomic/metatranscriptomic data processing and bioinformatics analyses
2b) Setup of microcosm experiments
2c) Write-up 2nd manuscript for publication
2d) Attend 1st scientific conference.

Year 3

3a) Microcosm experiments and consequent mercury geochemical analyses and molecular microbiological studies (Hg analyses at USGS Water Lab, WI, USA)

Year 3.5

3b) Write-up 3rd manuscript for publication
3c) Present culmination of doctoral research at 2nd scientific conference.

Training
& Skills

The PhD student will conduct ground-breaking research on the biogeochemistry of mercury in Scottish peatlands, a landscape that plays a key role in carbon storage and climate change mitigation. The PhD student will be trained in all aspects of the project by leaders in the fields of geomicrobiology and geochemistry. S/he will learn fieldwork planning, sampling and sample processing, mercury analytical chemistry, molecular biology, metagenomics, bioinformatics, experiment design, anaerobic culturing, conceptual modelling, data archiving, scientific writing and science communication. The student will have the opportunity to present her/his work at national and international scientific conferences.

References & further reading

Blythe, J.L., 2020. The effects of legacy sulphur deposition on methylmercury production in northern peatlands; geochemical and biological considerations.

Chen, C.Y., Driscoll, C.T., Eagles-Smith, C.A., Eckley, C.S., Gay, D.A., Hsu-Kim, H., Keane, S.E., Kirk, J.L., Mason, R.P., Obrist, D. and Selin, H., 2018. A critical time for mercury science to inform global policy.

Farmer, J.G., Anderson, P., Cloy, J.M., Graham, M.C., MacKenzie, A.B. and Cook, G.T., 2009. Historical accumulation rates of mercury in four Scottish ombrotrophic peat bogs over the past 2000 years. Science of the Total Environment, 407(21), pp.5578-5588.

Gionfriddo, C.M., Tate, M.T., Wick, R.R., Schultz, M.B., Zemla, A., Thelen, M.P., Schofield, R., Krabbenhoft, D.P., Holt, K.E. and Moreau, J.W., 2016. Microbial mercury methylation in Antarctic sea ice. Nature microbiology, 1(10), p.16127.

Liu, M., Zhang, Q., Cheng, M., He, Y., Chen, L., Zhang, H., Cao, H., Shen, H., Zhang, W., Tao, S. and Wang, X., 2019. Rice life cycle-based global mercury biotransport and human methylmercury exposure. Nature communications, 10(1), pp.1-14.

Moreau, J.W., Gionfriddo, C.M., Krabbenhoft, D.P., Ogorek, J.M., DeWild, J.F., Aiken, G.R. and Roden, E.E., 2015. The effect of natural organic matter on mercury methylation by Desulfobulbus propionicus 1pr3. Frontiers in microbiology, 6, p.1389.

Podar, M., Gilmour, C.C., Brandt, C.C., Soren, A., Brown, S.D., Crable, B.R., Palumbo, A.V., Somenahally, A.C. and Elias, D.A., 2015. Global prevalence and distribution of genes and microorganisms involved in mercury methylation. Science advances, 1(9), p.e1500675.

Further Information

Application procedure: For IAPETUS2 applications to the University of Glasgow please use the dedicated application portal: www.gla.ac.uk/ScholarshipApp (you will still need to submit your administrative details to the IAPETUS2 website as well).

John Moreau, john.moreau@glasgow.ac.uk, +44 141 330 5461

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