IAP2-22-405

Geomicrobiology and environmental metagenomics of mercury-cycling bacteria

Northern Scotland has been accumulating atmospherically-deposited mercury (Hg) for over two millennia [Farmer et al. 2009], from mining and fossil fuel burning since the height of the Roman Empire. What happens to all this mercury? We know that microorganisms and organic matter both strongly influence the mobility and toxicity of mercury in water and sediments. In particular, the combination of high organic matter and elevated sulphate in northern Scottish wetlands (peatlands) would promote conditions conducive for microbes to form the neurotoxin methylmercury (MeHg) [Blythe 2020]. Furthermore, climate change modelling [e.g., Chen et al. 2018] suggests that peatlands could become future sources of MeHg, to the detriment of terrestrial and marine food webs. However, this process also depends on the poorly understood association of mercury with peatland organic matter, which could actually decrease its bioavailability for microbial methylation [e.g., Moreau et al. 2015]. This project would involve fieldwork in the Scottish Highlands and Flow Country, as well as experiments in the Geobiology Lab at Glasgow University and The Lyell Centre at Heriot-Watt University, to:

• test hypotheses for peatland MeHg generation by microbes
• determine potential microbial sources for MeHg
• understand the effects of organic matter on Hg methylation

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Image Captions

Peatland at Doune Hill, Scotland; CCL, photo credit: Michal Klajban

Methodology

Students will work in Scottish peatland study areas to take sediment cores for total Hg (HgT) and MeHg quantification, DNA/RNA extractions, and laboratory cultivation 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 analyses will be conducted via collaboration with the U.S. Geological Survey. Metagenomic and metatranscriptomic analyses will be performed using local HPC cluster resources to characterise genes encoding for mercury methylation and demethylation in field and lab samples. Integrating results from the above workflows will allow the student to determine the potential for microbial Hg methylation in peatlands under various climate change scenarios, to test hypotheses about the potential for Scottish peatlands to act as net sources of environmental MeHg.

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 (UK).

Year 3

3a) Microcosm experiments with mercury geochemical analyses and molecular microbiological studies

Year 3.5

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

Training
& Skills

The PhD student will learn fieldwork planning, sampling and sample processing, mercury analytical chemistry, molecular biology, metagenomics, metatranscriptomics, bioinformatics, experimental design, anaerobic culturing, conceptual modelling, data archiving, scientific writing and science communication. The student will have the opportunity to present work at national and international scientific conferences.

References & further reading

See https://glasgowgeomicro.net for more info.

References:
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.

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