Manganese and its role in (de)stabilizing aquatic carbon from land to ocean

Biogeochemical Cycles



Dissolved organic matter (DOM) in aquatic environments is a fundamental part of the global carbon cycle supporting a variety of ecosystem services. DOM is operationally defined as organic matter that is not retained by filtration (pore sizes ranging from 0.2 to 0.7 um) and is a complex mixture of phenolic, carboxylic acid rich material that is often collectively referred to a humic substances (Hawkes et al., 2019). However, DOM concentration, composition, and chemistry are highly variable dependant on the source (e.g. allochthonous vs autochthonous) with a variety of other important DOM compounds also present including biopolymers, acids, and neutrals (Pereira et al., 2014). Various factors can influence the composition of DOM in water including temperature, ionic strength, pH, major cation composition, and the surface chemistry of co-transported sediments (e.g. Leenheer et al., 2003). Further transformations of DOM by photochemical and microbial activity increase its compositional variability (e.g. Battin et al., 2009; and Benner and Kaiser, 2011).

Manganese (Mn)-oxides are important mediators of DOM reactivity involved in both oxidation and reduction processes that under certain conditions can modify the composition (Johnson et al., 2015). For example, Mn additions to the humic substances pool of DOM can produce lower molecular weight (LMW) organic substrates that support microbial activity and wider river ecosystem functions (Sunda & Kieber, 1994). Mn oxidation by bacteria has also been shown to be an important process to CO2 into biomass (Yu & Leadbetter, 2020). However, while some key pathways have been identified looking specifically at the role of humic substances, there is a critical knowledge gap on how Mn interacts within the wider DOM pool and under which conditions are favourable for these reactions to occur in the aquatic continuum (Allard et al., 2017).

This PhD studentship will investigate for the first time the uncertainties in which Mn oxides impact DOM reactivity along a land-ocean transect from temperate (UK) and tropical (Guyana) waters. A key focus will be to investigate the partitioning of DOM-Mn interactions from soil, fresh, and estuarine/marine waters under changing physical conditions in the Lyell Centre climate-controlled facilities and examine the microbial response.

Based in Edinburgh, Scotland, at the Lyell Centre ( at Heriot-Watt university, the successful PhD candidate join the Carbon-Water Dynamics team and will undertake regular visits to Durham University ( ). This project offers the prospect to work in a world-leading laboratory for DOM analyses with a dynamic team working on global projects in the UK, Amazonia, and S.E Asia where the successful applicant has the opportunity to join ongoing expeditions. Under supervision the PhD candidate will lead experimental design and execution, and will be responsible for collecting new empirical data to test linkages of DOM-Mn transformations. This project has the opportunity to gain a unique blend of skills from organic, inorganic and microbial disciplines that offer a variety of potential futures in research, academia and industry.


The project will focus on identifying key DOM pools using the latest liquid chromatography organic carbon and nitrogen detection systems (LC-OCD-OND; Huber et al., 2011) and Mn-oxides using UV spectroscopy (Madison et al., 2011; Luther et al., 2015) techniques to investigate DOM and Mn interactions. Further controlled experiments will examine the role of changing environmental conditions (e.g. pH, temperature, redox) using the climate-controlled facilities at the Lyell Centre ( to investigate the role of microbial communities (qPCR, next-gen sequencing).

Project Timeline

Year 1

• Carry out literature review of aquatic DOM-Mn interactions and processes.
• Learn key skills in DOM, Mn, Fe and microbial techniques at Durham and Heriot-Watt universities.
• Meet both supervisors to finalise PhD objectives and plan fieldwork for year 2 in addition to regular project meetings.
• Identify training needs to complete ongoing workplan

Year 2

• Analyse the results on year 1 climate-controlled experiments
• Complete land-ocean transects in Guyana/Vietnam with support from overseas partners
• Continue new experiments for DOM transformations
• Meet supervisors to assess progress and plan for year 3
• Input new results into conceptual DOM-Mn model
• Present results at a national conference

Year 3

• Participate in additional fieldwork if necessary
• Complete outstanding experiments
• Make substantial progress towards thesis preparation and define a detailed timeline to ensure completion with 3.5 years
• Present results at a international conference
• Prepare first publication

Year 3.5

• Complete and submit thesis
• Prepare additional data for publication

& Skills

The supervisors will provide full training in their individual expertise areas, enabling the student to combine current best practice and develop the skills necessary to advance this science. Supervision will be provided jointly by Heriot-Watt (RP) and Durham (KJ/LB) universities. Directly relevant training will be provided by both institutes to equip the student with the skills necessary for all aspects of the project. IAPETUS fosters a strong sense of “community” that encourages students to organise a range of activities (e.g. annual conference) and identify additional training needs to be addressed via tailored opportunities. The student will have opportunities for additional training at the partner institution. The student should develop strong collaborations that will potentially result in further opportunities.

References & further reading

Hawkes et al (2019) Faraday Diss. 218, 52-71, DOI: 10.1039/C8FD00222C
Pereira et al (2014) GRL, 4, 1202-1208, DOI: 10.1002/2013GL058658
Leenheer et al (2003) Env. Sci. Tech., 37, 18A-26A, DOI: 10.1021/es032333c
Johnson et al (2015) Nat. Commun., 6, 7628, DOI: 10.1038/ncomms8628
Sunda & Kieber, (1994) Nature, 367, 62-64, DOI: 10.1038/367062a0
Yu & Leadbetter (2020) Nature 583, 453–458, DOI: 10.1038/s41586-020-2468-5
Battin et al (2009) Nat. Geosci. 2, 598-600, DOI: 10.1038/ngeo618
Benner & Kaiser, (2011). Biogeochem. 102, 209-222, DOI: 10.1007/s10533-010-9435-4

Further Information

Dr Ryan Pereira,
The Lyell Centre, Institute of Life and Earth Sciences, School of Energy, Geoscience and Society, Heriot Watt University, EH14 4AS.
Telephone: 0131 451 3537

Prof. Karen L. Johnson,
Department of Engineering, Durham University, Durham, DH1 3LE.

Dr Lynsay Blake
Postdoctoral Research Associate in the Department of Biosciences
Daphne Jackson Research Fellow in the Department of Engineering
Durham University, Durham, DH1 3LE.

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