Carbon-water cycle, inland waters, organic-inorganic interactions, manganese, organic matter

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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.