Abstract
Sulfate reduction plays an important role in altering dissolved organic matter (DOM) in estuarine and coastal sediments, although its role in the production of optically active chromophoric DOM (CDOM) and a subset of fluorescent DOM (FDOM) has not been previously investigated in detail. Freshwater sediment slurries were incubated anaerobically with added sulfate and acetate to promote sulfate-reducing bacteria. Ultraviolet visible (UV-Vis) absorbance and 3-dimensional excitation emission matrix (EEM) fluorescence spectra were measured over a five weeks anaerobic dark incubation period. Parallel Factor Analysis (PARAFAC) of FDOM determined components that increased significantly during dark and anaerobic incubation matching three components previously considered of terrestrially-derived or humic-like origin published in the OpenFluor database. The observed FDOM increase was strongly correlated (R2 = 0.96) with the reduction of sulfate. These results show a direct experimental link between sulfate reduction and FDOM production, which impacts our understanding of coastal FDOM sources and early sediment diagenesis. As 3D fluorescence techniques are commonly applied to diverse systems, these results provide increasing support that FDOM can have many diverse sources not consistently captured by common classifications such as “humic-like” fluorescence.
Highlights
Sulfate reduction plays an important role in altering dissolved organic matter (DOM) in estuarine and coastal sediments, its role in the production of optically active chromophoric DOM (CDOM) and a subset of fluorescent DOM (FDOM) has not been previously investigated in detail
Using Parallel Factor Analysis (PARAFAC) modeling of excitation emission matrix (EEM) fluorescence, we evaluated the production of FDOM directly related to sulfate-reducing bacteria and fermenting bacteria in anaerobic sediment pore waters
Dissolved organic carbon (DOC) and sulfate decreased over the five-week incubation for all treatments (Fig. 1, Supplementary Figs S2, S3)
Summary
Sulfate reduction plays an important role in altering dissolved organic matter (DOM) in estuarine and coastal sediments, its role in the production of optically active chromophoric DOM (CDOM) and a subset of fluorescent DOM (FDOM) has not been previously investigated in detail. Organic carbon mineralization in anaerobic sediments by dissimilatory sulfate reducing bacteria (SRB) is thought to be the main terminal respiratory process in continental margin sediments[13], and the rate of sulfate reduction depends on temperature and the quality and quantity of labile DOM5. Because of the dependence of SRB on the supply of sulfate, bioavailable carbon, and nutrients, the incorporation of sulfur in organic matter is linked to processes in near surface sediments with high rates of bacterial sulfur reduction and the concomitant formation of iron sulfides[15,16,17]. Fluorescent dissolved organic matter (FDOM), the component of DOM that can absorb and fluoresce, has been shown to increase with depth in sediment pore waters[19, 20]. Humic substances are thought to be preferentially preserved in anoxic sediments, as are sulfur containing humic substances that show humic-like fluorescence[9, 16, 19, 20]
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