Photochemistry Illuminates Ubiquitous Organic Matter Fluorescence Spectra

Abstract

Dissolved organic matter (DOM) in aquatic environments forms a vast reservoir of carbon present as a complex supermixture of compounds. An efficient approach to tracking the production and removal of specific DOM fractions is needed across disciplines, for purposes that range from improving global carbon budgets to optimizing water treatment in engineered systems. Although widely used to study DOM, fluorescence spectroscopy has yet to deliver specific fractions with known spectral properties and predictable distributions. Here, we mathematically isolate four visible-wavelength fluorescent fractions in samples from contrasting lake, river, and ocean environments. Using parallel factor analysis (PARAFAC), we show that most measured fluorescence in environmental samples can be explained by ubiquitous spectra with nearly stable optical properties and photodegradation behaviors over environmental pH gradients. Sample extraction changed bulk fluorescence spectra but not the number or shape of underlying PARAFAC components, while photobleaching preferentially removed the two longest-wavelength components. New approaches to analyzing fluorescence data sets incorporating these findings should improve the interpretation of DOM fluorescence and increase its utility for tracing organic matter biogeochemistry in aquatic systems.

Electrochemical characterization of natural organic matter by direct voltammetry in an aprotic solvent

“…Therefore, the distribution of the chemical composition of the samples used in this study is consistent with selectivity of the conventional (alkaline) extraction methods for similar redox active moieties. Similar conclusions have been drawn from other types of evidence, such as near edge X-ray absorption ne structure (NEXAFS) spectra, 158 collision induced dissociation coupled to FTICR MS, 159 solid phase extractions and FTICR MS, 22 uorescence spectroscopy coupled to PARAFAC analysis, 160 and statistical analysis of chemical composition data for hundreds of NOMs. 21,29 A few of the NOMs from this study distribute along a diagonal in Fig.…”

Depth‐Resolved Photochemical Lability of Dissolved Organic Matter in the Western Tropical Pacific Ocean

“…Note that all the validated models appear to capture a bit of the Rayleigh and Raman scattering despite our effort to completely remove it from the data set. After many attempts, it was found that we were losing the protein‐like signal if we tried to remove too much of the scattering (i.e., by enlarging the widths of the scattering band to be removed), similar to what has been alluded by Bro (1997) and Murphy et al (2018). This problem, due largely to the relatively high noise in our data set built primarily on blue water samples with low fluorescence intensities, at least partly explains the small emission elevations over 540–600 nm in C2 and C4 (Figure 2).…”

“…In particular, four of our components (C1–C4) were also identified by Jørgensen et al (2011) in marine water samples collected from both coastal regions and open oceans. Based on the results of previous studies (e.g., Chen et al, 2003; Coble, 1996; Jørgensen et al, 2011; Murphy et al, 2018), C2 and C3 are designated as humic‐like FDOM, and C1, C4, and C5 as protein‐like FDOM. Note that all the validated models appear to capture a bit of the Rayleigh and Raman scattering despite our effort to completely remove it from the data set.…”

Organic Matter Integration, Overprinting, and the Relative Fraction of Optically Active Organic Carbon in a Human-Impacted Watershed

“…Absorption and emission spectra from the optically active fraction of DOM relate to DOM biochemistry, and are widely used to estimate DOM composition (Stedmon et al, 2003;Hernes et al, 2009;Fellman et al, 2010a). Analyzing EEMs to discern biochemical characteristics carries distinct advantages: measurements are inexpensive, rapid, and sensitive, with capacity for large throughput.…”