Photochemistry Illuminates Ubiquitous Organic Matter Fluorescence Spectra

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.