Molecular heterogeneity in dissolved organic matter: Structural-state controls on fluorescence signals

Marine fluorescent dissolved organic matter, known as fDOM, is the fluorescently active fraction of dissolved organic matter, known as DOM, and is widely used as a tracer of organic matter sources and transformations. Yet frameworks based on bulk absorption or excitation emission matrix fluorescence spectroscopy, known as EEM, cannot readily disentangle the effects of concentration, molecular structure, and emission efficiency on fDOM fluorescence. Using samples from the Pearl River Estuary, we developed a jointly constrained structural state parameterization framework that integrates bulk EEM responses with nuclear magnetic resonance, known as NMR, functional group integrals. The framework represents fDOM variability using three effective structural state dimensions, namely aromaticity and conjugation, oxidation and carboxylation, and microenvironmental polarity and conformational relaxation, and reproduces EEM responses through their inferred modulation of absorption weights, fluorescence quantum yield, and emission spectral shape. The reconstructed fDOM fluorescence response was further embedded in a water column radiative transfer formulation to evaluate its potential contribution to remote sensing reflectance. Results showed that, along the salinity gradient, the NMR aromatic region integral decreased from 28.54% 0.98% to 18.81% 1.76%, humic like peaks A and C weakened, and protein like peak T increased, consistent with a shift from terrigenous humic dominated fDOM toward lower aromaticity, marine influenced structures. Compared with models constrained only by EEM or only by NMR, the joint constraint reduced structural parameter uncertainty and improved reconstruction stability in major fluorescence peak regions. The reconstructed fluorescence response produced a wavelength dependent contribution to remote sensing reflectance, suggesting that structural state variations may be reflected in water leaving optical signals under the modeled conditions. This framework provides a physically informed approach for linking DOM compositional variability, fluorescence behavior, and potential optical manifestations in estuarine waters.