Linking Optical and Molecular Properties of Dissolved Organic Matter in the Mediterranean Sea

The dissolved organic matter (DOM) composition of the major water masses of the Mediterranean Sea was characterized at the optical (fluorescence spectroscopy) and molecular level (Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, FT ICR MS). Water samples were taken from the deep chlorophyll maximum, Levantine Intermediate water, oxygen minimum layer and Eastern and Western Mediterranean Deep waters. Excitation-Emission Matrices (EEMs) were performed to deconvolve the main fluorophores present in the samples, which were correlated with molecular parameters of the solid phase extracted DOM. We observed that specific fluorescence of the humic-like components A (A*), C (C*) and M (M*) exhibited significant linear correlations with weighted average molecular parameters. Component C*showed significant relationships with the molecular weight (r² = 0.53, p < 0.0001), H/C ratio (r² = 0.62, p < 0.0001), double bond equivalent (r² = 0.71, p < 0.0001) and degradation index (r² = 0.78, p < 0.0001) as well as with highly unsaturated (r² = 0.63, p < 0.0001) and unsaturated aliphatic compounds (r² = 0.76, p < 0.0001). The statistical linkages between DOM molecular and optical properties illustrate that the simple, rapid and low-cost fluorescence spectroscopy technique provides valuable proxy information on the molecular composition of marine DOM.