Natural organic matter (NOM) from seawater is a complex mixture of compounds that has only been roughly characterized. In this study, advanced techniques such as fractionation based on adsorption–desorption dynamics (on XAD® resins), liquid chromatography–organic carbon detection (LC-OCD), excitation–emission matrix (EEM) fluorescence and membrane separation techniques were applied to the characterization of seawater NOM. Conventional analyses have shown that Mediterranean coastal seawater has a low NOM content, slightly aromatic and poorly biodegradability. Advanced analysis permitted better characterization by separating the NOM into different fractions. The NOM was found to be mainly hydrophilic (HPI) (70%) and slightly transphilic (TPI) (24%) from XAD fractionation. LC-OCD showed the seawater to be composed mainly of humic substances (HSs) and low molecular weight (LMW) neutrals that accounted for 37 and 40% of the dissolved organic carbon, respectively. The HSs contained hydrophobic, TPI and HPI compounds, revealing advanced hydrolysis, whereas the LMW neutrals were HPI compounds. Membrane separation revealed that most of the seawater NOM had a MW of <1 kDa, confirming that the HSs were in an advanced state of degradation. We have also shown that seawater NOM with a MW of >1 kDa contains mainly aromatic proteins, which are more biodegradable than whole seawater NOM.

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