Molecular characterization of dissolved organic matter (DOM): a critical review

TLDR: Advances in water chemistry in the last decade have improved knowledge about the genesis, composition, and structure of dissolved organic matter, and its effect on the environment, and improved purification methods have enabled facile desalting and concentration of freshly collected DOM samples, thereby complementing the analytical process.

Abstract: Advances in water chemistry in the last decade have improved our knowledge about the genesis, com- position, and structure of dissolved organic matter, and its effect on the environment. Improvements in analyti- cal technology, for example Fourier-transform ion cyclo- tron (FT-ICR) mass spectrometry (MS), homo and hetero-correlated multidimensional nuclear magnetic res- onance (NMR) spectroscopy, and excitation emission matrix fluorimetry (EEMF) with parallel factor (PARAFAC) analysis for UV-fluorescence spectroscopy have resulted in these advances. Improved purification methods, for example ultrafiltration and reverse osmosis, have enabled facile desalting and concentration of fresh- ly collected DOM samples, thereby complementing the analytical process. Although its molecular weight (MW) remains undefined, DOM is described as a complex mixture of low-MW substances and larger-MW biomo- lecules, for example proteins, polysaccharides, and exo- cellular macromolecules. There is a general consensus that marine DOM originates from terrestrial and marine sources. A combination of diagenetic and microbial processes contributes to its origin, resulting in refractory organic matter which acts as carbon sink in the ocean. Ocean DOM is derived partially from humified products of plants decay dissolved in fresh water and transported to the ocean, and partially from proteinaceous and poly- saccharide material from phytoplankton metabolism, which undergoes in-situ microbial processes, becoming refractory. Some of the DOM interacts with radiation and is, therefore, defined as chromophoric DOM (CDOM). CDOM is classified as terrestrial, marine, anthropogenic, or mixed, depending on its origin. Ter- restrial CDOM reaches the oceans via estuaries, whereas autochthonous CDOM is formed in sea water by micro- bial activity; anthropogenic CDOM is a result of human activity. CDOM also affects the quality of water, by shielding it from solar radiation, and constitutes a car- bon sink pool. Evidence in support of the hypothesis that part of marine DOM is of terrestrial origin, being the result of a long-term carbon sedimentation, has been obtained from several studies discussed herein.