Photooxidation of triglycerides and fatty acids in seawater: Implication toward the formation of marine humic substances

Unsaturated fatty acids rapidly photodecomposed in filtered seawater to aliphatic aldehydes and an omega-oxocarboxylic acid (CHO-(CH2)(n)-COOH) upon a 5-h exposure to ambient sunlight. The photoreactivity of fatty acids increased as their degree of unsaturation increased. Palmitic acid displayed no photodegradation while monounsaturated oleic acid photodecomposed to nonanal and 9-oxononanoic, respectively. Photooxidation of the polyunsaturated linoleic acid produced significantly higher concentrations, primarily hexanal, and a wider variety of aldehydes. The rate of photodegradation was more than 10 times greater for the polyunsaturated fatty acids than for the monounsaturated fatty acids. Photolysis of the triglyceride, trilinolein, gave an aldehyde profile similar to its component fatty acid, indicating each undergoes a similar mechanism of photodegradation. Twenty percent of the initial triglyceride was photodegraded during the 6-h irradiation. The rapid photoreactivity of unsaturated fatty acids may explain their low in situ levels relative to saturated fatty acids, the latter of which are found in the dissolved phase at relatively higher levels. Fluorescent hydrophobic dissolved organic matter, operationally defined as humic substances, increased after irradiation of trilinolein-treated seawater over a 14-d period. The concentration of humic products was enhanced by postphotolysis addition of ammonia. Dissolved ammonia concentrations declined as the dissolved organic nitrogen of the extracted humics increased, indicating the added ammonia was becoming incorporated into the organic structure.