PHOTOOXIDATION OF TRIGLYCERIDES AND FATTY-ACIDS IN SEAWATER - IMPLICATION TOWARD THE FORMATION OF MARINE HUMIC SUBSTANCES

Unsaturated fatty acids rapidly photodecomposed in filtered seawater t o aliphatic aldehydes and an omega-oxocarboxylic acid (CHO-(CH2)(n)-CO OH) upon a 5-h exposure to ambient sunlight. The photoreactivity of fa tty acids increased as their degree of unsaturation increased. Palmiti c acid displayed no photodegradation while monounsaturated oleic acid photodecomposed to nonanal and 9-oxononanoic, respectively. Photooxida tion of the polyunsaturated linoleic acid produced significantly highe r concentrations, primarily hexanal, and a wider variety of aldehydes. The rate of photodegradation was more than 10 times greater for the p olyunsaturated fatty acids than for the monounsaturated fatty acids. P hotolysis of the triglyceride, trilinolein, gave an aldehyde profile s imilar to its component fatty acid, indicating each undergoes a simila r mechanism of photodegradation. Twenty percent of the initial triglyc eride was photodegraded during the 6-h irradiation. The rapid photorea ctivity of unsaturated fatty acids may explain their low in situ level s relative to saturated fatty acids, the latter of which are found in the dissolved phase at relatively higher levels. Fluorescent hydrophob ic dissolved organic matter, operationally defined as humic substances , increased after irradiation of trilinolein-treated seawater over a 1 4-d period. The concentration of humic products was enhanced by postph otolysis addition of ammonia. Dissolved ammonia concentrations decline d as the dissolved organic nitrogen of the extracted humics increased, indicating the added ammonia was becoming incorporated into the organ ic structure.