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 o-oxocarboxylic acid (CHO-(CH,),,-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 linolcic 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 polqtnsaturated fatty acids than for the monounsaturated fatty acids. Photolysis of the triglyceride, trilinolein, gave En 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 concentraticn 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. Fatty acids are chemically and biologically labile compounds that comprise -3% of the total dissolved organic matter (DOM) in seawater (Kattner et al. 1983b). They, along with triglycerides, are involved in many biogeochemically significant processes occurring within the sea. Of particular importance is their potential role as precursors to several other important types of DOM, including aldehydes and humic substances. Several mechanisms have been proposed for the autooxidation of fatty acids to aldehydes in seawater. It has been suggested that oleic acid (A9 C,,,,) is autooxidized to octanal, nonanal, and decanal while palmitoleic acid (A9 C,,,,) is autooxidized to hexanal, heptanal, and octanal (Schauenstein et al. 1977; Gschwend 1979; Gschwend et al. 1980, 1982). o-Oxocarboxylic acids have been detected in marine aerosols and the dominance of the C, o-oxoacid (CHO-(CH,),COOH) was proposed to be due to photooxidation of A9 unsaturated precursors such as oleic (A9 C,,,,), palmitoleic (A9 C,,,,), linoleic (89,12 C,,:,), and linolenic (A9,12,15 C,,:,) fatty acids (Kawamura and Gagosian 1987). Although mechanisms for the photooxidation of fatty acids have been proposed, they have never been verified experimentally in seawater. This paper describes photoproduction of aldehydes and o-oxocarboxylic acids from fatty acids in seawater, under ambient sunlight conditions, as well as in situ aldehyde concentration in four coastal marine waters. Understanding the photolability of fatty acids and triglycerides in seawater is important for reasons beyond their role