Characterisation of photo-oxidation and autoxidation products of phytoplanktonic monounsaturated fatty acids in marine particulate matter and recent sediments

Oxidation products of phytoplanktonic monounsaturated fatty acids have been detected in particulate matter and recent sediment samples collected at the SOFI station (Gulf of Fos, Mediterranean Sea). The fluxes of these compounds are not negligible and correspond to 0.14–0.03% of the total organic carbon flux in the zone investigated. Oxidation of monounsaturated fatty acids results mainly in the production of isomeric allylic hydroperoxyacids, which are relatively stable in the water column. Saturated hydroperoxyacids, epoxyacids and dihydroxyacids, deriving probably from the addition of hydroperoxides to the double bond of monounsaturated acids, have been detected also. During early diagenesis, hydroperoxides undergo heterolytic cleavage to aldehydes and ω-oxocarboxylic or ω-hydroxycarboxylic acids, or homolytic cleavage and subsequent transformation to the corresponding alcohols or ketones. Due to the good preservation of their distribution pattern in Recent sediments, isomeric allylic hydroxyacids thus formed will constitute more useful and specific tracers of oxidative processes than ω-oxocarboxylic or ω-dicarboxylic acids (with C9 as the predominant species). These compounds will allow easy differentiation between autoxidative and photooxidative processes and could thus provide a useful indication of the residence time of particulate organic matter in the water column, the physiological state of phytoplanktonic communities and current environmental problems related to ozone depletion.