Photo- and autoxidation of unsaturated algal lipids in the marine environment: An overview of processes, their potential tracers, and limitations

Abstract

This paper provides an overview of results obtained in the course of recent studies dealing with abiotic degradation of algal lipids in the marine realm. In the first part, the photooxidation of the main lipid components (chlorophyll phytyl chain, carotenoids, Δ5-sterols, unsaturated fatty acids, alkenones and unsaturated alkenes) in senescent phytoplanktonic cells is examined. Type II photosensitized oxidation processes (i.e. involving singlet oxygen) play a key role in the photooxidation of these compounds. The second part of the paper describes free radical oxidation of the main lipids in senescent phytoplanktonic cells. These processes, which are believed to be induced by homolytic cleavage of photochemically produced hydroperoxides, involve the direct reaction of molecular oxygen in its triplet ground state with organic compounds under relatively mild conditions. Particular attention is given to the characterisation of pathway-specific tracers of the individual abiotic reactions described. Some of these compounds appear to have the potential to provide qualitative or semi-quantitative information about photooxidation, autoxidation, or the total abiotic degradation state of individual or groups of organisms. On the other hand, some other primary oxidation products are likely too susceptible to further degradation to permit reliable quantitative estimates, but can, nonetheless, provide a qualitative indication of photooxidation and autoxidation.