Photochemical oxidation and autoxidation of chlorophyll phytyl side chain in senescent phytoplanktonic cells: Potential sources of several acyclic isoprenoid compounds in the marine environment

Abstract

Visible light-induced degradation of the chlorophyll phytyl side chain was studied in senescent cells of two phytoplanktonic strains (Skeletonema costatum and Thalassiosira weissflogii). Particular attention was paid to the induction of autoxidation processes on the phytyl chain and its photoproducts by photochemically produced hydroperoxides. The combination of photochemical oxidation and autoxidation reactions resulted in the production of several acyclic isoprenoid compounds that have been unambiguously identified by comparison of their retention times and mass spectra with those of appropriate standards. Various mechanisms are proposed to explain the formation of these oxidation products. These processes appear to be potential sources of numerous oxidized acyclic isoprenoids that previously have been detected in lacustrine and marine environments. Some oxidation products newly described or whose presence in natural samples was never reported in the literature were then sought in particulate matter, sediment, and microbial mat samples. The results obtained supported the significance of photochemical oxidation and autoxidation of phytoplanktonic chlorophyll phytyl side chain in the marine environment.

Photo- and Free Radical-Mediated Oxidation of Lipid Components During the Senescence of Phototrophic Organisms

“…Compounds 22 and 23 (which are well specific markers of free radical oxidation) could be detected in particulate matter samples (Marchand et al, 2005) and E. huxleyi cells (Rontani et al, 2007a) attesting to the involvement of such processes in senescent phytoplanktonic cells. Free radical oxidation of chlorophyll phytyl chain appeared to be different in senescent cells of S. costatum (Rontani et al, 2003b). The differences observed were attributed to the well documented high chlorophyllase activity of this strain (Jeffrey and Hallegraeff, 1987) catalysing the hydrolysis of chlorophyll to free phytol and chlorophyllide.…”

“…Mechanisms of initiation for the free radical processes have been the subject of many studies. In senescent phytoplanktonic cells, initiation seems to result from the decomposition of hydroperoxides produced during photodegradation of cellular organic matter (Rontani et al, 2003b). Until now, autoxidative degradation in the marine environment has been largely ignored.…”

“…Until now, autoxidative degradation in the marine environment has been largely ignored. Specific markers of these reactions have been highlighted by in vitro studies (Frankel, 1998;Rontani et al, 2003b;. Using these markers, it was demonstrated in situ that autoxidation plays a very significant role in the degradation of particulate organic matter (Marchand et al, 2005;Rontani et al, 2006;Christodoulou et al, 2009;Rontani et al, 2011a).…”

“…Indeed, the mechanism of initiation of lipid radical oxidation, which has been debated for many years, seems to be the homolytic cleavage of photochemically produced hydroperoxides in phytodetritus (Rontani et al, 2003b). Redox-active metal ions are generally considered as the initiators of perhaps greatest importance for lipid oxidation in biological systems (Pokorny, 1987;Schaich, 1992).…”

Metal Ions and Hydroperoxide Content: Main Drivers of Coastal Lipid Autoxidation in Riverine Suspended Particulate Matter and Higher Plant Debris

“…In the initiation process, enzymes, heat, light and metal ions may play a role in the generation of radicals. The metal ion-catalyzed homolytic cleavage of photochemically-produced hydroperoxides [1] has been suggested to play an important role in the initiation of autoxidation reactions in phytodetritus [2], although this has not been firmly established experimentally. The chain reaction ends when free radicals collide and exchange electrons to form a new bond, hence rendering them unreactive.…”