Photoproducts of phytoplanktonic sterols: a potential source of hydroperoxides in marine sediments?

A derailed study of the lipid composition of Recent sediments of Carteau Ba y (Gulf of Fos, Mediterranean Sea) has made possible the detection of signi ficant amounts of Delta(5)-stenol photoproducts of phytoplanktonic origin. Photodegradation of Delta(5)-stenols in senescent phytoplanktonic cells see ms to play a role in the degradation of these compounds in the marine envir onment. These reactions lead to the production of Delta(4)-6 alpha/beta- an d Delta(6)-5 alpha-hydroperoxysterols mainly in esterified and bound forms, which appeared to be relatively well preserved in the sediments. This surp rising stability could be attributed to: (i) the weak reducing properties o f sulfides towards these hydroperoxides or (ii) the protection of these com pounds in intact phytoplanktonic debris. Destruction of hydroperoxides and allylic rearrangement of Delta(6)-5 alpha-hydroperoxysterols (to the corres ponding Delta(5)-7 alpha/beta- derivatives) takes place at the bottom of th e core analyzed. The detection of high amounts of 5,6-epoxy-24-ethylcholest an-3 beta-ol (52 ng/g dry sediment at 3.5 cm depth), (resulting probably fr om the oxidation of 24-ethylcholest-5-en-3 beta-ol by hydroperoxides in the absence of molecular oxygen) strongly suggests that hydroperoxysterols may play a Pole in the degradation of organic matter in anoxic sediments. Due to their greater stability in sediments, Delta(4)-6 alpha/beta-hydroperoxys terols will be more reliable in situ markers of type II photodegradation pr ocesses (i.e. those involving singlet oxygen) than Delta(6)-5 alpha-hydrope roxysterols. Dehydration of the reduced sterol photoproducts described in t he present work might constitute a potential source of steratrienes. which are often present in the sediments. (C) 2000 Elsevier Science Ltd. All righ ts reserved.