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

A detailed study of the lipid composition of Recent sediments of Carteau Bay (Gulf of Fos, Mediterranean Sea) has made possible the detection of significant amounts of Δ5-stenol photoproducts of phytoplanktonic origin. Photodegradation of Δ5-stenols in senescent phytoplanktonic cells seems to play a role in the degradation of these compounds in the marine environment. These reactions lead to the production of Δ4-6α /β- and Δ6-5α-hydroperoxysterols mainly in esterified and bound forms, which appeared to be relatively well preserved in the sediments. This surprising stability could be attributed to: (i) the weak reducing properties of sulfides towards these hydroperoxides or (ii) the protection of these compounds in intact phytoplanktonic debris. Destruction of hydroperoxides and allylic rearrangement of Δ6-5α-hydroperoxysterols (to the corresponding Δ5-7α/β- derivatives) takes place at the bottom of the core analyzed. The detection of high amounts of 5,6-epoxy-24-ethylcholestan-3β-ol (52 ng/g dry sediment at 3.5 cm depth), (resulting probably from the oxidation of 24-ethylcholest-5-en-3β-ol by hydroperoxides in the absence of molecular oxygen) strongly suggests that hydroperoxysterols may play a role in the degradation of organic matter in anoxic sediments. Due to their greater stability in sediments, Δ4-6α/β-hydroperoxysterols will be more reliable in situ markers of type II photodegradation processes (i.e. those involving singlet oxygen) than Δ6-5α-hydroperoxysterols. Dehydration of the reduced sterol photoproducts described in the present work might constitute a potential source of steratrienes, which are often present in the sediments.