Visible light-dependent degradation of lipidic phytoplanktonic components during senescence: a review

Though most of the organic components of phytoplankton are susceptible to b eing photodegraded during senescence, until recent years there has been ver y little research in this area. Recently, however, there have been a renewa l of interest, and the heterogeneous visible light-induced degradative reac tions of lipidic compounds associated with phytodetritus have been studied. The present paper reviews the results obtained in the course of these stud ies. In the first part, the production and quenching of excited states of c hlorophyll and toxic oxygen species (singlet oxygen, superoxide ion, hydrox yl radical and hydrogen peroxide) in healthy and senescent phytoplanktonic cells are discussed. Then, the photo-oxidation of the main lipidic cell com ponents (chlorophyll, chlorophyll phytyl chain, carotenoids, sterols, unsat urated fatty acids, alkenones and unsaturated alkenes) in senescent phytopl anktonic cells is examined. Emphasis is given to the degradation rates and the mechanisms of visible light-induced degradation of the main lipidic com ponents of phytoplankton and to the structure of the photoproducts formed. In each case, the selectivity of these compounds is discussed in order to s elect a "pool" of photoproducts able to act as tracers of photo-oxidative a lterations in the marine medium. This "pool" of photoproducts could provide very useful information concerning the physiological state of phytoplankto nic communities and current environmental problems related to ozone depleti on. (C) 2001 Elsevier Science Ltd. All rights reserved.