Senescence- and death-related alteration of chlorophylls and carotenoids in marine phytoplankton

This report extends previous work ([Louda et al., 1998a] and [Louda et al., 1998b]. Chlorophyll degradation during senescence and death. Organic Geochemistry 29, 1233–1251.) in which we detailed type-I (alteration) and -II (destruction) degradation of chlorophyll with representative fresh water phytoplankton. The present study covers similar experiments with marine phytoplankton, namely, a cyanobacterium (“ANA” Anacystis sp), a coccolithophore (“COC” Coccolithophora sp.), a dinoflagellate (“GYM” Gymnodinium sp.) and two diatoms (“CYC” Cyclotella meneghiniana and “THAL” Thalassiosira sp.). Mg loss (‘pheophytinizationʹ) was rapid and continuous in all species under room-oxic conditions and slow or sporadic under anoxic conditions. The proportion of dephytylated pigments (pheophorbides-a, chlorophyllides-a), relative to the phytylated forms (chlorophyll-a, pheophytins-a), increased over the first year under room-oxic conditions and in room-anoxic conditions only in “CYC”. Pheophorbide-a was converted to pyropheophorbide-a within 15 months only in “THAL” and “ANA”, and slightly in “COC”. After 9–15 months of oxic incubation, “COC” was found to contain traces of purpurin-18 phytyl ester. Consideration of carotenoid pigments is also included herein. All fucoxanthin containing species, except “THAL”, exhibited conversion of fucoxanthin to fucoxanthinol in room-oxic conditions. Diadinoxanthin was rapidly de-epoxidized to give diatoxanthin within the first 2–4 weeks. Diatoxanthin then disappeared from all species by 15 months with a concurrent increase in a pigment which we tentatively identify as a cis-zeaxanthin. Incubations of pure cultures are found to be an effective way by which to model the early type-I reactions for both chlorophylls and carotenoids. The influence of oxygen during senescence-death and the onset of early diagenesis is of paramount importance. The absence of oxygen and, by inference, aerobic microbiota, retards the breakdown of these pigments dramatically.