STABILITY OF LONG-CHAIN ALKENONES IN SENESCING CELLS OF EMILIANIA-HUXLEYI - EFFECT OF PHOTOCHEMICAL AND AEROBIC MICROBIAL-DEGRADATION ON THE ALKENONE UNSATURATION RATIO (U-37(K'))

Alkenones were photooxidized in pyridine solution of haematoporphyrin and in killed cells of Emiliania huxleyi. These ketones seem to be pro tected from photochemical damage in phytodetritus; this photochemical recalcitrance has been attributed to their chemical structure and loca lization in the cells. These results indicate that U-37(k') ratios are not likely to be strongly photochemically modified during senescence of Emiliania huxleyi (increase limited to 0.04 U-37(k') unit). In cont rast, the major sterol of this algae (24 alpha-methylcholesta-5,22E-di en-3 beta-ol) appeared to be very sensitive to photochemical processes . Non-sterile incubations of broken cells of Emiliania huxleyi showed a strong biodegradation of alkenones, indicating that aerobic microbia l processes must play an important role during senescence of this spec ies. However, this degradation takes place without dramatic variations in U-37(k') values (decrease limited to 0.05 U-37(k') unit). Several mechanisms are proposed in order to explain this strong and weakly sel ective biodegradation. (C) 1997 Elsevier Science Ltd.