Stability of long-chain alkenones in senescing cells of Emiliania huxleyi: effect of photochemical and aerobic microbial degradation on the alkenone unsaturation ratio (Uk′37)

Alkenones were photooxidized in pyridine solution of haematoporphyrin and in killed cells of Emiliania huxleyi. These ketones seem to be protected from photochemical damage in phytodetritus; this photochemical recalcitrance has been attributed to their chemical structure and localization in the cells. These results indicate that Uk′37 ratios are not likely to be strongly photochemically modified during senescence of Emiliania huxleyi (increase limited to 0.04 Uk′37 unit). In contrast, the major sterol of this algae (24α-methylcholesta-5,22E-dien-3β-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 microbial processes must play an important role during senescence of this species. However, this degradation takes place without dramatic variations in Uk′37 values (decrease limited to 0.05 U′37 unit). Several mechanisms are proposed in order to explain this strong and weakly selective biodegradation.