Résumé
The production and composition of acyclic, monocyclic and bicyclic C-34-C-37 botryococcenes in Lake Masoko sediments covering the last 32 ka show sharp and contrasting variations. Physicochemical conditions in the water column at ca. 32 ka and during the Younger Dryas (ca. 12 ka) appear to have been favourable for the development of Botryococcus braunii race B and apparently promoted methylation and cyclisation reactions during botryococcene biosynthesis, although both periods represent contrasting climatic conditions. The stable carbon isotopic composition of botryococcenes shows erratic variation, both for the same group of compounds (exhibiting close structural features) from one period to another, and between compounds within the same period. Overall, the results suggest complex physiological and/or physicochemical controls on the glacial/interglacial occurrence of botryococcenes in Lake Masoko, where local environmental parameters may have been more influential than global climatic changes.