Sexual reproduction of Brown algae occurs with the help of pheromones, chemical messengers involved in mating coordination. They are released by female gametes in order to attract and/or induce release of male conspecifics. These substances have been identified as olefines with linear or cyclic structure, composed mostly of eleven carbon atoms (C11 hydrocarbons), which are biosynthesised from C20 polyunsaturated fatty acids (C20 PUFAs). Some species of brown algae release cis-bisalkenylcyclopropane hydrocarbons as enantiopure compounds, which spontaneously rearrange to substituted cycloheptadienes. Other species of brown algae and certain diatoms, produce trans-bisalkenylcyclopropanes as defined enantiomeric mixtures. These diatoms simultaneously oxidise the C20 PUFA precursor. Aim of the project was to elucidate the stereochemical course of pheromone biosynthesis and the putative oxygenation of the fatty acid in a brown alga which produces an enantiomerically pure pheromone. These questions were addressed by incubation of female gametes of the brown alga E. siliculosus, with specifically deuterated precursors and mass spectrometry of their metabolites.