In a light-dependent reaction (3.5 kilolux) at pH 5, the evolution of hexanal, ethane, and ethylene has been established with cell suspensions of the diatom, Phaeodactylum tricornutum. During this process, chlorophyll and carotenoids are partially bleached. Addition of 25 millimolar α-linolenic acid or 12 millimolar docosahexaenoic acid yield total pigment destruction and enhancement of ethylene and ethane formation (by about 150 and 7,600%, respectively), whereas hexanal production decreases by 70%. Eicosapentaenoic acid, the major polyunsaturated fatty acid in diatoms, stimulates both ethane and hexanal formation (by about 1,400 and 130%, respectively), but reduces ethylene production (by about 60%). This competition suggests that the production of the volatile compounds is closely connected, although hexanal and ethylene obviously possess different unsaturated fatty acids as precursors. Both the kind of the fatty acids and their relative amounts seem to determine the pattern of the evolved hydrocarbons. The presence of 10 millimolar propylgallate inhibits the evolution of the volatile compounds by about 80%, indicating that radical formation might play a key role in this light-dependent cascade of reactions.