The algal excretion product geranylacetone was proved to be an effective and novel inhibitor of carotene synthesis in Synechoccus 6911. The application of geranylacetone resulted in an ac­ cumulation of phytofluene, indicating an inhibition of the conversion of this compound into fcarotene. The cts-isomer of geranylacetone was as effective as the natural irans-isomer. The chloro­ phyll synthesi...