Odour compounds of the diatom Cocconeis scutellum: effects on benthic herbivores living on Posidonia oceanica

Polyunsaturated aldehydes released by marine planktonic diatoms upon wounding have been extensively studied since the discovery that they reduce the hatching success of copepods. Diatoms are also abundant in benthic marine ecosystems. For this reason we investigated the presence and the possible ecological roles of Cocconeis scutellum parva, a diatom found tightly attached to the leaves of Posidonia oceanica. This diatom was previously demonstrated to play an important trophic role for the shrimp Hippolyte inermis, by influencing its sex reversal. In the present study, C. scutellum parva was isolated and cultivated and the volatile compounds released upon cell disintegration were identified by mass-spectrometric methods. We demonstrated the presence of unsaturated aldehydes with chain lengths from C 5 to C 10 . Unexpectedly, we found C 6 compounds represented by 3(Z)-hexenal, 2(E)-hexenal, hexanal and hexanol-1 that resemble the typical bouquet observed in higher plants. Compounds not described before for diatoms were, among others, pentane-2, 3-dione and octane-2, 3-dione. Food choice experiments performed on 17 animal species associated with P. oceanica meadows indicated that these grazers recognise the presence of the odour compounds, exhibiting complex patterns of reactions according to their life strategies. The mosaic of results obtained at various concentrations, in different species, indicated that wound-activated infochemicals generate a web of communication among diatoms and benthic invertebrates.