Introduction Recent progresses in silicon isotopic measurements using MC-ICP-MS technology have brought hopes among the biogeochemical scientific community in order to improve our knowledge on the silicon cycle (Cardinal et al., 2003). Results and Discussion The first silicon isotopic dissolved profiles obtained by this technique have confirmed the biological fractionation effect (De la Rocha et al., 2000) linked to the siliceous organism productivity both in marine (Antarctic Ocean) and fresh water (Lake Tanganyika) systems. This isotopic desequilibrium (δ29Si = 0.94±0.11 in surface and 0.62±0.06 in deep) is controlled by the Rayleigh distillation equation and allows to reconstruct the dissolved silicic acid utilisation based on the fractionation factor recorded in biogenic opal material. The fractionation observed in our studies between the dissolved Si pool in surface waters and the estimated diatom signature displays a δ29Si strikingly similar to what was measured on cultured batch (De la Rocha et al., 1997). These in-vivo measurements thus confirm the small impact in changes of temperature, salinity, pH and diatom species in the fractionation process. Spatial and temporal changes of the silicic acid utilisation will be discussed for the Tanganyika lake and Antarctic water samples along with the isotopic signature of silicon sources. Conclusions Several questions still need consideration: A better constrain of the Si isotopic fractionation factor and the parameters controlling it; Determining the δ29Si for silicic acid supply to the euphotic zone and its spatial variability; Deepening our perception of this tracer to provide valuable information on paleo-diatom rich productivities.