Mercury is a strong poison that poses significant and immediate hazards to human health. Due to its bioaccumulative properties, even small amounts of the metal are usually very poisonous or lethal when absorbed over long periods of time. Even though the possible dangers of mercury interactions with proteins are well understood, little is known about its uptake and dynamics within an organism. In particular, the concentration and distribution of the metal within a cell or a tissue are only poorly understood. In this study, we describe the application of a recently developed biosensor [Chapleau R.R., Blomberg R., Ford P.C., Sagermann M., 2008. Design of a highly specific and non-invasive biosensor suitable for real-time in vivo imaging of mercury(II) uptake. Protein Sci. 17(4), 614-622] that facilitates unprecedented non-invasive real-time imaging of ionic mercury uptake by an organism under in vivo conditions. Specifically, we here show that mercury ions can be taken up from the environment within minutes by prokaryotic as well as eukaryotic organisms. This rapid uptake can still be detected if the sensor expressing cells are shielded by layers of surrounding tissues suggesting that neither individual cell walls nor tissues provide a serious barrier for the metal. Furthermore, we show that this biosensor is suitable for the direct imaging of mercury uptake through the food chain. Our results suggest that ionic mercury remains available for extended periods of time and can rapidly contaminate surface as well as embedded tissue cells.