Monomeric carbohydrate uptake and structure-function coupling in stream biofilms.

We investigated the uptake of monomeric carbohydrates in stream biofilms under various hydrodynamic environments. Flow velocity and biofilm structure influenced the uptake of glucose (Glc) and arabinose (Ara), 2 carbohydrate monomers differing in bioavailability. While the uptake of Glc increased with flow velocity, the uptake of Ara showed a weaker and inverse relationship with flow velocity. Microautoradiography and confocal-laser-scanning-microscopy of biofilm cryosections revealed patterns of monomer uptake over biofilm depth. Glc was preferentially taken up in the biofilm canopy, and modelled depth gradients suggested that boundary layer thickness controls external Glc mass transfer to the biofilm. In contrast, Ara uptake was uniformly distributed or concentrated in deeper biofilm layers. Overall, Glc uptake was negatively correlated with Ara uptake. We suggest that the spatial separation of monomer uptake involves unidirectional supply flux to biofilms and selective use of carbon sources of differing quality. This ‘metabolic stratification’ can be interpreted as an expression of the long-known carbon catabolite repression phenomenon initially described as ‘diauxie’ by Jacques Monod in 1942.