"Recent findings concerning the role of inland waters in global carbon cycling is currently having a major impact of the view of the global carbon cycle. These findings highlight inland waters - such as streams, rivers and lakes - as major sites of carbon cycling, implying that they must be considered in the context of climate change. Microbial degradation of organic carbon is a process that is central to carbon cycling in all ecosystems. In soils, microbial degradation of recalcitrant carbon is often controlled by the availability of labile carbon sources. This is linked to the priming effect (PE). Mounting evidence suggests that PE is also important in aquatic ecosystems but it has yet to be explicitly addressed. Biofilms are vital components of aquatic ecosystems. In stream biofilms, heterotrophic bacteria and algae coexist in close proximity, exposing the bacteria to both recalcitrant organic carbon of terrestrial origin and labile organic carbon from the algae. This could make stream biofilms hotspots for PE. In PRIMA, I propose an innovative effort cutting across aquatic and terrestrial ecosystems, spanning single-cell to ecosystem scales, and combining methods from biogeochemistry and molecular microbiology to study PE in stream biofilms. Carbon flux in stream biofilm microcosms and in ecosystem scale stream mesocosms will be measured to quantify PE and its implications for carbon cycling in streams. The mechanisms of PE will be addressed on single-cell and community scales using cutting edge methods, such as NanoSIMS and 454-sequencing. I am an experienced researcher trained in Norway and Sweden. In PRIMA, I seek to combine my existing skills with the unique expertise and facilities of Prof. Tom J. Battin at the University of Vienna. The many conceptual and methodological training objectives of PRIMA, as well as its outstanding scientific quality, will strengthen my scientific skills and will enable me to reach my goals as an independent researcher."