Archaea are prokaryotic microorganims that constitute the third phylogenetic domain of life. Up to recently Archaea were thought to be mostly restricted to extreme environments but it has recently been established that the biodiversity and metabolic capabilities of the phylum Crenarchaeota is substantially larger than previously assumed.Recent evidence suggests that ubiquitous pelagic Crenarchaeota play an essential role in biogeochemical cycling in aquatic ecosystems. It is, however, unknown which crenarchaeotal groups are participating, to which extent, and under which environmental conditions.Recent findings suggest that marine and freshwater Crenarchaeota could act as chemoautotrophs and fix inorganic carbon in the dark. The project CRENARC will focus on the possible role of Crenarchaeota in fixing carbon in aquatic ecosystems using a fresh water lake and a coastal marine site as model ecosystems.The proposed research requires a molecular ecological approach using high-resolution technologies. We will perform stable isotopic probing (SIP) to monitor the assimilation of 13C-labelled substrate by Crenarchaeota and we will apply molecular ecological methods to analyse the labelled DNA/RNA. DNA/RNA sequences will be screened for both phylogenetic marker and functional genes.We will also develop metagenomic technologies to clone large labelled crenarchaeotal DNA fragments. Consecutive sequencing and gene screening combined with comparative genomics will increase our understanding of the ecological role and p hylogenetic diversity of Crenarchaeota.It is foreseen that CRENARC will result in a substantially increased understanding of the role of this enigmatic group of aquatic prokaryotes in biogeochemical cycling.