"Our knowledge on the marine nitrogen cycle has undergone some major revisions very recently with the discovery of novel pathways mediated by thus far unknown and partly unculturable prokaryotes. Among these new drivers of the nitrogen cycle are non-thermophilic Crenarchaea and Euryarchaea. Recently, Crenarchaea have been shown to harbor the amoA gene, responsible for the expression of ammonia monooxygenase, which represents a key enzyme in the nitrification step. The project aims at determining the dynamics in abundance and activity of these Archaea in European coastal seas. Specific emphasis is paid to differentiate between the free-living, planktonic Archaea and those attached to the two major types of suspended particles in coastal areas, lithogenic vs biogenic particles. We will assess the phylogenetic composition of the free-living and particle-attached crenarchaeal and euryarchaeal communities and compare them with those of beta- and gammaproteobacterial communities as these two bacterial groups are also known to harbor nitrifying phylotypes. Conventional fingerprinting techniques using the 16 S rRNA gene and terminal-restriction fragment length polymorphism, cloning and sequencing will be combined with the pyro-sequencing approach, the latter allowing an unsurpassed resolution of the total number of phylotypes present. Quantitative PCR on the 16S rRNA level and on crenarchaeal and beta- and gammaproteobacterial amoA gene level will be used to determine the number of crenarchaea and bacteria putatively oxidizing ammonia. This approach will be combined with fluorescence in situ hybridization and autoradiography to allow insights into the microspatial distribution of Crenarchaea on suspended particles. These molecular approaches to determine the abundance and phylogeny of nitrifiers in ambient water and on suspended particles are complemented by nitrification rate measurements differentiating, for the first time, bacterial from crenarchaeal nitrification using"