"The recent discovery of a large number of low abundance bacterial phylotypes, coined the ‘rare biosphere’, coexisting next to a few abundant phylotypes raises a number of fundamental questions concerning their role in the microbial food web and their functioning in the biogeochemical cycles, mainly driven by prokaryotes in the ocean. HERA addresses some of these basic questions concerning the role of the ‘rare biosphere’ in coastal European waters using a set of different approaches and recent technological advances. The first approach is to use fractionation of the complex bacterial community by flow cytometry and high speed cell sorting and cell sorting by capillary electrophoresis, the latter based on the specific surface charge of bacteria. This fractionated community will then be subjected to PCR and two fingerprinting techniques, T-RFLP and ARISA. The other approach, to obtain more detailed information on the phylogeny of the ‘rare biosphere’ vs. abundant phylotypes is based on cell sorting of individual populations, enumerating their abundance and using highly multiple displacement amplification (MDA) and subsequently, cloning and sequencing. With this approach, the phylogeny and the abundance of the individual components of the ‘rare biosphere’ will be compared with that of the abundant phylotypes. Leucine incorporation in bacteria and subsequent fractionation will allow comparing the cell-specific growth rates of abundant vs. rare phylotypes. A similar approach is used to determine the phylotype-specific ectoenzymatic activity. Taken together, the combination of approaches will allow us answering some of the fundamental but unsolved questions on the role of the rare biosphere in the microbial food web and in the biogeochemical cycles in coastal seas. The study will be performed over a seasonal cycle hence, we will be able to resolve the enigma whether members of the ‘rare biosphere’ can attain high or low abundance shaping to the organic matter cycling."