Deep-Sea Research I 54 (2007) 1721–1743 Microbial community structure and function in the Levantine Basin of the eastern Mediterranean Tsuneo Tanaka a,b, , Tamar Zohary c , Michael D. Krom d , Cliff S. Law e,f , Paraskevi Pitta g , Stella Psarra g , Fereidoun Rassoulzadegan a , T. Frede Thingstad b , Anastasios Tselepides g , E. Malcolm S. Woodward f , Gro Anita Fonnes Flaten b,1 , Evy Foss Skjoldal b , George Zodiatis h a Laboratoire d’Oce´anographie de Villefranche (LOV), Station Zoologique, Universite´Pierre et Marie Curie (Paris VI)—CNRS, 06234 Villefranche-sur-Mer, France b Marine Microbiology Research Group (MMRG), Department of Biology, University of Bergen, Bergen, Norway c Israel Oceanographic Limnological Research, the Kinneret Limnological Laboratory, Israel d Earth and Biosphere Institute, School of Earth and Environmental Sciences, Leeds University, Leeds, UK e National Institute of Water and Atmospheric Research, Wellington, New Zealand f Plymouth Marine Laboratory, Prospect Place, Plymouth, UK g Hellenic Centre for Marine Research, Oceanography Institute, Heraklion, Crete, Greece h Oceanography Centre, University of Cyprus, Nicosia, Cyprus Received 13 September 2006; received in revised form 29 June 2007; accepted 29 June 2007 Available online 10 July 2007 Abstract During May 2001 and May 2002, the structure and function of the microbial community within and outside the Cyprus quasi-stationary warm-core eddy in the Levantine Basin of the eastern Mediterranean was studied down to the depth of the bathypelagic layer. We present here the detailed description of the microbial food web in one of the most oligotrophic and P-starved marine systems on earth. The isothermal layer was at the depth between 20 and 260/300 m at the core of the eddy, and between 20 and 100/110 m outside. Nitrate and phosphate were found at higher concentration between 100 and 500/800 m outside the eddy compared within the core of the eddy, but the vertical diffusive flux of nitrate and phosphate across the pycnocline was higher within the core of the eddy. There were only minor differences in microbial abundance in the euphotic layers of the two sites. It is suggested that the differences in the areal supply of nutrients to the isothermal layer, between the two sites, resulted in essentially a similar volumetric supply of nutrients to the euphotic layer. This suggests that the results of this study can be applied to describe the microbial food web within the euphotic layer over the larger area of the Levantine Basin, which exhibits ultra-oligotrophic and P-starved conditions. Primary production and abundances of the microbial community were somewhat higher in May 2001 than in May 2002, possibly because of higher nutrient fluxes in the euphotic layer, which are probably the result of deeper winter mixing in 2001, although a later onset of winter mixing or increased dust supply could not be discounted. In the euphotic layer, heterotrophs (bacteria, heterotrophic nanoflagellates (HNF), and ciliates) dominated (60–70%) the microbial carbon biomass. Heterotrophic ciliates were found to be much more abundant in the upper 50 m of the water column, while no consistent pattern was ARTICLE IN PRESS www.elsevier.com/locate/dsri 0967-0637/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2007.06.008 Corresponding author. Laboratoire d’Oce´anographie et de Bioge´ochimie, UMR6535-CNRS, Centre d’Oce´anologie de Marseille, Campus de Luminy-Case 901, 13288 Marseille Cedex 09, France. Tel.: +33 04 91 82 91 07; fax: +33 04 91 82 19 91. E-mail address: tsuneo.tanaka@univmed.fr (T. Tanaka). 1 Present address: Bergen University College, Nyga˚rdsgaten 112, Postboks 7030, N-5020 Bergen, Norway.