Taxonomic composition of the particle-attached and free-living bacterial assemblages in the Northwest Mediterranean Sea analyzed by pyrosequencing of the 16S rRNA

12 pages, 8 figures, 2 tablesFree-living (FL) and particle-attached (PA) bacterial assemblages in the Northwest Mediterranean Sea were studied using pyrosequencing data of the 16S rRNA. We have described and compared the richness, the distribution of Operational Taxonomic Units (OTUs) within the two fractions, the spatial distribution, and the taxonomic composition of FL and PA bacterial assemblages. The number of OTUs in the present work was two orders of magnitude higher than in previous studies. Only 25% of the total OTUs were common to both fractions, whereas 49% OTUs were exclusive to the PA fraction and 26% to the FL fraction. The OTUs exclusively present in PA or FL assemblages were very low in abundance (6% of total abundance). Detection of the rare OTUs revealed the larger richness of PA bacteria that was hidden in previous studies. Alpha-Proteobacteria dominated the FL bacterial assemblage and gamma-Proteobacteria dominated the PA fraction. Bacteroidetes were important in the PA fraction mainly at the coast. The high number of sequences in this study detected additional phyla from the PA fraction, such as Actinobacteria, Firmicutes, and VerrucomicrobiaThe Modivus cruise was supported by the Spanish MICINN project MODIVUS (CTM2005-04795/MAR). We thank the captain and the crew of the RV García del Cid. We thank V. Balagué for help with sampling and P. J. Wallhead for statistical advice. We thank two anonymous reviewers for their valuable comments. Pyrosequencing was carried out at the Josephine Bay Paul Center, Marine Biological Laboratory, Woods Hole, MA, U.S.A., under the auspices of the International Census of Marine Microbes (ICoMM) via a grant from the Keck Foundation. Initial analysis of the data was carried out at the VAMPS portal, supported by the Josephine Bay Paul -Center and the Alfred P. Sloan Foundation. Additional work was supported by Spanish MICINN grant MarineGems (CTM2010-20361). B. G. C. was supported by Juan de la Cierva contract from the Spanish “Ministerio de Ciencia e Innovación”. B. F.-G. was supported by a predoctoral I3P Grant from CSIC. This is a contribution to the International Census of Marine Microbes (ICoMM)Peer reviewe