We studied the composition of free-living and particle-associated bacterial communities during the course of the phytoplankton succession in spring and early summer in the Wadden Sea, a tidal flat ecosystem in the southern North Sea. We applied the denaturing gradient gel electrophoresis (DGGE) approach based on PCR-amplified 16S rRNA gene fragments and, in addition to Bacteriaspecific primers, used primer sets targeting Alphaproteobacteria, the Roseobacter-clade and the Bacteroidetes phylum. Even though the application of the primer sets targeting Bacteria and Alphaproteobacteria detected some changes, they were most pronounced with those targeting the Roseobacter-clade and Bacteroidetes. The changes were supported by a correspondence analysis, which showed a statistically significant correlation of the DGGE banding patterns of the Roseobacter-specific PCR with the composition of the phytoplankton (p = 0.03). This indicates that changes in the phytoplankton composition in this habitat are not reflected by changes in the most abundant or most readily amplifiable phylotypes. The findings rather suggest that few, specialized, heterotrophic bacteria are most responsive to the organic matter supplied by distinct phytoplankton communities and that the main part of organic matter in the Wadden Sea is utilized by generalists. Sequence analyses of excised bands revealed a high diversity for the Bacteria-and Bacteroidetes-targeted approaches. The bacterial community detected by the primer set targeting Alphaproteobacteria, however, was mainly composed of bacteria affiliated to the Roseobacter-clade.KEY WORDS: Free-living bacteria · Particle-attached bacteria · Bacteroidetes · Roseobacter · Phytoplankton · Denaturing gradient gel electrophoresis · Wadden Sea Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 48: [47][48][49][50][51][52][53][54][55][56][57][58][59][60] 2007 tions (LeBaron et al. 1999, Riemann et al. 2000, Schäfer et al. 2001, Pinhassi et al. 2004, Abell & Bowman 2005. The specific organic matter profile of various algae appears also to be an important selection factor for distinct bacterial communities and populations evolving in the phycosphere of algae (Grossart 1999, Grossart et al. 2005. In fact, Alphaproteobacteria, in particular the Roseobacter-clade, and the Sphingobacteria/Flavobacteria group appear to be most responsive to inputs of phytoplankton-born DOC and organic sulfur compounds (Riemann et al. 2000, Fandino et al. 2001, Schäfer et al. 2001, Moran et al. 2003, Pinhassi et al. 2004, Grossart et al. 2005.It is also well established that the community composition of particle-associated (PA) bacteria differs from that of free-living (FL) bacteria. Several studies have shown that Sphingobacteria and Flavobacteria preferentially colonize particles whereas Alpha-and Gammaproteobacteria mainly dwell in free-living marine bacterial communities (Fandino et al. 2001, Simon et al. 2002, Grossart et al. 2005. Our knowledge on the development and su...