Seasonal changes in the taxonomic composition of bacterioplankton in a coastal oligotrophic system

The succession of taxa within the bacterioplankton assemblage was followed over a whole seasonal cycle in Blanes Bay, an oligotrophic coastal system (average chl a: 0.5 mug l(-1)) in the NW Mediterranean Sea. Phylogenetic composition of the bacterioplankton was studied by means of DGGE (denaturing gradient gel electrophoresis) and sequencing of predominant bands. Analysis of DGGE fingerprints showed gradual changes in the dominant bacterial members over the sampling period. The time scale during which a single population appeared in significant amounts in the system ranged between weeks and months. Most of the DGGE bands sequenced showed high similarities to sequences of uncultured marine bacteria. The bacterial assemblage appeared to be dominated by members of alpha-Proteobacteria (mostly from the Roseobacter clade), Cytophaga-Flavobacteria-Bacteroides (CFB) and cyanobacteria throughout the year. Two bands related to prasinophyte chloroplasts were detected, whereas no bands related to SAR11 (alpha-Proteobacteria) or SAR86 (gamma-Proteobacteria) clusters were found. Contrasting with the relative stability of broad phylogenetic groups, examination of bands belonging to alpha-Proteobacteria, CFB and cyanobacteria revealed a substitution of closely related phylotypes during the seasonal cycle within each one of these 3 groups. Overall, the taxonomic composition of the bacterioplankton in this coastal marine system appeared to be rather stable in time, showing gradual changes throughout the year.