One of the first comparisons of a natural iron fertilized bloom with a high-nutrient low-chlorophyll (HNLC) site was undertaken during the Kerguelen ocean and plateau compared study (KEOPS) cruise. To understand better the bacteria-phytoplankton relationship in the context of natural iron fertilization, bacterial diversity and activity was investigated in the bloom and in the adjacent HNLC region by 16S rDNA clone libraries and by single strand conformation polymorphism (SSCP) analysis. Both libraries were dominated by Alphaproteobacteria, Gammaproteobacteria and the Cytophaga-Flavobacteria-Bacteroides group. Cluster analysis at 99% sequence similarity yielded several microdiverse clusters and revealed striking differences between the two libraries. In the bloom, the dominant operational taxonomic units (OTUs) were the Roseobacter NAC11-7 cluster, SAR92 and a Cytophaga-Flavobacteria-Bacteroides cluster related to the agg58 group, whereas in the HNLC region, SAR11, Roseobacter RCA and Polaribacter dominated. SSCP analysis of 16S rDNA and 16S rRNA revealed contrasting dynamics of three different Roseobacter OTUs. Roseobacter NAC11-7 and NAC11-6 had higher relative abundances and activities in the bloom compared with the HNLC site and NAC11-6 was only detected at the decline of the bloom concomitant with a shift in phytoplankton composi tion. In contrast, Roseobacter RCA was relatively abundant and active both inside and outside of the bloom. These results suggest that the different OTUs within the Roseobacter group represent functional groups that each play an important role in the cycling of carbon.