A previous study of microbial diversity in the Southern Ocean during the Keops1 cruise showed an important response of the bacterial community to the fe-induced phytoplankton bloom. Results from bottle incubations indicated that the bacteria were responding indirectly to Fe fertilization through their response to phytoplankton-derived organic matter. Results from our diversity study and from single-cell probing approaches showed a change in the specific bacterial groups during the decline of the bloom and a decrease in their relative activity. The Keops2 cruise offered an important opportunity to sample at the beginning of the phytoplankton bloom at the same Keops1 station that was dominated by diatoms, but also to sample at additional bloom stations with contrasting phytoplankton compositions. The major aims of this study were to: i) determine if the bacterial groups present in the early stages of the bloom were the same as those which showed high activity in the peak of the Keops1 bloom, ii) determine if the same bacterial groups were reoccurring at the same bloom and HNLC stations between Keops1 and Keops2, iii) determine in parallel the phytoplankton diversity and activity using molecular approaches coupled with traditional taxonomy and iv) attempt to link bacterial and phytoplankton diversity and activity. To achieve these objectives, heterotrophic bacteria and phytoplankton diversity and activity were assessed by next generation sequencing of 16S rRNA gene and 16S cDNA fragments for bacteria, and rbcL gene and rbcL cDNA fragments for phytoplankton from depths samples at contrasting bloom stations and the reference HNLC station. Since there is now a large body of research showing the complex interactions between heterotrophic bacteria and phytoplankton with often the attachment of the former, phytoplankton and bacterial community diversity was assessed in the particulate fractions 0.8-3 µm and 3-10 µm.