The response of heterotrophic bacteria (Bacteria and Archaea) to the spring phytoplankton bloom that occurs annually above the Kerguelen Plateau (Southern Ocean) due to natural iron fertilization was investigated during the KErguelen Ocean and Plateau compared Study (KEOPS) cruise in January–February 2005. In surface waters (upper 100 m) in the core of the phytoplankton bloom, heterotrophic bacteria were, on an average, 3-fold more abundant and revealed rates of production ([3H] leucine incorporation) and respiration (<0.8 μm size fraction) that exceeded those in surrounding high-nutrient low-chlorophyll (HNLC) waters by factors of 6 and 5, respectively. These differences in bacterial metabolic activities were attributable to high-nucleic-a...