The project KEOPS studies the impact of natural iron fertilization of the Southern Ocean on the functioning of the ecosystem and on the biogeochemical cycles. In this context the region around Kerguelen is an exceptional natural laboratory where in situ observations and experimentations could be conducted both in iron fertilized and non-fertilized regions. The main objectives are (i) to identify and quantify the sources and the pathways for iron fertilization, (ii) to study and quantify the impact of iron fertilization on the biogeochemical processes occurring in the water column and at the interfaces with air and sediment, (iii) to study the impact of iron fertilization on the structure of the ecosystem, from viruses to zooplankton, and to investigate how changes in biology affect biogeochemical fluxes. This special issue will gather publications of the results of the second oceanographic campaign (KEOPS2) that took place in October-November 2011. It will include comparisons with the results of the first campaign (January-February 2005) and with other previous natural or artificial iron fertilization experiments in the Southern Ocean.