"The bioavailability of iron (Fe) has been shown to limit primary production in up to 50% of the ocean’s waters. As a result of its function in modulating Earth’s climate, the role of Fe supply on the oceanic carbon cycle has received widespread attention over the last two decades. However, much of the research to date has focused new sources Fe, and little attention has been directed toward controls on the supply of recycled Fe. Within the mesopelagic zone (the layer underlying the euphotic zone and extending to 1000 m), heterotrophic bacterial activity results in a dramatic decrease in the organic carbon exported from surface waters. For this depth range of importance for predictions of future changes in the ocean’s carbon cycle, bacterial remineralization and the fate of particulate iron (PFe) are, paradoxically, poorly understood. The proposed project seeks to better understand the functioning of the ocean’s Fe cycle through investigations of the bacterial remineralization of PFe within the mesopelagic zone.Using an innovative technical approach, the first part of the project will consist of the simultaneous and in situ measuring of bacterial remineralization rates and dissolved iron (DFe) release within the mesopelagic zone. Since the speciation of released DFe has major consequences on the bioavailability of upwelled fluxes of Fe, the second part of the project will consist of determining, using complementary in vitro experiments, the concurrent release of iron-binding ligands, as well as their size class (soluble or colloidal), during bacterial particle remineralization.This experimental work will be performed at different times of the year within two oceanic regions (the Southern Ocean and the Mediterranean Sea) that are characterized by contrasting Fe supply mechanisms in order to determine whether the fate of PFe at depth is primarily driven by particle properties that are imprinted within the surface ocean or in mesopelagic particle transformations."