Biogeochemical cycle of iron : distribution and speciation in the North Atlantic Ocean (GA01) and the Southern Ocean (GIpr05) (GEOTRACES)

It is now recognized that iron (Fe) availability dictates the efficiency of the global biological carbon pump such that any perturbation of Fe sources will lead to changes in the carbon cycles with consequences on both other major nutrient cycles and the climate system, controlling about 50% of the worldwide ocean primary production. However, the underlying processes themselves that affect the pathways releasing and trapping Fe, and the relative predominance of Fe sources among the different ocean basins are still poorly constrained. More importantly, the extent to which both the chemical and the physical speciation of Fe are available and accessible for marine organisms, once it enters the ocean, remains uncertain. The reactivity of Fe within the marine environment will depend on its redox and complexation state, with DFe generally considered the most bioavailable form for phytoplankton and Fe-binding organic ligands likely increasing the residence time of Fe that enables enhanced DFe concentrations way above its inorganic solubility in seawater (c.a. 10 pmol L\(^{-1}\)). In this context and as part of the international GEOTRACES program, this thesis aims at improving our knowledge on Fe biogeochemical cycle in the ocean and its interactions with the phytoplankton community structure to better constrain the bioavailable forms of Fe. The objectives of this thesis revolve around three scientific questions: 1) What are the distributions, sources, and sinks of dissolved iron? 2) What is the link between the phytoplankton community structure and dissolved iron concentrations? 3) How the organic speciation of dissolved iron affects its concentrations and bioavailability for the phytoplankton community? These three questions were investigated through two contrasted areas: the North Atlantic Ocean (GEOVIDE, GA01 GEOTRACES voyage, PIs G. Sarthou and P. Lherminier) and the Southern Ocean (HEOBI, GIpr05 GEOTRACES voyage, PIs A. Bowie, T. Trull, Z. Chase) the former being occasionally seasonally depleted in Fe, the latter permanently. The distribution of DFe in the North Atlantic revealed the importance of external sources such as meteoric water in the subpolar gyre close to the Newfoundland and Greenland margins and the Tagus river inputs above the Iberian Margin. The most striking feature was the increasing DFe concentrations observed along the flow path of the Labrador Sea Water, which were likely explained by a combination of two processes: on one side, the dissolution of Newfoundland sediments and, on the other side, the potential bacteria-mediated Fe-binding organic ligand production. Pigment samples were also collected during this voyage and ran in the CHEMTAX model to estimate the composition of phytoplankton classes. They were further used to assess potential limitation of Fe, together with the main nutrients. The goal of the HEOBI Southern Ocean voyage was to study the impact of submarine volcanoes on primary production in waters close to Heard and McDonald Islands (central Kerguelen Plateau). More specifically for this thesis, emphasis was given to Fe-binding organic ligands and biological link within the B-transect and to the size partitioning of both DFe and dissolved Fe-binding ligands