Congratulations to Caroline Lory (CYBELE) who defended her thesis on Tuesday 21 June 2022

On the following topic:

"Role of iron in controlling atmospheric nitrogen fixation in the southwestern tropical Pacific Ocean"


"Role of iron in the control of atmospheric nitrogen fixation in the Western

Tropical South Pacific Ocean".

Biological nitrogen (N2) fixation by diazotrophs supports new primary production and carbon export in ~50% of the global ocean. The nitrogenase enzyme that catalyzes N2 fixation places a high demand on iron (Fe), whose low concentrations in the ocean often limit the growth of these organisms. This thesis work focuses on the interactions between diazotrophs and Fe in the tropical Southwest Pacific Ocean (WTSP), an area accounting for ~20% of global diazotrophy. The objectives of this thesis are:

1) to identify key environmental factors that structure the activity and biogeographic distribution of diazotrophs in this region,

2) to quantify in situ the Fe demand of diazotrophs, and to determine their contribution to Fe uptake relative to the rest of the microbial community

3) to evaluate the bioavailability of Fe complexed to extracellular polymeric substances (EPS) produced by a diazotroph (Crocosphaera) compared to Fe complexed to other ligands and to inorganic Fe.

(1) The high N2 fixation rates measured during the TONGA mission confirm that the WTSP is a diazotrophic hot spot, exhibiting the highest rates at the Tonga volcanic arc. Statistical analysis of data from previous missions, shows that Fe is the main parameter that structures the N2 fixation activity between the South Pacific gyre (east of the arc) and the WTSP (west of the arc). While UCYN-A dominate the diazotrophic community in early austral summer, the volcanic arc area is an ecological niche for Trichodesmium. Statistical relationships between metals and diazotroph abundance suggest that in addition to Fe, other trace metals may influence diazotrophy in this region.

(2) Fe demand of major diazotrophs (Trichodesmium, Crocosphaera) and non-diazotrophic organisms (Synechococcus, Prochlorococcus, picoeukaryotes, and heterotrophic bacteria) was quantified by a new group-specific approach based on coupled in situ measurements of 55Fe uptake and cell sorting by flow cytometry. The contribution of diazotrophs to total biological Fe uptake is significant (~30%), although these organisms are in the minority, but lower than that of picoplankton (53%, fraction 0.2-2µm), of which Prochlorococcus and heterotrophic bacteria are the main contributors in this study. Fe uptake rates and resulting quotas are higher for Trichodesmium than for Crocosphaera, resulting in a distinct biogeographic distribution of these two phylotypes in the WTSP. This group-specific approach improves our understanding of the ecophysiological role of diazotrophs and other functional groups in the ocean.

(3) We demonstrate that EPS produced by Crocosphaera are weak ligands for Fe. By measuring the uptake of Fe previously complexed with intermediate (a humic substance) and strong (a siderophore) ligands, we show that the EPS produced by Crocosphaera provide it with highly bioavailable Fe, uptake at higher rates but close to those measured for inorganic Fe and humic substances, and significantly higher than for the siderophore. Crocosphaera produces large amounts of EPS compared to other plankton functional groups, probably contributing to its ecological success in Fe-depleted waters. These EPS provide highly bioavailable Fe, providing an additional facet of the role of diazotrophs like Crocosphaera in biogeochemical cycles.

Keywords: N2 fixation, iron, diazotroph, hydrothermal micronutrients, carbon sequestration, biogeochemistry