Chenopodium quinoa Willd., an ancient Andean pseudocereal almost exclusively consumed in South America, jumped onto the global stage when Western cultures noted quinoa's advantageous nutritional profile. Quinoa seed's high protein content, nutritionally balanced amino acid profile, low glycemic index, and high fiber, vitamin, and mineral content, make it a highly sought-after 'superfood'. Pitseed goosefoot (C. berlandieri Moq.), a closely related North American species sharing quinoa's genome composition (AABB), grows across the North American continent, inhabiting diverse environments including the saline coastal soils of the Gulf of Texas and the drought-prone regions of the Southwest. Quinoa and pitseed goosefoot, along with South American avian goosefoot (C. hircinum Schrad.), make up the Allotetraploid Goosefoot Complex (ATGC). We hypothesize that an ancient hybridization event between A- and B-genome diploids, with a subsequent whole-genome duplication, gave rise to the common ancestor of the ATGC. Prior data indicate that allopolyploidization most likely occurred within North America, with long-range dispersal of the ATGC to South America. We have sequenced the genome of the North American AA-genome diploid C. watsonii and identified via DNA marker analyses the closest extant species to the AA-genome diploid ancestor of the ATGC from among a panel of 41 AA-genome diploid resequenced accessions, encompassing 30 putative AA-genome diploid species, from North and South America. We also present evidence for reciprocal long-range dispersal of Chenopodium diploids between North and South America.

La fixation de N2, ou diazotrophie, est un processus biogéochimique majeur en raison de son apport en azote nouveau dans la couche de surface de l’océan. Cependant ses facteurs de contrôle restent mal connus. Le fer, dont les concentrations de surface sont faibles, est un élément potentiellement limitant de la fixation de N2 du fait du contenu en fer important de la nitrogénase. En raison de leur découverte récente, peu d’études ont été menées sur les cyanobactéries diazotrophes unicellulaires (UCYN) pouvant être responsables de ~50% de la fixation de N2 à l’échelle globale. Des expériences en culture ont permis de caractériser et quantifier pour la première fois la réponse d’une UCYN, Crocosphaera watsonii, face à la limitation en fer. En condition de limitation en fer, il a été observé une réduction de la croissance et des taux de fixation de N2 ainsi qu’une stratégie d’adaptation des cellules avec une diminution de leur volume. La stimulation de la croissance et de l’activité de C. watsonii cultivées en condition de limitation en fer suite à l’ajout d’une pluie saharienne artificielle a permis de mettre en évidence qu’une partie au moins du fer issu de poussières désertiques est biodisponible. En Atlantique subtropical Nord, où nous avons déterminé une forte contribution de la fixation de N2 à la production nouvelle, nous avons observé une limitation de la fixation de N2 et de la production primaire principalement par les phosphates et mis en évidence le rôle des métaux traces dans le contrôle de la fixation de N2. Un ajout de pluie saharienne a permis de stimuler systématiquement la fixation de N2 et la production primaire. / Despite the biogeochemical importance of N2 fixation, which represents the largest source of newly-Fixed nitrogen to the open ocean, some uncertainties remain about its controlling factors. Iron (Fe) is widely suspected as a key controlling factor due to the high Fe content of the nitrogenase complex and to its low concentration in oceanic surface seawaters. N2 fixation rates associated with unicellular N2 fixing cyanobacteria (UCYN) were estimated to be ~50% of the total N2 fixation at global scale, but as they have been recently discovered few studies have been conducted on these organisms. We performed culture experiments in order to quantify for the first time the response of an UCYN, Crocosphaera watsonii, to Fe limitation. Reduction of ambient Fe concentration led to significant decreases in growth rate and N2 fixation rates per cell and we observed an adaptive strategy to Fe limitation with a cell volume reduction. Then, the enhancement of growth and activity of C. watsonii under Fe limitation condition after artificial Saharan rain addition highlighted that at least a part of the Fe released by the dust is bioavailable. In subtropical North Atlantic, an important contribution of N2 fixation to new production was observed and we showed that primary production and N2 fixation were globally P-Limited. We revealed that trace metals play a key role in controlling N2 fixation in this area. Saharan rain addition stimulated N2 fixation, presumably by supplying these nutrients. All these results contribute to our knowledge of the control of oceanic N2 fixation and provide new insight about interactions between Fe, nitrogen and carbon biogeochemical cycles.