Seasonal dynamics of unicellular diazotrophs were investigated in the upstream Kuroshio and the northern South China Sea (SCS). Unicellular diazotrophs had been postulated as an important N2-fixing contributor for the phenomenon of N* in the SCS where abundances of filamentous Trichodesmium and Richelia were scarced. Samples were collected during four cruises between August 2008 and August 2009 in summer (CR1310 and CR910), winter (CR886), and late spring (CR899), respectively. Sampling stations located between 21¢XN-22¢XN and 116¢XE-122¢XE in the upstream Kuroshio off southeast Taiwan and covering the shelf and basin waters of the northern SCS. The abundance of the unicellular diazotrophs was determined using whole-cell immunocytochemical method in which antibody of nitrogenase was used as the probe. Cells containing nitrogenase can be visualized and counted after the antigen-antibody reaction under microscope. Unicellular diazotrophs were classified to four types according to their sizes and shapes. For diameters of those with 1-3 £gm and in coccoid shape are called 1-3 £gm C, diameters of 1-3 £gm and in rod shape are called 1-3 £gm R, diameters of >3-10 £gm and in coccoid shape are called >3 £gm C, and diameters of >3-10 £gm and in rod shape are called >3 £gm R. Surface abundance of the unicellular diazotrophs was highest in winter in both the Kuroshio and the SCS, followed by summer, and was least in late spring. Among four cell types, 1-3 £gm C usually was the most abundant group, followed by 1-3 £gm R and >3 £gm R, and was least for the group of >3 £gm C. The abundances between groups of 1-3 £gm C and 1-3 £gm R were positively correlated. Likewise, the abundances between >3 £gm C and >3 £gm R were positively correlated. However, the total abundance of small cells (1-3 £gm C+R) was not significantly related to the large cells (>3 £gm C+R). During summer and late spring, the abundance of unicellular diazotrophs in the SCS was 1.3-2 times of that in the Kuroshio. However, in winter the abundance in the Kuroshio was 1.2 times of that in the SCS. Surface water temperature was found negatively correlated to the abundance of 1-3 £gm C, >3 £gm C, >3 £gm R, and large cells (>3 £gm C+R), respectively. Significant correlations among surface water temperature and surface chlorophyll a, [NO2+NO3], SRP and N:P ratio implicated that the dynamics of cell abundances could be attributed to the correlated ecological variables of surface water temperature. The dynamics for the abundances of >3 £gm C, >3 £gm R, and large cells (>3 £gm C+R) were suggested to relate with the fluctuation of SRP concentration. Unicellular diazotrophs accounted for 60-90 % of total unicellular cells in terms of cell number. Vertical distributions of unicellular diazotrophs in the Kuroshio and the SCS were in similar trends, with maximum abundance in deep water during summer and late spring, and on surface water during winter.

Ce travail de thèse a pour but d'étudier la fixation d'azote marine dans les environnements riches mais déficitaires en azote (N), comparé au phosphore (P) dans un rapport N:P<16, grâce à l'utilisation d'approches complémentaire en culture in vitro et sur le terrain in situ. La première partie de ce travail a consisté à évaluer la réponse de la fixation d'azote de la cyanobactérie unicellulaire diazotrophe Crocosphaera watsonii face à des concentrations micromolaires en azote inorganique dissous (DIN) supposées inhiber l'activité de fixation d'azote : - suite à un apport sporadique, ou, - après une longue période d'acclimatation. Les résultats de ces études n'ont pas permis d'observer une inhibition des activités de fixation d'azote de cet organisme, laissant supposer que ce processus pourrait être actif dans une zone de l'Océan ayant ces même caractéristiques biogéochimiques : le Sud Est Tropical de l'Océan Pacifique (ETSP). En effet, cette zone est une des trois plus grandes zones de minimum d'oxygène (OMZ) de l'Océan et d'intenses processus de pertes de N (dénitrification et anammox) y ont lieu, résultant en un déficit de N par rapport au P. Des études présumaient que des processus inverses, de gains de N par la fixation d'azote, pourraient y être actifs mais aucune mesure à l'échelle du bassin n'y avait été faite car la fixation d'azote n'était supposée se produire que dans les environnements oligotrophes, comme les gyres subtropicaux. Dans le cadre d'un projet international, des missions océanographiques ont pu avoir lieu dans cette zone en Février 2010 pendant un évènement El Niño et en Mars-Avril 2011 pendant un évènement La Niña. / The objectif of these thesis was to study dinitrogen fixation in marine environments rich but deficient of nitrogen (N) compared to phosphorus (P) in a ratio N:P<16, by using complementary approaches in culture in vitro and in the field in situ. The first part of this work was to evaluate the response of nitrogen-fixing unicellular Cyanobacteria Crocosphaera watsonii faced with micromolar concentrations of dissolved inorganic nitrogen (DIN) supposed to inhibit nitrogen fixation activity : - after sporadic input, or, - after a long period of acclimatization. The results of these studies have failed to observe the inhibition of nitrogen fixation activities of this organism, suggesting that this process could be active in an area of the Ocean with these same biogeochemical characteristics : the Eastern Tropical South Pacific (ETSP). Indeed, this area is one of the three largest oxygen minimum zones (OMZ) of the Ocean, where intense processes of N losses (denitrification and anammox) took place, resulting in a deficit of N compared to P. Studies assumed that the inverse process, gain of N by nitrogen fixation, could be active in the ETSP but no measurements across the basin have been performed because nitrogen fixation was assumed to occur only in oligotrophic environments, such as the subtropical gyres. In the framework of an international project, cruises took place in this area in February 2010 during a El Niño event and in March-April 2011 during a La Niña event. Results of these two cruises have confirmed that nitrogen fixation was unexpectedly active with an intensity comparable to those reported in oligotrophic areas.