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In this paper we obtain precise asymptotics for certain families of graphs, namely circulant graphs and degenerating discrete tori. The asymptotics contain interesting constants from number theory among which some can be interpreted as corresponding values for continuous limiting objects. We answer one question formulated in a paper from Atajan, Yong, and Inaba in [1] and formulate a conjecture in relation to the paper from Zhang, Yong, and Golin [23]. A crucial ingredient in the proof is to use the matrix tree theorem and express the combinatorial Laplacian determinant in terms of Bessel functions. A non-standard Poisson summation formula and limiting properties of theta functions are then used to evaluate the asymptotics.

In coastal areas and estuaries, such as those encountered in the western part of France (Brittany region), the recycling of carbon and nutrients from sediments can participate in the development of micro and macro-algal blooms with harmful consequences for these ecosystems. One of the main processes controlling this recycling is the microbial mineralization of sedimentary organic matter (SOM). Mineralization is controlled by the origin, quantity and accessibility of the SOM, three factors whose relative importance remain, however, poorly quantified, mainly due to the great diversity of OM sources in coastal areas. The first objective of the present work was to assess the variability of the SOM origin at the regional scale representative of the complexity of the sources likely to be involved. The second objective was to determine the link between the SOM origin and its biodegradability, and how the OM sources can drive nutrient dynamics at the sediment-water interface. To this end, a broad sediment sampling campaign was carried out on Brittany mudflats, particularly affected by the eutrophication, during the spring period. A total of 200 samples were collected at 45 sites. They were characterized by their porosity and grain-size, as well as their chemical composition through elemental, isotopic and molecular biomarker analysis. A wide range of OM sources were identified in the sediments, including both natural (bacteria, algae, macrophytes, terrestrial plants), and anthropogenic (combustion products, crude oil, petroleum products – e.g. from the processing of crude oil at refineries- and fecal matter) sources. Sediment slurry incubations were carried out to determine the spatial variability of potential mineralization rates under oxic conditions. In addition, the measurements of NH4+ and PO4 fluxes at the sediment-water interface were made from sediment core incubations under realistic redox conditions of sediment. The physical and chemical sedimentary characteristics explained 58 % of the variability of mineralization rates under oxic conditions, with a negligible independent effect of the SOM origin (3 %). Conversely, under insitu redox conditions, the prevalent role of SOM origin over quantity/accessibility on the sediment biodegradability was highlighted with a significant effect 5 and 1.5 fold higher on the PO4 and NH4+ fluxes respectively. The anthropogenic inputs from the watershed to the coastal sediment, through agricultural runoff and/or sewages discharge, seem to significantly drive the nutrient dynamics at the sediment-water interface. Higher values of NH4+ and PO4 fluxes were measured for the sediment with a chemical composition impacted by human activities.

National audience

FEUTURE Online Paper No. 14 This FEUTURE paper examines global (Russian and US press) and regional (Egyptian and Georgian press) perspectives on EU-Turkey relations since 1999 with respect to identity and culture. Using the Critical Discourse Analysis methodology, the research traces the evolution of Turkey and Europe’s identity representations in foreign media, therefore providing an outlook of the way significant Others make sense of the EU-Turkey relationship in the context of Turkey’s EU bid. While the more dynamic (and positive) Egyptian and American press coverage initially contrasted with Georgian and Russian newspapers’ static portrayal of Europe and Turkey’s respective identities as antithetical, the prominence of certain identity markers in recent drivers contributed to shifts in identity representations supporting the degradation of EU-Turkey relations toward conflict., This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 692976. This publication reflects the views only of the authors, and the Commission cannot be held responsible for any use which may be made of the information contained therein.

N2 fixation by the genus Trichodesmium is predicted to support a large proportion of the primary productivity across the oligotrophic oceans, regions that are considered among the largest biomes on Earth. Many of these environments remain poorly sampled, limiting our understanding of Trichodesmium physiological ecology in these critical oligotrophic regions. Trichodesmium colonies, communities that consist of the Trichodesmium host and their associated microbiome, were collected across the oligotrophic western tropical South Pacific (WTSP). These samples were used to assess host clade distribution, host and microbiome metabolic potential, and functional gene expression, with a focus on identifying Trichodesmium physiological ecology in this region. Genes sets related to phosphorus, iron, and phosphorus–iron co-limitation were dynamically expressed across the WTSP transect, suggestive of the importance of these resources in driving Trichodesmium physiological ecology in this region. A gene cassette for phosphonate biosynthesis was detected in Trichodesmium, the expression of which co-varied with the abundance of Trichodesmium Clade III, which was unusually abundant relative to Clade I in this environment. Coincident with the expression of the gene cassette, phosphate reduction to phosphite and low-molecular-weight phosphonate compounds was measured in Trichodesmium colonies. The expression of genes that enable use of such reduced-phosphorus compounds were also measured in both Trichodesmium and the microbiome. Overall, these results highlight physiological strategies employed by consortia in an undersampled region of the oligotrophic WTSP and reveal the molecular mechanisms underlying previously observed high rates of phosphorus reduction in Trichodesmium colonies.

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International audience; The evolution of organic carbon export to the deep ocean, under anthropogenic forcing such as ocean warming and acidification, needs to be investigated in order to evaluate potential positive or negative feedbacks on atmospheric CO 2 concentrations, and therefore on climate. As such, modifications of aggregation processes driven by transparent exopolymer particles (TEP) formation have the potential to affect carbon export. The objectives of this study were to experimentally assess the dynamics of organic matter, after the simulation of a Saharan dust deposition event, through the measurement over one week of TEP abundance and size, and to evaluate the effects of ocean acidification on TEP formation and carbon export following a dust deposition event. Three experiments were performed in the laboratory using 300 L tanks filled with filtered seawater collected in the Mediterranean Sea, during two 'no bloom' periods (spring at the start of the stratification period and autumn at the end of this stratification period) and during the winter bloom period. For each experiment, one of the two tanks was acidified to reach pH conditions slightly below values projected for 2100 (~ 7.6–7.8). In both tanks, a dust deposition event of 10 g m-2 was simulated at the surface. Our results suggest that Saharan dust deposition triggered the abiotic formation of TEP, leading to the formation of organic-mineral aggregates. The amount of particulate organic carbon (POC) exported was proportional to the flux of lithogenic particles to the sediment traps. Depending on the season, the POC flux following artificial dust deposition ranged between 38 and 90 mg m-2 over six experimental days. Such variability is likely linked to the seasonal differences in the quality and quantity of TEP-precursors initially present in seawater. Finally, these export fluxes were not significantly different at the completion of the three experiments between the two pH conditions.

FEUTURE Online Paper No. 11 Since the early 2000s, major transformations in the Southern Mediterranean political landscape have undeniably tested the limits of the EU and Turkey’s foreign policies. To a lesser extent, drivers from the eastern neighbourhood have strained, and in some instances, strengthened relations. Overall, drivers from the southern neighbourhood are likely to lead to pragmatic cooperation, while drivers from the eastern neighbourhood, although less influential, have the potential to lead to conflict as well as cooperation in areas of mutual interest. For the foreseeable future, convergence is clearly off the table, and no driver from the Eastern or Southern neighbourhood is likely to change this., This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 692976. This publication reflects the views only of the authors, and the Commission cannot be held responsible for any use which may be made of the information contained therein.

In this thesis we study the number of spanning trees in some classes of graphs. This is made possible by the famous matrix tree theorem established by Kirchhoff in 1847 which states that the number of spanning trees in a finite graph is given by the product of the non-zero eigenvalues of the combinatorial Laplacian of the graph divided by the number of vertices. We adapt techniques derived by Chinta, Jorgenson and Karlsson in 2010 for d-dimensional discrete tori to circulant graphs with first generator equals to 1 and to d-dimensional degenerating discrete tori. They are degenerating in the sense that d-p sides of the tori are tending to infinity at the same rate while the p other sides tend to infinity sublinearly with respect to the d-p sides. Furthermore, the results on d-dimensional discrete tori enable to derive asymptotics for the number of spanning trees on d-dimensional orthotope square lattices. Other results obtained in this thesis concern closed formulas for the number of spanning trees in directed and non-directed circulant graphs where the generators vary, that is, they linearly depend on the number of vertices.

The objective of this thesis was to assess the biogeochemical effect of two forcings: the ocean acidification and the dust deposition. Firstly, we showed that in the Low Nutrient Low Chlorophyll (LNLC) area the effect of the ocean acidification on the marine cycle of nitrogen (N), phosphorus (P) and iron (Fe) will be likely negligible. The pH decrease of the surface waters depleted in N and P would not affect the speciation of these elements, and the absence of effect on the composition of the plankton community would not lead to a shift in the dynamic of inorganic nutrients. Secondly, the experiments performed during this thesis allowed a better parametrization of the post-depositional processes of atmospheric nutrients in the surface seawater after a Saharan event. The exchanges dissolved/particulate occurred in the short timescale. They were controlled by both the particles dynamic and the nature of the dissolved organic matter (DOM) that can aggregate with the lithogenic particles. Under ocean acidification conditions, while the dissolution of anthropogenic elements adsorbed onto dust, such as the inorganic nitrogen, will not be affected, the dissolution of the crustal elements (P and Fe) could be higher and faster according the formation of organic-dust aggregates. The major role of the aggregation process on the availability of atmospheric nutrients was highlighted by the rapid and large abiotic formation of transparent exopolymeric particles (TEP) following the dust deposition to the surface waters., L'objectif de cette thèse a été d'évaluer l'effet biogéochimique de deux forçages : l'acidification de l'océan et les dépôts de poussières sahariennes. Dans un premier temps, nous avons montré que dans une région dite Low Nutrient Low Chlorophyll (LNLC) l'effet de l'acidification de l'océan sur les cycles marins de l'azote (N), du phosphore (P) et du fer (Fe) sera surement négligeable. L'acidification des eaux de surface appauvries en N et P n'affecterait pas la spéciation de ces éléments, et la composition de la communauté planctonique, n'étant pas significativement modifiée, n'induirait pas de changement dans la dynamique des nutriments inorganiques. Dans un deuxième temps, les expériences menées pendant cette thèse nous ont permis de mieux paramétriser les processus post-dépôts des nutriments atmosphériques à la surface de l'eau de mer lors d'un évènement saharien. Les échanges dissous/particulaire ont lieu sur une courte échelle de temps. Ils sont à la fois contrôlés par la dynamique verticale des particules et la nature de la matière organique dissoute (DOM) pouvant s'agréger sur les particules lithogéniques. Dans des conditions d'acidification de l'océan, alors que la dissolution des éléments anthropiques adsorbés sur les poussières sahariennes, tels que l'azote inorganique, ne sera pas affectée, celle des éléments d'origine crustale (P et Fe) pourrait être accrue ou accélérée selon les agrégats organique-minéral formés. Le rôle majeur du processus d'agrégation dans la mise à disposition des nutriments atmosphériques a été mis en évidence par une rapide et importante formation abiotique de particules exopolymériques transparentes juste après le dépôt de poussières.