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Sargassum leachate initially inhibits then stimulates phytoplankton growth in coral reef waters
No full textInternational audienceMassive Sargassum accumulations in the Atlantic form the Great Atlantic Sargassum Belt, which fuels recurrent coastal inundation events that disrupt ecosystems by reducing light availability for phytoplankton and enriching nearshore waters with nutrients released during decomposition. This study examined the short-term effects of decaying Sargassum leachate on phytoplankton productivity and growth in Guadeloupe’s coral reef ecosystems using several photosynthesis measurements based on fluorescence and 13C. A range of leachate concentrations were applied, under controlled conditions, to a natural phytoplankton community. The study revealed two major effects of Sargassum leachate on phytoplankton. First, leachate at concentrations ≥1 % initially inhibited photosynthesis and growth up to Day 2, likely due to allelopathic substances such as polyphenols. This was evidenced by near-zero Fv/Fm values and disrupted electron transport in PSII. By Day 3, these inhibitory effectsdiminished, suggesting degradation of labile inhibitory compounds or community modification. Second, from Day 3 onward, leachate addition stimulated phytoplankton growth, as seen in increased biomass, primary production (JVIImax), and carbon fixation. This nutrient-driven response was accompanied by reduced C/Chl a ratios and improved photosynthetic efficiency (lower Фe,C values), indicating alleviation of nutrient limitations, particularly nitrogen and phosphorus. These findings underscore the dual role of Sargassum leachate: an initial suppressive impact through allelopathy and a subsequent nutrient enrichment effect driving phytoplankton blooms. Such dynamics highlight the significant and complex influence of Sargassum strandings, combining ecological stress with nutrient-driven productivity changes
Crystallisation in biomineral mollusc shell studied by 3D Bragg ptychography
No full textInternational audience3D investigation of the crystalline properties of a mollusc shell biomineral evidences a strain–size relationship of the individual coherent domains
Transcriptomic reaction norms highlight metabolic depression as a divergence in phenotypic plasticity between oyster species under ocean acidification
No full textInternational audienceOcean acidification is occurring at a rate unprecedented for millions of years, forcing sessile organisms, such as oysters, to respond in the short term by relying on their phenotypic plasticity. But phenotypic plasticity has limits, tipping points, beyond which species will have to adapt or disappear. These limits could be related to the adaptation of species to different habitat variabilities. Here, we exposed juvenile pearl oysters, Pinctada margaritifera, to a broad pH range and determined the response at the gross physiological, lipidome and transcriptome levels. Thus, we identified its high tolerance with low pH tipping points at pH 7.3–6.8 below which most physiological parameters are impacted. We then compared the transcriptomic reaction norms of the tropical subtidal P. margaritifera with those of an intertidal temperate oyster, Crassostrea gigas, reusing data from a previous study. Despite showing similar tipping points to C. gigas, P. margaritifera exhibits strong mortality and depletion of energy reserves below these tipping points, which is not the case for C. gigas. This divergence relies mainly on the induction of metabolic depression, an adaptation to intertidal habitats in C. gigas but not P. margaritifera. Our method makes it possible to detect divergence in phenotypic plasticity, probably linked to the species' specific life-history strategies related to different habitats, which will determine the survival of species in the face of ongoing global changes. Such an approach is particularly relevant for studying the physiology of species in a world where physiological tipping points will increasingly be exceeded
La règle du forfait de pension s'applique aux fonctionnaires… même lorsqu'ils ne bénéficient pas d'une pension ou d'une allocation temporaire d'invalidité
No full textInternational audienceNote sous CAA Bordeaux, 24 septembre 2024, no 22BX02412 (C+
Late Holocene landscape reconstruction of the lower ‘Ōpūnohu Valley, Moʻorea Island (Society Islands, South Pacific) and its geoarchaeological implications
No full textInternational audienceA multidisciplinary study was conducted in the high volcanic island of Moʻorea (South Pacific, French Polynesia)in order to reconstruct its long-term environmental evolution together with its history of human occupation. TheOpunohu Valley, in particular, records one of the longest and best documented human histories of the islanddating from the first Polynesian (Ma'ohi) occupation, from roughly the 11th to the 18th Cent. CE. In order toreconstruct past landscape dynamics that include both the depositional history of the sediments and the shoreline mobility, we have studied six sedimentary profiles from the foothills and from the modern coastal floodplain. Laboratory work includes loss on ignition measurements, laser granulometry analyses and pollen/NPP identification. Chronostratigraphy was based on a series of five radiocarbon datings performed on charcoals, organic sediment, and coral debris. Palaeoenvironmental results reveal two major phases of detrital input related to deforestation (fires) and changes in land use, first during the Ma'ohi period, and secondly at the onset of the European period. The substantial delivery of these fine-grained sediments into Opunohu Bay has contributed to the formation of a delta since the onset of the 2nd millennium CE. Our results bring to light the contribution of anthropogenic actions and natural factors to explain the specific landscape evolution of the Opunohu coastal plain since the beginning of human settlement on the island. Finally, our work is of archaeological interest since it helps to determine primary settlement of the M¯aʻohi people in the context of a rapidly shifting shoreline
Elasmobranch species richness and assemblage composition in the world’s largest shark sanctuary
No full textInternational audienceIndo-Pacific coral reefs host diverse assemblages of elasmobranchs from small-bodied mesopredators to apex predators that may vary in the amount of time they spend on reefs. Reef sharks and rays as a group are threatened by human activities and are facing widespread population declines, primarily due to fishing. These human factors may affect not only elasmobranch abundance, but also their assemblage composition. Thus, a better understanding of the factors associated with differences in species-specific abundances and assemblage structure across multiple spatial scales in relatively undisturbed systems could enhance the conservation of shark and ray populations on reefs generally. Here, we used baited remote underwater video stations to examine species richness and assemblage composition of elasmobranchs across forereefs in French Polynesia, the world’s largest shark sanctuary. Boosted regression tree models revealed that island group, latitude, and island geomorphology had the greatest effect on elasmobranch species richness. Assemblages at most islands were dominated by blacktip reef sharks Carcharhinus melanop terus and grey reef sharks C. amblyrhynchos , while rays were generally rare, although there was significant spatial variation in elasmobranch assemblage composition. This variation was not associated with human factors, and appears to reflect species interactions and species-specific responses to environmental variation. Further studies on species interactions (facilitation, competition, and predation) among elasmobranchs will provide a better functional understanding of drivers of elasmobranch species composition on individual coral reefs
Contestation des élections des représentants du personnel : la « czabajisation » obéit à un délai raisonnable spécial… mais lequel ?
No full textInternational audienceNote sous TA Melun, 4 juillet 2024, no 2308298 (C+
Modules over orders, conjugacy classes of integral matrices, and abelian varieties over finite fields
No full textInternational audienceAbstract We give an algorithm to compute representatives of the conjugacy classes of semisimple square integral matrices with given minimal and characteristic polynomials. We also give an algorithm to compute the F q -isomorphism classes of abelian varieties over a finite field F q which belong to an isogeny class determined by a characteristic polynomial h of Frobenius when h is ordinary, or q is prime and h has no real roots
The Use of New Biologically Based Technological Development in the Management of Crown-of-Thorns Sea Stars
No full textInternational audienceCrown-of-thorns sea star (CoTS) outbreaks represent one of the biggest threats to scleractinian coral reefs across the Indo-Pacific, and this threat is further exacerbated by ocean warming, cyclones and ocean acidification. There is currently a large collective effort by stakeholders across Indo-Pacific nations to develop technologies to accurately manage (predict, monitor and reduce) these outbreaks. Emerging technologies include: low-cost and high-throughput eDNA profiling of seawater (biosensors) to accurately count the abundance of CoTS in a local area; developing chemosensory attractants and dispersants to herd CoTS for efficient removal; desynchronising CoTS reproduction by altering the timing of CoTS reproductive cues; producing CoTS with lower fecundity through gene editing/contraceptives; using machine learning technologies and robots to automatically detect and cull CoTS; making CoTS more susceptible to diseases through microbiome manipulation to reduce population sizes; strategically planting corals more resilient to CoTS predation, e.g., more venomous corals; and facilitating the predation of CoTS by increasing the numbers of CoTS predators through marine protected area (MPA) designation and habitat restoration. In this chapter, we discuss the emerging technologies and considerations that should be taken into account to effectively manage CoTS outbreaks for future reef resilience
Scleractinian Coral Skeletons as Natural Archives of Global Change: A Cross-Disciplinary Investigation
No full textInternational audienceScleractinian coral skeletons serve as valuable environmental archives, offering key insights into past climate and ocean variability. Understanding the mechanisms underlying coral calcification is essential for reconstructing accurate paleoenvironmental records. However, calcification is a complex process regulated by both biological and environmental factors. This study explores the interplay between coral skeletal geochemistry and physiological processes, focusing on species-specific growth patterns, biomineralization dynamics, and the impact of diagenesis. We highlight the use of elemental ratios (e.g., Mg/Ca, Sr/Ca, Li/Mg) as proxies for environmental reconstructions while addressing biases introduced by skeletal heterogeneity, bioerosion, and secondary mineral precipitation. Additionally, we discuss how physiological traits, such as symbiosis, nutrient uptake, and respiration, influence calcification and may affect the preservation of geochemical archives over time. The integration of high-resolution geochemical analyses with biological insights is of great importance for refining paleoenvironmental proxies and enhancing the accuracy of climate reconstructions. This multidisciplinary approach provides a broader perspective on coral biomineralization, a critical issue for assessing reef resilience in a changing ocean