Coral reefs are important biodiversity hotspots and major ecological reserves. They are also critically important to the many countries living nearby. With global warming, extreme thermal events called Marine Heat Waves (MHWs), have devastating effects on coral reefs, inducing massive bleaching (i.e. symbiosis disruption between corals and their Symbiodiniaceae algae, depriving the coral of its main food source). Bleaching can lead to coral death if the stress persists, unless corals can rely on their heterotrophic nutrient acquisition, by consuming organic matter or planktonic preys. There is evidence that some coral communities, living in mesotrophic reefs (rich in plankton and nutrients) are less sensitive to bleaching. In laboratory, corals supplied with plankton are more resistant to heat stress but fewer studies have been conducted in the field. While MHWs are becoming more frequent and intense, areas rich in plankton and organic matter (hereafter called mesotrophic reefs) can be key to coral survival by allowing corals to obtain external energy sources. They can serve as coral refuge in the face of climate change. BOOST gathers four UMRs (ENTROPIE, LEMAR, MIO, LOMIC) and three international partners, the CSM, Duke University and KAUST on a highly multidisciplinary project merging ecophysiology, biogeochemistry, oceanography and remote sensing. Laboratory and in situ approaches are applied in BOOST to: (1) (a)Determine whether mesotrophic reefs show higher metabolic performances, and (b) whether corals from oligotrophic reefs can adapt to mesotrophic conditions, by measuring in particular their productivity and calcification with innovative equipment using high-frequency sampling, and by transplanting corals from oligo- to mesotrophic reefs and assess their physiological parameters; (2) Confirm, under in situ conditions, that corals from mesotrophic reefs are more resistant to bleaching by performing short-term acute heat stress on corals collected either in meso- or oligotrophic reefs and transplanted from an oligotrophic reef; (3) Assess that coral tissue properties reflect the seawater nutrient properties and enable the determination of coral heterotrophic levels in situ by measuring new heterotrophic markers (bulk isotopic d13C and d15N values, some d15N-compound-specific amino acid values and a fatty acid biomarker (cis-gondoic acid)) calibrated in corals cultivated in laboratory conditions, under different diets; (4) Localize other mesotrophic reefs, where corals may be more resistant to future MHWs, by analyzing satellite images of surface chlorophyll-a around New Caledonia. BOOST will provide new tools to help policymakers and environmental managers decide where to focus their efforts to preserve areas more resilient to climate change, and thus essential for reef protection and restoration. BOOST results can be combined with other “Nature Based Solution” to improve reef restoration strategies and may even make it possible to consider seeding some coral reef portions with plankton or organic matter.

Understanding and forecasting the proliferation of holopelagic Sargassum in the tropical Atlantic requires progress in modeling the transport and biology of the macroalgae. Current models focus mostly on the transport part and base the biological part on relatively outdated knowledge for Sargassum fluitans or S. natans indistinctively. However, there is now strong evidence of the presence of three co-occurring morphotypes of Sargassum, with possibly different physiologies, and that the nutrients controling their growth may vary considerably among regions. Therefore we need to gain knowledge in the morphotype-specific response to varying environmental conditions and on the origin of nutrients in order to improve our capacity to simulate and forecast Sargassum biomass. The main objectives of BIOMAS are to acquire the necessary knowledge to build an individual-based model of Sargassum growth, to include that model into a drift model, and finally perform simulations of the integrated growth-drift model in order to forecast Sargassum morphotypes proliferation at seasonal scale. These objectives will be achieved through a strong synergy between laboratory experiments and modeling. The project is structured under : WP1, development of a growth model of Sargassum morphotyes based on the Dynamic Energy Budget (DEB) theory; WP2, laboratory experiments for calibrating the DEB model; WP3 in situ monitoring focusing on morphotypes and elemental composition in the Western and Eastern Atlantic; WP4, integration of the DEB model into a drift model and simulations. The consortium has expertise and experience in Sargassum biology, laboratory and field works, DEB and drift modeling, and is already involved in Sargassum research. Pan-Atlantic sampling will occur through the involvment of Brazilian, Mexican and African partners. The general public, including schools, will be engaged through a smartphone app that will serve both citizen science and results dissemination.

Thanks to their many advantages plastics have become ubiquitous in our society. Their low recyclability and poor end-of-life management lead to 10 to 20 million tons released into the environment each year. Because plastics degrade slowly, they persist in the environment where they are fragmented into smaller particle notably through UV-degradation. Over the last decades, micro/nano-plastics (MP/NP) have contaminated the Ocean and marine species at all levels of the food chain, from one pole to another down to the deep sea. Polyethylene (PE) is the most widely used plastic which accounts for 90% of plastic waste accumulated in the environment, yet most research on MP/NP uses commercially available polystyrene particles. The lack of environmentally realistic NP/MP models is a major and under-studied technical obstacle in this research field. Currently no method can easily produce PE particles because of the gaseous nature of the monomer and the poor solubility of the polymer. Three strategies are proposed for the production of PE MP/NP: 1) by cryo-grinding 2) by micro-emulsification and 3) by radical polymerization of the ethylene in aqueous emulsion. The first method will produce micro-sized MP, formulated with a known additive in order to identify its toxicity. However, grinding particles down to 10 μm from pristine polymers remain challenging and will be improved through a preliminary UV-irradiation of the pellets which will also help to reproduce weathering of the polymer. In the second method, the PE is solubilized in hot toluene followed by emulsification in water under vigorous stirring in the presence of biosurfactant (i.e. polysaccharides secreted by microalgae). This process allows producing particles ranging from 0.8 to a few μm which will be optimized by varying the molar masses (reduction of dispersed phase viscosity) and the oxidation state of the PE (less hydrophobic). The latter method makes it possible to produce particles below 500 nm and narrow size dispersity to precisely study the effect of size on toxicity. In this case, the use of a biosurfactant is incompatible with the polymerization conditions (degradation of the polysaccharide at high temperature). Therefore, a non-ionic polar surfactant will be used and later exchanged with the biosurfactant by dialysis. In addition, a small amount of polar comonomer will be introduced in order to simulate aging, thanks to their greater UV sensitivity. The expertise of polymer chemists and physical chemists will enable to develop a wide range of realistic PE MP/NP by reproducing the alteration of the properties of the polymer (photodegradation induced by UV) as well as the formation of a biofilm. The presence of a biofilm on the surface of MP/NP can have a significant impact on their buoyancy and aggregation. Thus, the study of the colloidal stability of PE particles coated with a biofilm will be carried out in order to better understand their behavior in the marine environment and their bioaccumulation. The production of stable aggregates of NP is a great asset to study their toxicity below the range 2-200 μm ingested by the filtering model organism chosen here for toxicity studies: the oyster. The assessment of the ecotoxicological effects of NP/MP will greatly benefit from the synthesis of such tailor-made PE particles by focusing on the two main mechanisms of toxicity known to date: i) identifying the most toxic additives (e.g. endocrine disruptor) in order to consider their substitution and ii) the nanotoxicity as NP increases physical interaction notably damages to biological membranes induced by their large specific surface.

Primary producers form the basis of marine food webs, control population sizes at higher trophic levels and fish stock recruitment. Marine photoautotrophic organisms are also responsible for nearly half of the global net primary production, i.e., they replenish the ocean (and atmosphere) with oxygen and fix substantial amounts of carbon. Despite its outstanding relevance for the functioning of marine ecosystems and global climate, past primary production dynamics and mechanisms controlling them are not well characterized. This is particularly true for nearshore coastal environments and for times prior to significant human perturbation of biogeochemical cycles. Available data sources for changes in marine primary production (i) do not provide the necessary temporal resolution to resolve short-lived and spatially restricted phytoplankton blooms, specifically in shallow waters, (ii) are too short to distinguish trends from low-frequency cycles of primary production and (iii) do not cover the entirety of photoautotroph taxa which include more than just phytoplankton and cyanobacteria, i.e. microphytobenthos and macroalgae. Therefore, this project will develop an innovative technique that can provide reliable, temporally well-constrained, seasonally to inter-annually resolved data on past primary production dynamics in coastal nearshore environments based on shells of bivalve mollusks. For this purpose, we will test and refine existing proxies (surrogates) for primary production, and develop new proxies and integrate them in a multiproxy approach. In order to obtain a mechanistic understanding of how information on the species composition and number of marine photoautotrophs is recorded in chemical properties (Ba/Ca, Mo/Ca, Li/Ca, stable isotopes of carbon and nitrogen, triple isotope composition of oxygen, pigments) and color (hue and saturation index) of the shells, field and tank experiments will be conducted during which environmental variables can be closely monitored and manipulated. Since the study involves experiments with living bivalves we chose the fast-growing species, Pecten maximus, and an ecosystem that has been studied in great detail, the Bay of Brest, France. The multiproxy approach will subsequently by applied to subfossil shells collected from an archaeological site to determine the human impact on primary production dynamics of the Bay of Brest including (i) the seasonal occurrence of photoautotrophs as well as the intensity, frequency and seasonal timing of phytoplankton blooms, (ii) shifts in species composition and biomass through time and (iii) changes in the link between organisms inhabiting the sea floor and those living near sea surface. Results of this study will significantly advance marine sciences including paleoecology, paleoclimatology and fisheries sciences.

HABs are now considered as a major environmental, societal and economic concern for the sustainability of marine ecosystems and their uses and appear as a scientific challenge in national and international research strategies (Paris Agreement, UNESCO). HAB affect coastal ecosystem ecology, structure of marine communities and life-history traits of ecological and economic important species and thus the food web they support and related socio-ecosystems with significant associated costs, of for example more than 800 million euros/year in Europe alone.Often synchronized with the reproduction period of bivalves, these HABs are suspected to be responsible for the recruitment defects of bivalves regularly observed along the French coast. These recruitment anomalies could, in the long term, modify the structure of wild populations and affect aquaculture production, thus the sustainability of the resources of exploited species. Until recently, the HAB species mostly studied were those causing paralytic, amnesic or diarrheic syndromes in humans following the consumption of contaminated shellfish. But some of these species, as well as other so-called ichthyotoxic species (e.g. Kareniacae) regularly observed or emerging on European coasts produce bioactive compounds. Their chemical nature is unknown but recent studies suggest that these compounds are extremely deleterious for marine species. In a context of global changes associated with HAB intensification and the need to develop aquaculture to support the food needs of a growing world population, HABIS project will analyze the vulnerability of exploited bivalves to regularly blooming or emerging HAB species along French coasts. We will focus on consequences of HAB on 3 life-traits determining bivalve stock renewal: reproduction, development and recruitment of selected bivalve species (WP2&4) and the identification of causative agents responsible for HAB toxicity (WP3). Our funnel-shaped strategy relies first on in vitro large screening of HAB toxicity using bioassays on gametes (developed in HABIS) and larvae (existing) (WP2). The HAB/bivalve pairs for which toxicity is highest are selected for WP3&4. For these pairs, the toxic compounds of HAB will be researched (WP3). Meanwhile, the cellular (flow cytometry, imaging) and molecular mechanisms (transcriptomics, epigenomics) of toxicity on bivalve reproduction and development will be studied by an integrative physiological approach from individual to gene, over a generation and the offspring thanks to a strong experimental force of the consortium (WP4). Results from WP2 to 4 will implement bioenergetic models to prediction purposes (WP4). HABIS will actively participate to public awareness of the consequences of increasing human ecological footprints and global change on the development of HAB and their impacts on human health and marine ecosystems by developing classes, art and communication towards general public and transfer knowledge to stakeholders (WP5). This multidisciplinary project will bring together biologists, physiologists and chemists, and will also involve some professional partners, such as a private hatchery, the local aquaculture and fisheries committee and artists involved in communications actions. HABIS will participate to a better knowledge on HAB toxicity and their consequences on marine resources, especially on reproduction, a key component of population fitness. This basic knowledge is critical to further determine the vulnerability of molluscan populations in a scenario of global change. HABIS will also participate to a better consideration of HAB phenomenon by public, stakeholders and decision-makers through communication actions and research in environmental law. The approaches used in HABIS will be applicable to other biological models enabling future improvement of management of shellfish resources for sustainability of productive ecosystems.