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The present work is based on a dataset comprised of 31 Biogeochemical (BGC) Argo floats that collected 0-1000 m vertical profiles of biogeochemical and optical data from 2012 to 2016 in the Mediterranean Sea. The dataset was integrated in 1-dimensional model simulations following the trajectories of each float and considering measured photosynthetically available radiation (PAR) profiles as the reference light parameterization. The simulations were aimed to be consistent with data measured by float sensors, especially in terms of the deep chlorophyll maximum (DCM) depth. Moreover, we tested several light models in order to estimate their impact on modeled biogeochemical properties, including self-shading dynamics based on chlorophyll and colored dissolved organic matter (CDOM) concentrations. The results, evaluated with the corresponding in-situ BGC-Argo chlorophyll data, indicate that the proposed approach allows to properly simulate the chlorophyll dynamics and illustrate how PAR and vertical mixing are essential environmental regulation factors driving primary producers dynamics. The higher skills are reached using in-situ PAR, but some of the alternative bio-optical models here presented show comparable skill in reproducing DCM depth spatial variability. Simulation results show that during the stratification phase the diel cycle has significant impact on the surface chlorophyll regimes. The approach here presented serves as a computationally smooth solution to analyse BGC-Argo floats data and to corroborate hypotheses on their spatio-temporal variability. [ABSTRACT FROM AUTHOR]

The HYdrological cycle in the Mediterranean Experiment (HyMeX) Special Observing Period 2 (SOP2, January 27-March 15, 2013) was dedicated to the study of dense water formation in the Gulf of Lion in the northwestern Mediterranean. This paper outlines the deep convection of winter 2012-2013 and the meteorological conditions that produced it. Alternating phases of mixing and restratification are related to periods of high and low heat losses, respectively. High-resolution, realistic, three-dimensional models are essential for assessing the intricacy of buoyancy fluxes, horizontal advection, and convective processes. At the submesoscale, vertical velocities resulting from symmetric instabilities of the density front bounding the convection zone are crucial for ventilating the deep ocean. Finally, concomitant atmospheric and oceanic data extracted from the comprehensive SOP2 data set highlight the rapid, coupled evolution of oceanic and atmospheric boundary layer characteristics during a strong wind event. [ABSTRACT FROM AUTHOR]

A large set of biogeochemical (nutrients, dissolved and particulate organic matter), phytoplanktonic (biomass and photosynthetic activity) and bacterial (bacterial diversity and ectoenzymatic activities) parameters were determined to understand how the severe drought period relative to the 2009-2010 El Niño Modoki episode influenced biogeochemical cycling and phyto- and bacterio-plankton communities in Terminos Lagoon (Mexico) potentially prefiguring future environmental conditions due to expected trends in climate change. During the study period, the water column of Terminos Lagoon functioned globally as a sink, and especially as a "nitrogen assimilator", because of high production of particulate and dissolved organic matter although exportation of autochthonous matter to the Gulf of Mexico was weak. Coupling between top-down and bottom-up controls accounted for the diverse responses in phytoplankton productivity. Nitrogen and phosphorus stoichiometry mostly accounted for the heterogeneity in phytoplankton and bacteria distribution in the lagoon. In the Eastern part, we found a clear decoupling between areas enriched in dissolved inorganic nitrogen in the North close to Puerto real coastal inlet and areas enriched in phosphate (PO4) in the South close to the Candelaria estuary. Such a decoupling limited the potential for primary production resulting in an accumulation of dissolved organic carbon and nitrogen (DOC and DON, respectively) close to the river mouth. In the Western part of the lagoon, maximal phytoplankton development resulted from the coupling between Palizada river inputs of nitrate (NO3) and particulate organic phosphorus -PP- (but depleted in PO4) and bacterial activity transforming PP and dissolved organic phosphorus (DOP) to available PO4. The Chumpan River only marginally contributed to PO4 inputs due to its very low contribution to overall river inputs. We also found that a complex array of biogeochemical and phytoplanktonic parameters were the driving force behind the geographical distribution of bacterial community structure and activities. Finally, we showed that nutrients brought by the Palizada River supported an abundant bacterial community of polycyclic aromatic hydrocarbon (PAH)-degraders, which are of significance in this important oil production zone. [ABSTRACT FROM AUTHOR]

We evaluated the relative impact of anthropogenic polycyclic aromatic hydrocarbons (PAHs) among biogeochemical variables on total, metabolically active, and PAH bacterial communities in summer and winter in surface microlayer (SML) and subsurface seawaters (SSW) across short transects along the NW Mediterranean coast from three harbors, one wastewater effluent, and one nearshore observatory reference site. At both seasons, significant correlations were found between dissolved total PAH concentrations and PAH-degrading bacteria that formed a gradient from the shore to nearshore waters. Accumulation of PAH degraders was particularly high in the SML, where PAHs accumulated. Harbors and wastewater outfalls influenced drastically and in a different way the total and active bacterial community structure, but they only impacted the communities from the nearshore zone (<2 km from the shore). By using direct multivariate statistical analysis, we confirmed the significant effect of PAH concentrations on the spatial and temporal dynamic of total and active communities in this area, but this effect was putted in perspective by the importance of other biogeochemical variables. [ABSTRACT FROM AUTHOR]

We measured phytoplankton primary production and heterotrophic bacterial activities on microplastics and seawater in the Northwestern Mediterranean Sea during two 3-month spring periods over 2 consecutive years. Microorganisms growing on a 5 mm diameter low density polyethylene films (LDPE; 200 μm thick) faced two contrasting conditions depending on the year. Spring 2018 was characterized by consistent nutrient inputs and bloom development. In spring 2019, nutrient inputs and bloom were low. For the first time, we observed a clear coupling between primary production and heterotrophic prokaryote production on microplastics during both years, but with different intensity between years that reflected the crucial role of the trophic environmental conditions (nutrient supply) in shaping microbial activities on plastics. We found that high primary production on plastics could support the whole (net autotrophy) or the majority of the bacterial carbon demand needed for heterotrophic activities, supplemented by other carbon sources if surrounding waters are highly productive. We propose that microbial activity on plastics influences the microbial community in the surrounding seawater, especially when the environmental conditions are less favorable. An illustrative image of the role of plastics in the environment could be that of an inverter in an electrical circuit that mitigates both positive and negative variations. Our results highlight the potential role of the plastisphere in shaping biogeochemical cycles in the context of increasing amounts of plastic particles in the marine environment. [Display omitted] • Microplastics are the seat of strong coupled autotrophic and heterotrophic activities. • Microplastics can be either a sink or a source of organic matter for the environment. • Microplastic biofilms are relatively unaffected by environmental nutrient depletion. • Marine primary production is enhanced by material released from the plastic biofilms. [ABSTRACT FROM AUTHOR]

Total alkalinity (A T) and dissolved inorganic carbon (C T) in the oceans are important properties to understand the ocean carbon cycle and its link with global change (ocean carbon sinks and sources, ocean acidification) and ultimately find carbon based solutions or mitigation procedures (marine carbon removal). We present an extended database (SNAPO-CO2, Metzl et al, 2024d) with 24700 new additional data for the period 2002 to 2023. The full database now includes more than 67000 A T and C T observations along with basic ancillary data (time and space location, depth, temperature and salinity) in various oceanic regions obtained since 1993 mainly in the frame of French research projects. This includes both surface and water columns data acquired in open oceans, coastal zones, rivers and in the Mediterranean Sea and either from time-series or punctual cruises. Most A T and C T data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ± 4 µmol kg-1 for both A T and C T. The same technique was used onboard for underway measurements during cruises conducted in the Southern Indian and Southern Oceans. The A T and C T data from these cruises are also added in this synthesis. The data are provided in one dataset for the global ocean (https://doi.org/10.17882/102337) that offers a direct use for regional or global purposes, e.g. A T/Salinity relationships, long-term C T estimates, constraint and validation of diagnostics C T and A T reconstructed fields or ocean carbon and coupled climate/carbon models simulations, as well as data derived from Biogeochemical-Argo (BGC-Argo) floats. These data can also be used to calculate pH, fugacity of CO2 (f CO2) and other carbon system properties to derive ocean acidification rates or air-sea CO2 fluxes. [ABSTRACT FROM AUTHOR]

We investigated spatiotemporal variations of nutrients, dissolved organic pools (C, N, P), phosphomonoesterase (PME) and phosphodiesterase (PDE) activities, heterotrophic prokaryotic production and planktonic microorganisms within the mixed layer (ML) in the Eastern Mediterranean Sea. We characterized two contrasted situations: autumn 2018 (highly stratified period, deep chlorophyll maximum within 100 m depth) and winter 2019 (including a bloom period). We compared the distribution of biogeochemical variables within the mixed layer and hydrological vertical structure between the different stations using a principal component analysis. Six groups of stations were identified (one group in autumn, 5 in winter), based on variable physical descriptors but also environmental biogeochemical conditions related to i) the seasonal aspect (for instance, all stations sampled within the Ierapetra anticyclone in autumn clustered in one, single group); ii) transitions between cyclonic and anticyclonic structures with a large range of ML depths (18–269 m) and indications of intense, preceding winter convection events: iii) progression of a high phytoplankton bloom during the winter cruise inferred from a series of observations: a strong nitrate drawdown, important growth of Synechococcus , pico and nano eukaryotes, accumulation of chlorophyll a (>60 mg m−2), primary production rates up to 509 mg C m−2 d−1, changes in the pigments' diversity, increase in biomass-specific ectoenzymatic activities and of heterotrophic prokaryotic production, all in conjunction with the vicinity of the Rhodes gyre. Here, we studied the distribution of biological and biogeochemical properties within the mixed layer, in particular by employing sensitive methods for the detection of low phosphate concentrations and of the labile dissolved organic phosphorus pool. From this data set, we demonstrate that the surface mixed layer classically considered as a P-depleted and uniform layer in the Eastern Mediterranean Sea was highly biologically dynamic, and prone to rapid spatio-temporal changes in phosphatase activities and phytoplankton dynamics. Altogether, these data reveal a strong short-term population dynamics. The results highlight the role of mixing episodes in winter, which provide pulsed supplies of phosphate and/or nitrate from the deeper layer to the euphotic zone, triggering transient blooms that often go undetected by satellites. • Significant role of mixing episodes in winter in eastern Mediterranean Sea. • Meso-scale variability in the mixed layer (P-depleted) affects P cycle and populations. • Phosphomonoesterase and phosphodiesterase kinetics tracers of P availability. • An important, winter phytoplankton bloom evidenced in the vicinity of the Rhodes gyre. • Highest cell specific phosphatases were seen in the Rhodes gyre. [ABSTRACT FROM AUTHOR]

Biosourced and biodegradable plastics offer a promising solution to reduce environmental impacts of plastics for specific applications. Here, we report a novel bacterium named Alteromonas plasticoclasticus MED1 isolated from the marine plastisphere that forms biofilms on foils of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Experiments of degradation halo, plastic matrix weight loss, bacterial oxygen consumption and heterotrophic biosynthetic activity showed that the bacterial isolate MED1 is able to degrade PHBV and to use it as carbon and energy source. The likely entire metabolic pathway specifically expressed by this bacterium grown on PHBV matrices was shown by further genomic and transcriptomic analysis. In addition to a gene coding for a probable secreted depolymerase, a gene cluster was located that encodes characteristic enzymes involved in the complete depolymerization of PHBV, the transport of oligomers, and in the conversion of the monomers into intermediates of central carbon metabolism. The transcriptomic experiments showed the activation of the glyoxylate shunt during PHBV degradation, setting the isocitrate dehydrogenase activity as regulated branching point of the carbon flow entering the tricarboxylic acid cycle. Our study also shows the potential of exploring the natural plastisphere to discover new bacteria with promising metabolic capabilities. [Display omitted] ● A. plasticoclasticus MED1, a novel bacteria isolated from the plastisphere. ● MED1 is a PHBV-degrader forming biofilms. ● Complete metabolic pathway of MED1 for PHBV degradation. ● Activation of the glyoxylate shunt during PHBV degradation. [ABSTRACT FROM AUTHOR]

Over recent years, biodegradable polymers have been proposed to reduce environmental impacts of plastics for specific applications. The production of polyhydroxyalkanoates (PHA) by using diverse carbon sources provides further benefits for the sustainable development of biodegradable plastics. Here, we present the first study evaluating the impact of physical, chemical and biological factors driving the biodegradability of various tailor-made PHAs in the marine environment. Our multidisciplinary approach demonstrated that the chemical structure of the polymer (i.e. the side chain size for short- vs. medium-chain PHA) which was intrinsically correlated to the physico-chemical properties, together with the specificity of the biofilm growing on plastic films (i.e., the associated 'plastisphere') were the main drivers of the PHA biodegradation in the marine environment. [Display omitted] • Marine biodegradation is much higher on scl- than mcl- PHA. • PHA types mainly influence biodegradation. • Identification of distinct bacteria involved in scl- or mcl- PHA biodegradation. [ABSTRACT FROM AUTHOR]

• The biodegradation of 7 polymers in the marine environment was investigated to challenge their application as potential substitutes of the conventional microbeads used in cosmetics and personal care products. • Innovative 2-stepwise experiments were designed to mimic realistic natural conditions. • The four biodegradation steps were characterized, including biodeterioration, biofragmentation, bioassimilation and mineralization. • The biosourced PHBV is proposed as the best biodegradable polymer tested to replace the conventional microbeads. Plastic microbeads have been produced at very large scales since several decades for cosmetics and personal care products, a large amount of which is being directly transported in the oceans. To better understand their biodegradability in marine environment and evaluate their possible replacement by biodegradable polymers, seven polymer types were studied including three conventional petroleum-based polymers (PE, PMMA, and PCL), two bio-based polymers (PLA and PHBV), and two natural products (rice seeds and apricot kernel). We used several innovative approaches by both the experimental design and the set of multidisciplinary techniques to follow the successive steps of biodegradation together with abiotic degradation under seawater conditions. Congruent signs of biodegradability were observed by oxygen consumption, weight loss, and modification of molecular weight, 1H NMR, and mass spectrometry profiles. We found that microparticles made of PHBV, PCL, rice, and to a lesser extent apricot were biodegradable under our conditions, while not microparticles made of PE, PMMA, and PLA. Taking into consideration of the industrial application, PHBV could be the potential material to replace the conventional microbeads, since the material is resistant to hydrolysis, but susceptible to microbial degradation, as shown by the 1H NMR results. This study provides the first time arguments for the use of biodegradable substitutes to replace conventional microbeads to support the recent legislative rules aiming to reduce the pollution by primary microplastics in the oceans. [Display omitted] [ABSTRACT FROM AUTHOR]

The thin film of life that inhabits all plastics in the oceans, so-called "plastisphere," has multiple effects on the fate and impacts of plastic in the marine environment. Here, we aimed to evaluate the relative influence of the plastic size, shape, chemical composition, and environmental changes such as a phytoplankton bloom in shaping the plastisphere abundance, diversity and activity. Polyethylene (PE) and polylactide acid (PLA) together with glass controls in the forms of meso-debris (18 mm diameter) and large-microplastics (LMP; 3 mm diameter), as well as small-microplastics (SMP) of 100 μm diameter with spherical or irregular shapes were immerged in seawater during 2 months. Results of bacterial abundance (confocal microscopy) and diversity (16S rRNA Illumina sequencing) indicated that the three classical biofilm colonization phases (primo-colonization after 3 days; growing phase after 10 days; maturation phase after 30 days) were not influenced by the size and the shape of the materials, even when a diatom bloom ( Pseudo-nitzschia sp.) occurred after the first month of incubation. However, plastic size and shape had an effect on bacterial activity ( 3 H leucine incorporation). Bacterial communities associated with the material of 100 μm size fraction showed the highest activity compared to all other material sizes. A mature biofilm developed within 30 days on all material types, with higher bacterial abundance on the plastics compared to glass, and distinct bacterial assemblages were detected on each material type. The diatom bloom event had a great impact on the plastisphere of all materials, resulting in a drastic change in diversity and activity. Our results showed that the plastic size and shape had relatively low influence on the plastisphere abundance, diversity, and activity, as compared to the plastic composition or the presence of a phytoplankton bloom., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Cheng, Jacquin, Conan, Pujo-Pay, Barbe, George, Fabre, Bruzaud, Ter Halle, Meistertzheim and Ghiglione.)

Diatoms are one of the major primary producers in the ocean, responsible annually for ~20% of photosynthetically fixed CO 2 on Earth. In oceanic models, they are typically represented as large (>20 µm) microphytoplankton. However, many diatoms belong to the nanophytoplankton (2-20 µm) and a few species even overlap with the picoplanktonic size-class (<2 µm). Due to their minute size and difficulty of detection they are poorly characterized. Here we describe a massive spring bloom of the smallest known diatom (Minidiscus) in the northwestern Mediterranean Sea. Analysis of Tara Oceans data, together with literature review, reveal a general oversight of the significance of these small diatoms at the global scale. We further evidence that they can reach the seafloor at high sinking rates, implying the need to revise our classical binary vision of pico- and nanoplanktonic cells fueling the microbial loop, while only microphytoplankton sustain secondary trophic levels and carbon export.